Astrophysics and Space Science
Following the repair of the SALT optics in late 2010, the telescope has reentered the commissioning phase with its principal scientific instruments, SALTICAM and RSS. Both these instruments have undergone major refurbishments in their own right, and their current status and that of the telescope will be described, together with an update on its performance.
The advent of international wideband communication by optical fibre has produced a revolution in communications and the use of the internet. Many African countries are now connected to undersea fibre linking them to other African countries and to other continents. Previously international communication was by microwave links through geostationary satellites. These are becoming redundant in some countries as optical fibre takes over, as this provides 1000 times the bandwidth of the satellite links. In the 1970's and 1980's some two dozen large (30m diameter class) antennas were built in various African countries to provide the satellite links. As these antennas become redundant, the possibility exists to convert them for radio astronomy at a cost of roughly one tenth that of a new antenna of similar size. HartRAO and the SA SKA Project have started exploring this possibility with some of the African countries.
ThunderKAT is one of ten accepted MeerKAT Large Survey Projects. ThunderKAT will study all aspects of transient radio emission associated with accretion and explosive events. Through a comprehensive and complementary programme of monitoring Galactic synchrotron transients (across a range of compact accretors and a range of other explosive phenomena) and exploring distinct populations of extragalactic synchrotron transients (microquasars, supernovae (SNe) and possibly yet unknown transient phenomena) we will revolutionise our understanding of the dynamic and explosive transient radio sky. We will give an overview of ThunderKAT and discuss the prospects of studying the transient sky with KAT-7.
With the birth of the new generation ground-based gamma-ray imaging atmospheric Cherenkov telescope arrays (such as H.E.S.S., MAGIC and VERITAS), there is a growing need to probe X-ray binary stars for very high energy gamma-ray emissions. Since the discovery of the first extra-solar X-ray binary (XRB), namely the Scorpius X-1 in 1962, XRBs are now well-established systems in the realm of X-ray astronomy. Today Scorpius X-1, with X-ray output energy 100 000 times greater than the total radiation of the Sun at all wavelengths, is today known as a microquasar. In order to foster the multi-wavelength campaigns of the day, we selected candidate XRBs from the Galaxy that were observed by H.E.S.S. for purposes of searching for significant TeV gamma-ray emission from these. Paredes (2008) confirmed four XRBs to be candidates that can be listed on the gamma-ray sky map. These were PSR B1259-63, LS I +61 303, LS 5039 and Cygnus X-1. From a survey of 125 known XRBs, Dickinson (2009) reported no conclusive detections of TeV emissions. The present study presents the Cherenkov technique, a briefing on X-ray binaries, known radiation mechanisms, relevant analysis techniques, and report on the preliminary results of Galactic XRBs that do not add any of the candidate XRBs to the envisaged catalogue of TeV gamma-ray binaries. It is an astrophysical hope that the envisaged CTA (Cherenkov Telescope Array), which may be hosted by South Africa, will provide the anticipated discoveries, providing a deep insight of the TeV gamma-ray sky.
The Fermi Large Area Telescope (LAT) has revolutionized high-energy (HE) astronomy, and is making enormous contributions particularly to gamma-ray pulsar science. As a result of the many new pulsar discoveries, the gamma-ray pulsar population is now approaching 100. Some very famous millisecond pulsars (MSPs) have also been detected: J1939+2134 (a.k.a. B1937+21), the first MSP ever discovered, as well as J1959+2048 (a.k.a. J1957+20), the first black widow pulsar system. These along with other MSPs such as PSR J0034-0534 and J2214+3000 (and also including the Crab pulsar), are unique among the pulsar population in that they exhibit nearly phase-aligned radio and gamma-ray light curves (LCs). Traditionally, pulsar LCs have been modeled using standard HE models in conjunction with low-altitude conal beam radio models. However, a different approach is needed to account for phase-aligned LCs. We explored two scenarios: one where both the radio and gamma-ray emission originate in the outer magnetosphere, and one where the emission comes from near the polar caps on the stellar surface. We find best-fit LCs using a Markov Chain Monte Carlo technique for the first class of models. The first scenario seems to be somewhat preferred, as is also hinted at by the radio polarization data. This implies that the phase-aligned LCs are possibly of caustic origin produced in the outer magnetosphere, in contrast to the usual lower-altitude conal beam radio models. We lastly constrain the emission altitudes with typical uncertainties of 10% of the light cylinder radius. The modelled pulsars are members of a third gamma-ray MSP subclass, in addition to two others with non-aligned radio and gamma-ray LCs.
Since the launch of the Large Area Telescope (LAT) on board the Fermi spacecraft in June 2008, the number of observed gamma-ray pulsars has increased dramatically. A large number of these are also observed at radio frequencies. Weltevrede et al. (2010) derived constraints on the viewing geometries of 6 gamma-ray pulsars exhibiting single-peaked gamma-ray profiles using high-quality radio polarization data. We obtain independent constraints on the viewing geometries by using a geometric emission code to model the Fermi-LAT and radio light curves. We find fits for the magnetic inclination and observer angles by searching the solution space by eye. Our results are generally consistent with those obtained by Weltevrede et al. (2010), although we do find differences in some cases. We will indicate how the gamma-ray and radio pulse shapes as well as their relative phase lags lead to constraints in the solution space. A more rigorous approach, the Monte Carlo Markov Chain technique, is able to statistically find best-fit light curves in addition to constraining several model parameters and estimating errors on these.
Applied and Industrial Physics
High concentration photovoltaic (CPV) systems utilize non-imaging optics to concentrate and distribute the solar flux uniformly onto a solar receiver to achieve maximum performance and power output from a CPV module. However, in many cases due to mechanical misalignment, tracker error and imperfections in the optical material, the optimum performance of the module is compromised. A LabVIEW programme employing visualization was used to determine the main contributing factor for current generation, i.e. position and intensity of the distribution. The topography was determined by multiple roster scans with a spectroradiometer and optical fiber in the plane of the reflective secondary’s aperture where the cell would be placed. The results showed different currents been generated at different points on the cell surface. These results were put into a CPV cell current-voltage (I-V) characteristic simulator to extract I-V curve at each point. These were then compared with measured I-V curves obtained from the CPV system. The results showed that there was a non-uniform current densities (Jsc) distribution due to non-uniform spectral and intensity distribution across the cell surface.
Efficient thermal management of low concentrator photovoltaic (LCPV) modules ensures that the maximum power capabilities of the LCPV system are harnessed, and may substantially prolong the operating lifetime of the photovoltaic (PV) cells. A general understanding of the thermal transfer properties of PV modules is thus necessary to effectively design, construct, and implement viable LCPV systems. A basic thermal model based on one-dimensional heat transfer was developed and includes various energy dissipation mechanisms, such as convection and radiation. Panel fans were used to simulate air flow across the surface of the PV module and temperature measurements of the front and back surface of the PV module were recorded to analyse energy dissipation in accordance with the basic thermal model. Initial results indicate that 50% of the incoming irradiance is dissipated through convection and radiation. Optical losses will also be incurred through reflection from the surface of the PV module. The basic thermal model manages to account for most of the energy incident on a PV module, and thus effectively illustrates the principle of energy conservation within the PV system.
The benefit of concentrating sunlight onto solar cells is that expensive solar cell material can be reduced and replaced with relatively inexpensive optical elements, which has the potential to reduce the cost of solar energy. In low concentration photovoltaics (LCPV), solar cells are subjected to higher irradiance levels. The electrical output, and hence efficiency of a LCPV module is dependent on the amount of irradiance, temperature and more importantly the uniformity of illumination across the solar cells. This paper discusses the design and characterization of the optical subsystem of a LCPV concentrator. By optimizing the aperture area of the LCPV concentrator while still maintaining a uniform illumination intensity across the solar cells, the module’s electrical performance can be maximized. In the study a mathematical model was developed to design and evaluate suitable optical elements for LCPV application. This model was based on a facetted reflector that was designed to meet a predetermined set of boundary conditions. Initial evaluation of an experimental LCPV concentrator, based on this type of design, will be discussed in this paper. Results from the model illustrate the angles and positions of receiver and reflectors for maximum geometric concentration ratio as well as the best illumination profile across the receiver.
Recent developments in electricity regulatory environment and the possible inclusion of small, kW-scale, photovoltaic (PV) systems in the feed-in tariff framework has led to an increased interest in these systems. In South Africa, grid reliability issues are not fully mitigated by employing a straight grid-tie PV system, with users sometimes wanting some battery backup for load-shed periods at night. In this study a system comprising a PV array, maximum power point tracker, a battery bank and inverter was monitored using a custom-build data acquisition system. The monitored parameters include; environmental conditions, DC and AC power, and energy flow within the system. This paper will discuss the influence of environmental parameters and load requirements on the performance of the system. Issues such as soiling and other loss mechanisms are also discussed.
In order for solar energy to become an alternative to traditional fossil fuel energy it is important that defects in photovoltaic (PV) modules can be easily identified. Typically a module is characterised by measuring the current-voltage (I-V) characteristics under standard test conditions. In addition to this electroluminescence (EL) and thermal imaging can be used to identify the location of defects in the module. A cooled Si CCD camera is used to detect the EL emitted from a forward biased PV module. The intensity of this light is related to the minority carrier concentration and the emitted EL thus provides a visual representation of defects in the cell. Infrared imaging can be used to identify irregular heating patterns which are indicative of defective cells or contacts. In this study we used both techniques in conjunction to assess the defects present in a 36 cell custom-made single crystalline silicon module. Defects that were identified in this study showed that areas around the contacts appeared very bright in EL images and this corresponded with hot areas in the infrared images. These results show that the electrical contacts in these areas are poor.
We consider a solar trough system in which cylindrical parabolic mirrors focus the sun’s radiation onto a receiver pipe, heating it. Conventionally, the pipe is enclosed in a glass cover under vacuum and the dominant radiation losses are reduced by the use of a selective coating on the receiver pipe. We study the suitability of applying a ‘hot mirror’ coating on the glass cover instead, which transmits in the visible but reflects well in the infra-red. We compare the performance of the ‘selective’ with a ‘hot mirror’ coating using the results from simulations for a general heat transfer model. It is seen that a hot mirror is a viable alternative, and certainly allows higher temperatures of the working fluid and therefore higher Carnot efficiency. The optimum is a hybrid system, with selective coating applied at the low temperature end of the receiver pipe and the hot mirror being used at higher temperatures.
Carbon nanoparticles embedded in thin NiO layers have shown an increasing interest for application in spectrally selective solar absorbers that can be used for low temperature photothermal applications. It is expected that the size and shape of carbon nanoparticles in the NiO matrix is extremely important in tuning the selectivity of the coatings. In this work, multiwall carbon nanotubes (MWCNT)/NiO composite films of different CNT contents were prepared by sol-gel technique, and their structural and optical properties were investigated by scanning electron microscopy, x-ray diffraction, Raman spectroscopy, UV-Vis spectroscopy and emissometer. Preliminary durability study on these coatings will also be presented. The results demonstrate promising spectrally selective properties indicating that the CNT/NiO composite is an excellent choice for solar water heating applications.
We report on studies of thermodynamic, structural and electronic properties of substitutional defects involving C, B, N and vacancies in models of single-walled carbon and boron nitride nanotubes. Using the generalized gradient approximation (GGA) for the exchange-correlation functional, we perform first principles calculations within the framework of density functional theory to optimize fully the geometries of the systems in their ground states. We give detailed accounts of the relaxed geometries. We compare the heats of formation of the various point defects, and we draw conclusions about the relative stability of the these defects. We study the changes to the electronic structure for these defect systems, and further investigate the detailed nature of the defects. We make a comparative study of the C and BN nanotube systems.Where data is available, comparisons are made.
The adsorption of Sb on the (111) plane of noble metals surfaces such as copper and silver has long been of practical interest in the fields of surface science and technology. The electronic properties of such surface alloys are important for several surface and interface related applications like metal-metal or semiconductor-metal heteroepitaxy, heterogeneous catalysis, sensors and spintronics applications. The structure of these monolayers on metal substrates is a complex interplay of electronic, stress and geometric effects, all related to each other. Theoretical calculations have suggested that the energetics of the Sb/Cu(111) system are such that in the ordered 0.33 ML Cu(111) (√3x√3) R30°-Sb phase the Sb atoms substitute one-third of the outermost Cu atoms to produce an ordered surface alloy after annealing. Due to its ability to act as a surfactant (low surface energy), Sb segregates from the bulk of the substrate to remain on the surface, thus forming a surface alloy. This work present an in situ Variable Temperature Scanning Tunneling Microscopy (VT-STM) study of the segregation and dissolution kinetics of Sb/Cu(111) studied at various temperatures. The study shows the growth mechanism of Sb mediated by the kinetics and thermodynamics at the substrate surface. After deposition of ~0.3 ML Sb on a clean Cu(111) surface, STM images exhibit Sb atoms as bright spots surrounded by six copper atoms at the surface with perturbed atomic positions resulting in a broken structural inversion symmetry at the surface.. The atomic arrangements of Sb remain stable during several STM scans.
Most properties of polycrystalline materials are controlled by the structure and interfaces they possess – e.g., grain boundaries. When a Cu surface is alloyed with antimony, the Sb atoms appear to diffuse to the growth surface and continuing to act as a surfactant throughout the growth process. Copper alloys are important commercial materials, which are often used at temperatures where segregation and diffusion processes has a huge influence on their properties. An important characteristic of these alloys is that segregation of one component to the alloy surface causes the surface composition to differ significantly from the bulk composition. This study utilizes an ultrahigh vaccum Variable temperature Scanning Tunneling Microscopy (UHV-VTSTM) and Low-Energy Electron Diffraction-Auger Electron Spectroscopy (LEED-AES) to determine the segregation and dissolution temperatures of Sb on Cu polycrystals and the growth mechanism. The VT-STM images of the Cu surfaces showed localization of Sb atoms in the vicinity of the step edges. The STM data showed that alloying initially occurred at the monatomic steps and a homogeneously spread Cu-Sb surface alloy was formed by the migration of Sb atoms.
We carried out ab initio self-consistent calculations to study the modification of the band offsets by polar layers. We considered a double line of C in a monolayer of hexagonal BN, also known as Boronitrene. This effectively introduces a line of dipoles at the interface. We considered BN/C zigzag and armchair configurations. The BN/C zigzag-short is found to be energetically most stable and with the smallest (1.51eV). We show that a two dimensional diode can be constructed from this structure, with changes to the electronic and optical properties of the host system.
Black chromium-chromium (III) oxide cermet (Ceramic-Metal) nano-composites exhibit attractive solar absorbance characteristics. They are used widely as selective solar heat thermal collector nano-coatings. These black chromium-chromium (III) oxide selective solar absorbers are produced by various physical vapor deposition techniques such as DC and RF reactive sputtering, pulsed sputtering method, and e-beam and thermal evaporation techniques. This contribution reports on large surface coatings of black chromium-chromium (III) oxide cermet by using a novel cost effective physical-chemical method: the Aqueous Chemical Growth (ACG). More specifically, the present study addresses the growth mechanism of the uniform fine (in the range of micron to nano)-spherical particles of α-Cr2O3 using the ACG method at low temperature. The growth kinetic of the quasi-mono-dispersed spherical particles of chromium (III) oxide, α-Cr2O3, in aqueous solution was studied as a function of ageing time. The small fine spherical particles of α-Cr2O3 revealed by SEM micrographs suggests a growth mechanism based on the diffusion-limited Ostwald ripening process on the basis of LSW theory. The experimental data were analyzed in the frame of existing coarsening and particle growth mechanisms, i.e. Ostwald ripening. A good fits of the growth of α-Cr2O3 particles with d3 model confirms the growth mechanism to be mainly diffusion-controlled.Two major configurations have been considered initially: Cr black coated α-Cr2O3 nano-particles coatings/Cupper substrate and Cr black coated α-Cr2O3 nano-particles coatings/discontinuous ultra-thin Cr layer/Cupper substrate. The optimized configurations exhibit a low diffuse reflectance as well as low transmittance in the VIS-NIR spectrum sustaining therefore their high solar absorbance property.
Control of diameter, length and level of nitrogen doping in synthesis of nitrogen doped carbon nanotubes (N-CNTs) can be achieved by careful selection of catalyst, precursor mixture and chemical vapour deposition (CVD) synthesis temperature. Solutions of ethanol and acetonitrile (20% acetonitrile) were pyrolysed over a Fe/Al/MgO catalyst in a CVD reactor at temperatures from 700 OC to 1000 OC. The products were characterised by TGA, TEM and Raman spectroscopy. N-CNTs with well graphitised morphology, small diameter, and narrow diameter distribution were obtained at CVD temperatures from 850 OC to 900 OC. Below 800 OC the N-CNTs with large seized diameters were formed and above 900 OC the product yield became very low and the carbon nanotubes were malformed with wide diameter distribution. By pyrolysing precursor mixtures of acetonitrile and alcohols (methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol and octanol) we found out that short, 500 nm in length N-CNTs were obtained by the pyrolysis of long chain alcohols, (heptanol and octanol) and acetonitrile solution whilst longer N-CNTs were formed from pyrolysis of earlier alcohols (methanol, ethanol and propanol). Short and thin carbon nanotubes can be useful in carbon based nanoelectronics and photovoltaic devices since there is less entanglement of N-CNTs which will result in good dispersion of carbon nanotubes in solutions and polymer matrix. Preliminary results of the devices made from N-CNTs will be presented.
Covellite copper monosulphide (CuS) is a representative I–VI chalcogenide semiconductor with the unique photoelectricity transformation characteristic. Due to quantum confinement effect, CuS nanoparticles exhibit optical and electrical properties as compared to the bulk materials. CuS is an indirect semiconductor with a bandgap around 1.7 eV. At high temperature CuS exhibits fast ion conduction and shows metal-like electrical conductivity and is an important semiconductor because of its excellent optical and electronic functionality. Colloidal method was used to develop CuS nanoparticles from copper acetate monohydrate and Thioacetamide in water and methanol using alanine as a stabilizing agent at pH 10 at different low temperatures. Water soluble CuS quantum dots in the form of rods and spherical shaped with an average diameter of 3-10 nm has been successfully obtained. The as-obtained CuS nanoparticles were characterized by X-ray diffractometry (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), Infrared spectra (FT-IR), UV-visible spectroscopy (UV) and Photoluminescence (PL). Effect of temperature was investigated for the stability of nanoparticles
We report on the synthesis and electrical characterization of nano-crystalline diamond films grown by hot filament chemical vapor deposition. Low temperature magnetoresistance measurements have shown a crossover in conductivity from hopping, activated to semi-metallic conduction as the nitrogen level in the CVD chamber is increased from 10% - 15%, 20% to 22% respectively. Raman spectroscopy studies and SEM micrograph indicate a change in morphology with deposition parameters such as pressure, temperature and gas chemistry while AFM micrograph suggests a columnar growth process. In the semi-metallic regime the conduction is characterized by anisotropic 3D weak localization mechanism with a weak temperature dependence of the dephasing time (D) approximately (D) T-0.7. Such behavior has been reported in Superlattice structures. Understanding of the transport properties in nano-crystalline diamond films is a perquisite for the application of diamond films in nano-electronics for fast switching and field emission devices.
Long afterglow calcium aluminate phosphors doped with Eu2+ and co-doped with Nd3+ /or Dy3+ (CaAl2O4:Eu2+, Nd3+, Dy3+) were fabricated by urea-nitrate solution – combustion method at 500 °C. The effects of varying concentrations of Ca:Al and co-dopants molar ratio (Nd3+:Dy3+) on the structural and luminescent properties of the phosphor were investigated. It was observed that Ca:Al molar ratios greatly affect the crystalline structure of the material. The results of the X-ray diffraction (XRD) analysis reveal that the formation of several crystalline phases depends on the molar ratios of the host material. The peaks show the presence of other phases such as Ca3Al2O6 and CaAl4O7 but the predominant phase formed was that of CaAl2O4. As the concentration of Ca ions increase, the peak intensity increase which led to formation of monoclinic CaAl2O4 as a single phase for the 1.5% Ca. However it was found that the crystalline structure is generally not affected by variation of the co-dopants concentration. PL studies revealed a general rise in intensity with increase in the mole ratio of Ca:Al . The highest PL intensity was observed with 1.5% Ca but was weak with 0.1% Ca. Also it is evident that as the fraction of Nd3+ increase and that of Dy3+ decrease proportionally good emission intensities are formed in the region 400nm-500nm at the ratio 1:1. The initial luminescent intensity and rate of decay vary from each other when co-doped with various proportions of Nd3+ and Dy3+. Low Nd3+ exhibited high initial luminescent intensity but low decay rate because of insufficient trap vacancies in the host matrix.
The magnetic and electrical properties of nitrogen doped carbon nanospheres can be probed using Electron Magnetic Resonance (EMR) techniques. Previous Raman spectroscopy studies show that both the pristine and doped spheres are graphitic in character. However, XRD measurement showed that these spheres consist of graphitic crystallites embedded in an amorphous phase. Previous ESR measurements showed large peak on nitrogen doped spheres, implying that the nitrogen occupies substitutional sites in graphitic lattice. The EMR measurements clearly showed large Curie-type paramagnetism at low temperatures for all the samples investigated. The EMR peak-to-peak linewidth have provided evidence of the phonon bottleneck effect in one of the sample investigated, meaning to say that there is a strong interaction between localized and conduction electrons. The broadening of linewidth may be manifested by a Korringa type interaction whereby the spin-lattice relaxation time T1 is inversely proportional to the temperature. The sample with the highest concentration of nitrogen behaves very differently from the other samples studied in th temperature range 77 K to 300 K, and this behaviour is not well understood at present. This may involve contributions from electron scattering with stationary nitrogen impurities, and therefore different spin relaxation mechanisms which are related to the Elliot and Yafet mechanisms.
Laser pyrolysis has been used to synthesize tungsten oxide (WO3-x) nanostructures, and here we report on the production of six-sided nanostars. The proposed mechanism used to explain the growth of the stars is the concentration difference and gradient mechanism which speculates that a high local concentration of one reactant mixed with a low concentration of another reactant under ambient conditions, and the high concentration favoured the thermodynamic conditions for crystal growth and the low concentration resulted in a diffusion-controlled kinetic environment for growth of hierarchical structures. Apart from precursor concentration, further analysis was carried out to determine the influence of varying laser wavelengths and power densities in such experiments. The laser wavelength was varied between 9.22-10.82 m at a fixed power density of 51.2 W/cm2, and the laser power density was varied between 17-110 W/cm2 at a fixed wavelength of 10.6 m. Annealing the samples at 450oC in argon atmosphere for 17 hours appeared to be an essential step for further growth of nanostructures. Particle size and morphology were determined by scanning and transmission electron microscopy and the chemical composition was determined by x-ray diffraction studies in conjunction with Raman spectroscopy and energy dispersive x-ray spectroscopy to confirm the tungsten oxide phase as a function of the laser parameters.
This study explores learners’ knowledge of measuring instruments (Ammeter and Voltmeter) in a direct current resistive circuit. It investigates learners’ understanding of how instruments should be connected and why they have to be connected in a particular way. Ammeter and voltmeter are used to determine the behavior of a circuit by indicating the readings of current and voltage, respectively. An educator sees an ammeter as an “invisible” component because it does not change the characteristics of a circuit. However, a learner views an ammeter differently. A paper-and-pencil questionnaire was administerd to grade 12 learners. Group interviews were conducted in order to validate and support the results of the questionnaire. The results indicate that learners lacked the basic understanding of the role played by meters (why we need them in electrical circuits). It was also evident that due to lack of practical experience with real circuits, learners did not know how these meters should be connected. This paper will report on some of the results and their implications for teaching/learning of electricity.
Mathematics is an essential tool in studying physics, i.e., it will be difficult to study Physics without the sound basics of Mathematics. It is even called the “language of physics” (Redish, 2005). Physicists blend conceptual physics with mathematical skills and use them to solve and interpret equations and graphs. For instance, in kinematics, different aspects from mathematics such as knowledge of functions and the solving of equations are combined with physics concepts. Many introductory physics students perform poorly on the use of mathematical skills and interpretations of graphs in physics. Two possible reasons may be: (1) Students lack the necessary mathematical skills needed to solve the physics problems and (2) Students do not know how to apply and relate their mathematical skills in the context of physics. These possible reasons were investigated in a Masters Research project which is still in progress, which probed first year university students’ interpretations of graphs in kinematics and in mathematics. This paper will use the idea of Beichner’s standardized questionnaire on kinematic graphs. From this questionnaire, an equivalent questionnaire was devised in the context of Mathematical equations and graphs. The results of the investigation tend to indicate the deficiencies in the students’ mathematical conceptual knowledge as well as in the transfer of mathematical skills that they possess to solve kinematic equations and graphs.
An Investigation of students’ expectations when they are accepted to study at the University of Pretoria, how they study for physics and their reactions after the first semester test. We find that most students are shocked after their first semester test, because they expected to obtain high marks and don’t, or the other way around.
The purpose of this study was to investigate the pre-knowledge of the Physics I students on vectors. The study was conducted on 234 Physics I students from the University of Limpopo (Medunsa campus). The sample was divided into four groups, where the first group (n = 119) did their grade 12 in 2010, the second group (n = 46) did their grade 12 prior 2010, the third group (n = 42) did Foundation Physics in 2010 and the fourth group (n = 27) were those repeating Physics I. An ex post facto research design was chosen for the study whereby all the groups were given a vector test at the beginning. The test was divided into three questions, testing definitions, classifications, drawing and interpretations of graphs. Results showed a less difference between average percentage of the groups whereby the group before 2010 (47.4%), foundation group (47.3%), the repeaters (44.3%) and those who did grade 12 in 2010 (44.1%).
A simple pinhole camera was used to demonstrate to students that light travels in a straight line. The one to one correspondence between a point on the object and a point on the image was used to explain why the image was upside down and reversed from left to right.
The investigation reflects on the causes of unsatisfactory students’ performance on the section involving forces during tests. It has been realized that more often than not students tend to be excited after undertaking a test on forces. However, subsequent feedback usually paints a different picture and this has become a worrying trend. The results of the investigation in this regard seem to suggest that students have inadequate understanding of sizes, directions and points of actions on the body. In addition, students also appear to encounter difficulties in distinguishing between the pushing and the pulling interactions.
Lasers, Optics and Spectroscopy
A dielectric barrier discharge CO2 TEA laser excited by a thyratron driven power supply has been developed and characterized. Laser output was observed at frequencies up to 25 Hz for an electrode separation of 10 mm with 1.8 mm Suprasil glass covering the electrodes. At this gap separation, pulsed power output of about 9 W was detected for gas pressures between 100 and 400 mbar. Changing the electrode gap separation to 5 mm and using 1.4 mm Suprasil glass dielectric increased the output power to 23 W and enabled laser output to be observed at gas pressures up to 700 mbar and maximum pulse excitation frequencies of up to 400 Hz. The developed laser does not require water cooling since the system operates in burst pulse mode.
A method of experimentally decomposing a field within an apertured plano-concave resonator cavity into a forward propagating field (plane to concave) and a backward propagating field (concave to plane) is presented. The use of an intra-cavity aperture which is a simple method of laser beam shaping is a means of mode selection as higher-order modes are actively discriminated. This apertured cavity is modelled by two fundamental resonator theories, namely, Fox-Li analysis and Laguerre-Gaussian decomposition where they are used in the determination of the respective beam profiles at a specific plane. These theories traditionally differ from one another for an apertured cavity where the Gaussian beam within the cavity is truncated. A preliminary set-up is characterized for Gaussian propagation in an attempt to verify that the cavity is viable and accurate. A comparison of experimental data with the theories is presented.
Passively Q-switched Nd:YAG lasers with Cr4+:YAG as a saturable absorber that can operate with pulse repetition frequencies(PRF) in the hundreds of kilohertz typically have problems with pulse to pulse fluctuations of pulse energy and width and the associated timing jitter as well as drift in PRF. One technique for stabilizing the PRF and reducing fluctuations is modulation of pump power. This technique has been simulated using a plane wave rate equation model with square wave pump modulation at frequency of 100 kHz. Locking of the pulse repetition rate is achieved at the modulation frequency over a range of pump powers. This locking range is dependent on the modulation function, a square wave with a 50% duty cycle and a modulation depth of 0.5 produced a 13% locking range. Additionally smaller regions of stability also occur at higher pulse repetition frequencies at higher pump powers. The effect on the stability of pulse energy and width, and the timing rate jitter of this technique have been investigated.
We have previously demonstrated a novel laser pulse energy control system based on two analog controllers to control a Q-switched Nd:YLF laser. We now present the improved technique based on a custom-designed high-speed FPGA (field programmable gate array) digital logic controller, together with experimental results from controlling a high-energy Ho:YLF ring laser. Laser pulses of constant and predictable energies are required in most applications, especially materials processing, micro-machining and medical laser surgery. Furthermore, for these applications the time between individual laser pulses may be purposely varied, which result in significant pulse-to-pulse variations in energy. These variations can cause damage and/or non-uniform processing of optics and target materials. Simultaneous electronic control of both an intra-cavity loss modulator (acousto-optic modulator) and the laser pump source (laser diode) provides the means to accurately stabilise and control the output pulse energy. This “dual loop” electronic feedback technique has been successfully demonstrated using two analog PID (proportional, integral and derivative) controllers. We have now implemented the control in a high-speed digital controller, adding several advantages in terms of computer programmability, flexibility and accuracy. The digital electronics consist of a FPGA processing unit (which can calculate programmed control algorithms at high-speed in parallel) sandwiched between high-speed analog-to-digital and digital-to-analog converters. The digital dual loop pulse control system was connected to a custom developed 2 micron single-frequency Ho:YLF ring laser. Normally, injection-seeding and resonator-length control (based on the Pound-Drever-Hall technique) are used to achieve stable single-frequency pulsed operation. The digital pulse control system provided a less expensive and more compact method for stable operation, and could selectively produce between 15 – 45mJ pulses set via a computer control interface. The control could also maintain constant pulse energies (within 4.3% of the set-point) across a range of pulse repetition rates.
Lasers and amplifiers emitting 2µm single-frequency pulses are used for spectroscopy, remote sensing, and defence applications. The laser sources group at the CSIR National Laser Centre has developed a number of 2µm lasers and amplifiers over the course of five years. We report on three of these, all of which achieved record breaking output energies. To generate the high energy 2µm pulses, we developed a 70mJ single-frequency, Q-switched Ho:YLF ring MOPA. Both the ring laser and pre-amplifier were pumped by a single commercial 80W, 1940nm, Tm:fibre laser from IPG. The seed laser system delivered up to 73mJ per pulse at 50Hz, with a pulse duration of ~365ns in a diffraction limited beam. This system set two world records: it delivered the highest energy from a Ho doped laser, pumped with one Tm:fibre laser, and it was also the highest single-frequency singly-doped Ho:YLF laser. In order to scale these pulse energies even further, we developed a Tm:YLF pumped slab amplifier system. Amplified single-frequency pulses of up to 210mJ were generated from a 43mm long Brewster cut Ho:YLF and a 20mm long Ho:LuLF slab crystal. However, numerical simulations indicated that longer Ho doped crystals between 80 to 120mm in length would perform significantly better. A new Ho:YLF slab amplifier was therefore built which consisted of two 50mm long crystals placed close together in series in a double seed pass configuration. This amplifier delivered >330mJ of single-frequency pulses and a small signal gain in excess of 40. This is the highest reported single-frequency 2µm energy from a Ho:YLF slab MOPA system. The high-energy 2µm pulses were then used to pump a 4µm molecular HBr MOPA, which will also be reported at this conference.
Optically pumped molecular lasers are attractive alternatives to generate high energy pulses in the mid-infrared wavelength range, compared to non-linear conversion that can suffer from optical damage. One such laser is based on HBr, for which it has been experimentally demonstrated to emit laser light in the 4 µm wavelength range when pumped at 1.3 µm or at 2 µm. Prior to the work presented here the highest reported output energy from an optically pumped HBr oscillator was 2.5 mJ in a 132 ns pulse at 50 Hz repetition rate when pumped at 2.064 µm. To scale the output energy of this technology we have designed and implemented an HBr master oscillator power amplifier (MOPA) system which was pumped by a Ho:YLF slab amplifier (pumped by a Tm:YLF slab laser) which amplified the 70 mJ output of our single longitudinal mode 2.064 µm Ho:YLF oscillator (pumped by a Tm:fibre laser) to 330 mJ per pulse at 50 Hz pulse repetition rate. Initially all the available pump energy from the Ho:YLF amplifier was coupled into the HBr oscillator which produced up to 5.5 mJ per pulse for 200 mJ incident energy, at which point optical damage of the HBr oscillator cell window was observed due to the high intensity of the pump light in the double-pass configuration. The oscillator was subsequently operated at a reduced input energy of 50 – 60 mJ at the point where the HBr laser was most efficient with respect to incident energy. The HBr MOPA system produced 9 mJ per pulse when seeded with 2.3 mJ from the HBr oscillator and pumped with 200 mJ incident energy, of which approximately 75% was absorbed when the HBr amplifier cell pressure was 60 mBar. The measured output wavelengths of the HBr MOPA were 4.20 µm and 4.34 µm.
To our knowledge, we have demonstrated the first optically pumped tunable HBr oscillator lasing on both the P- and the R-branches. Through the implementation of an intra-cavity diffractive grating, we could tune the wavelength over a range of 538 nm from 3872 nm to 4410 nm. A single-frequency Ho:YLF ring laser and amplifier system was used as pump source. The oscillator was pumped in a double pass configuration with 50 mJ, 2064 nm pulses at a repetition rate of 50 Hz. The HBr was kept at 52 mBar, 20 degrees Celsius in a 510 mm tube. The resonator cavity consisted of a flat output coupler mirror with a reflectivity of R=80% at 4 μm, and a high-reflective coating at 2 μm. The input coupler mirror had a concave curvature of 5 m and was coated to be highly reflective at 4 μm and highly transmissive at 2 μm to allow pumping of the HBr. The input coupler mirror was slightly tilted to reflect light unto a flat mirror which in turn reflected unto the diffractive grating which was blazed at 5 μm. The diffractive holographic grating therefore acted as the backreflector mirror, and the wavelength could be selected by rotating the grating. Lasing was demonstrated on eight lines of the P-branch with wavelengths ranging from 4102 nm to 4410 nm, and five lines of the R-branch ranging from 3872 nm to 3999 nm. The highest output energy for the given pump power was 1.1 mJ at 4131 nm.
Nuclear, Particle and Radiation Physics
Quantum condensation of alpha particles is believed to occur in cluster states of the self-conjugate alpha nuclei 12C and 16O. This phenomenon is similar to the Bose-Einstein condensation observed through the laser cooling of atoms such as Rb or Na. A state of this nature, which must be of 0+ character, is expected to exist just above the 5 alpha decay threshold in 20Ne. In this region, only high spin states Jiπ≥4+ are known. This is mainly due to the types of reactions usually used to populate states in this nucleus. Alpha-particle inelastic scattering will be performed close to zero degrees with the K600 magnetic spectrometer at iThemba LABS. This will provide a sensitive probe for states of the 0+ character which may be associated with the 5 alpha particle condensate state, while enhancing the L = 0 angular momentum peaks. Additionally, measurements will be made at two larger angles in order to demonstrate the L = 0 character.
Motivated by several theoretical cluster calculations that predict the existence of the 06+ state in 16O located at 15.1 MeV as a very good candidate for the equivalence of the Hoyle state in 12C, a high resolution measurement of 16O spectrum in coincidence with the 16O decay products was proposed using a (p,t) reaction at 200 MeV and 0o. The decay products of the excited 16O recoil nucleus were detected by means of Double Sided Silicon Detectors and the outgoing tritons were identified in the K600 magnetic spectrometer focal plane. Data were collected with targets containing 18O, 16O, 12C and 7Li. First results on precise measurement of energy spectra of the corresponding recoil nuclei using the K600 as the master trigger and extracted cross sections will be discussed.
In our experiments devoted to studying of a new ternary decay of low excited heavy nuclei called “collinear cluster tri-partition” (CCT) a specific CCT mode was observed based on double magic 132Sn cluster. Pre-scission configuration which presumably gives rise to the mode under discussion can be obtained. The Sn cluster can "move" as a whole along the cylinder-like configuration that consists of residual nucleons. Two light fragments accompanying this cluster and marked by symbols M1 and M2 were actually detected in previous experiments. The value of M2 lies between 0amu and the difference between the initial mass of 252Cf and the detected fragments. M1 cannot assume any value less than 95amu (deformed magic 95Rb). The question that arises is whether 132Sn can also be changed by double magic 208Pb. This would lead to a new type of lead radioactivity. Searching for such a mode is one of the goals of our forthcoming experiment, which will require better statistics and more precise time-of-flights measurements. Testing of a specially designed setup aimed at addressing and solving these problems is one of our current plans.
Cluster-like structures in the shell-model description of the ground state of nuclei can be conveniently studied by means of knockout reactions. Of these the (p, p alpha) reaction is perhaps the simplest, especially from the viewpoint of the tractability of theoretical calculations used to interpret experimental results. The distorted wave impulse approximation (DWIA) is a versatile theory which is applicable to a knockout reaction. Fortunately its results appear to be reasonably insensitive to uncertainties in the exact ingredients, such as distorting optical potentials, which are obtained from unrelated elastic scattering studies. It will be shown that a number of surprisingly simple approximations in the DWIA hold for alpha-cluster knockout from light nuclei. Furthermore, results for a medium-mass nuclear target such as 40Ca are also consistent with expectation. Extracted spectroscopic factors are in reasonable agreement with shell-model estimates.
In heavy ion reactions the emission of Intermediate Mass Fragments (IMFs) at forward angles is dominated by the direct break-up process. To improve our understanding of this process in the reaction mechanisms involving the interaction of light projectiles with light to heavy target nuclei, a coincidence measurement was performed at iThemba LABS. Standard ΔE-E detector telescopes were used to identify and measure the energies of the of the correlated 8Be and alpha particles produced in the binary break-up of 12C projectiles at an incident energy of 400 MeV. While the 8Be fragments were detected in their ground state at a fixed angle of 9º, the correlated alpha particles were measured on the opposite side of the beam, covering an angular range from 16º to 26º. Two dimensional energy spectra were generated for each alpha-particle angle in order to distinguish quasi-elastic events from inelastic break-up events. These spectra also allowed to identify events originating from the interaction of the 12C beam with an H contaminant on the target foils and to subsequently correct for these events in the extraction of angular distributions for alpha particles in coincidence with quasi-elastic 8Be particles. The angular distributions obtained from the interaction of 12C with 93Nb and 197Au show a smooth decreasing trend with respect to the alpha-particle angle, which suggests that the binary break-up of 12C seems to be independent of the target nucleus. A deviation from this trend is however observed for the 12C target. As a first attempt to interpret these results a comparison between the measured angular distributions and GEANT4 simulations will be presented.
The fusion evaporation reaction is mainly used to populate high spin states in atomic nuclei. Using stable beams, a large number of β + unstable nuclei were studied i.e. those nuclei on the neutron deficient side of the valley of stability. A plethora of experimental data is available in a wide range of mass A ∼ 20 to A ∼ 250 and was used to test the statistical model codes such as PACE, CASCADE, GEMINI. With the advent of radioactive beams, a new landscape will be available to provide further tests to the models. However, a study of fusion evaporation reactions will only be possible with sufficient beam intensities. In this research β − unstable nuclei were populated with one of the few reactions where nuclei approaching the neutron rich region can be populated with stable beams. Due to the relatively low Coulomb barrier the evaporation of charged particles is a relatively strong channel. The DIAMANT light charged particle detector was set in coincidence with the AFRODITE γ-ray spectrometer to trigger between light charged particles and γ-rays. The coincident data allows the evaporation channels involving p, d, t and particles to be identified due to excellent particle identification with the CsI charged particle detectors while the A, Z of the residual nuclei are identified with the gamma transitions measured with germanium detectors. In this work, the cross sections of the residual nuclei were calculated with the statistical model code CACARIZO, a Monte Carlo version of CASCADE based on the Hauser-Feshbach formalism. Experimental and theoretical results and charged particle energy spectra are compared.
Theoretical and Computational Physics
A symmetrized and dynamical density matrix renormalization group is used to study 1-dimensional extended Peierls-Hubbard model at half-filling. We have investigated the optical conductivity spectrum and electro-absorption spectrum for low-lying optical exciton with strong, intermediate and weak coupling parameter sets for the on-site and neighbor interactions. We were able to capture the Stark effect in the case of strong coupling under static electric field. The intermediate coupling was found to be clear for weak electric field and destroyed for strong one. We were not able to resolve the splitting in the case of weak coupling due to the small binding energy so that small value of electric field could distroy the exciton.
The origin of numerical errors, which in certain cases lead to complete failure of the direct method for solving the 1-dimensional Marchenko integral equation, are investigated. Bargmann and block potentials, for which exact analytical expressions exist, are used to compare the accuracy of numerical results. In particular, numerical results from the Nystrom method are compared to those of two novel collocation methods. The results indicate that, for roughly the same computational effort, the three method may be ranked in order of increasing accuracy: Nystrom method, equidistant collocation method and non-equidistant collocation method. In all three cases the origin of the failure, which occurs for barrier-like potentials of relatively large width and height, may be attributed to the numerical instability introduced in both the Fourier inversion and the linearization steps. Subsequently these errors produce perturbations on the matrix elements of the inversion matrix, a matrix which is found to become increasingly ill-conditioned as the height and width of the potential increases. At the current limits of successful inversion, we attribute the "early" onset of inversion failures, to the aforementioned matrix element perturbations. However at still higher potential height and width, the inherent ill-conditioning of the inversion matrix alone, accounts for the inversion failures. We therefore conclude that there is an inherent upper-bound on the application of the direct inversion method for inverting relatively high and wide potentials.
The pressure induced B3-B1 phase transition has been studied using both density functional theory (DFT) and quantum Monte Carlo (QMC) methods. We present results obtained using the local density approximation (LDA), PW91-GGA generalized gradient approximation, hybrid density functionals and QMC. The changes in the equation of state has also been investigated using the different functionals and form the results obtained, we find that the choice of functional significantly effect the to the equation of state. The results of the B3-B1 phase transition pressure for DFT using the different functionals and QMC are reported and they demonstrate good agreement with experimental data.
Understanding the magnetic effects of ultra-dense, cold nuclear matter is of particular importance when investigating the magnetic properties of dense matter systems such as neutron stars. One property that we are interested in is the possibility of generating a magnetic field in nuclear matter as the central density increases. We investigate this possibility by employing a relativistic, self-consistent calculation to capture the interaction of the neutrons, protons and electrons with the magnetic field. In this talk we present this model for describing ferromagnetism in dense nuclear matter systems. Our current results will also be presented.
The theory of continuous measurement provides a tool to monitor the evolution of the wave function of a single quantum system in real time. We derive the master equation in the non-selective regime for the dynamics of the wave function of a particle in an external potential which is subject to continuous measurement of position. In the derivation we view continuous measurement as the limit of a sequence of unsharp position measurements. Unsharp position measurements are described by generalised measurement observables, or in mathematical terms, positive operator valued measures (POVM) rather than the standard von Neumann projection operators which are a special class of the sub-class of POVM's called projection valued measures (PVM). We discuss a method which introduces a commutative algebra for non-commuting operators in order to carry out the summation of the corresponding measurement operators. In addition we reveal the stochastic Ito equations for the selective regime of measurement.
We investigate the scattering of a wave-packet off a soliton in the (1+1) dimensional kink model. We solve the classical, time-dependent field equation numerically subject to the initial condition that the wave-packet is widely separated from the kink soliton at very early times and propagates towards the soliton. After some time the wave-packet interacts with the static soliton and departs from it at later times. At very late times the wave-packet is finally again separated from the soliton. We then extract the scattering matrix from the distorted wave-packet and compare it to the known result from the static scattering calculation. This investigation constitutes a first step towards studying crossing symmetry in soliton models, i.e. in a framework beyond perturbation theory.
Quantum walks of particles obeying bose statistics are introduced. In such a quantum walks the conditional shift operation is performed with the single coin tossing for the whole lattice. An explicit form for the transition probabilities in a single step is derived. This allow to describe the evolution of an arbitrary state and an arbitrary number of steps. This model easily embrace the concepts such as the join probability, the counting statistics and the high order correlations. It also presents the computational challenges arising from the exponential increase in the number of basis states entering into the lattice state as a function of the number quantum walkers and the number of steps. Possible solutions are proposed in some applications of the model to quantum walks on finite graphs.
Astrophysics and Space Science
The highly transient novalike variable AE Aqr is perhaps an ideal laboratory to study accretion related astrophysical fluid dynamics. It consists of a fast spinning highly magnetic white dwarf (WD) orbiting, and accreting mass, from a low-mass main sequence companion. The system emits, and has been detected in almost all wavelengths. AE Aqr is in a propeller state, and most of its emission properties are associated with the turbulent interaction between the accretion flow and the fast rotating magnetosphere of the WD. We have analysed its X-ray spectra using contemporaneous Chandra and Swift X-ray data. The results of this study show that the X-ray emission has both thermal and non-thermal characteristics. The thermal X-ray emission is modeled to be the result of bremsstrahlung emission of heated mass outflow above the polar caps, whereas the non-thermal X-ray emission is the result of synchrotron radiation of accelerated electrons outside the light cylinder radius of the WD. In this paper, some of the results, based on the constraints of the proposed thermal and non-thermal emission mechanisms, will be presented.
Some eclipsing contact binary stars of the W UMa-type are known to undergo changes in orbital period. These changes can be as a result of the light travel time effect if the contact binary is a member of a multiple stellar system or due to some intrinsic phenomena that are poorly understood at this stage. Analysing systems that appear to be undergoing changes in orbital period may shed some light on the important physical processes that occur in close binary systems. Pilecki et al. (2007) searched through the All Sky Automated Survey (ASAS) database for semi-detached and contact binary systems with high period change rates. They present 31 interacting binaries whose periods either increased (10) or decreased (21) in a five year interval of observations. Using data from the Wide Angle Search for Planets (SuperWASP) project, it has been possible to do period analyses using O – C diagrams which provide a more reliable measure of period change. The results of the analyses will be presented.
Kazarovets et al. (1999) gave General Catalogue of Variable Star (GCVS) designations to 3157 variable stars that had been identified from observations made using the Hipparcos satellite. The variables were classified into standard GCVS categories according to their photometric light curves, but no other parameters of the variables were documented. V-band photometric data for many of the Hipparcos-identified eclipsing binaries can be found in the All Sky Automated Survey (ASAS) and Wide Angle Search for Planets (SuperWASP) databases. In several cases the ASAS classifications, which were done semi-automatically and without cross referencing other databases, differ from those presented in IBVS 4659. Using the ASAS and/or SuperWASP data, the classifications are being checked, orbital periods are being determined and, where possible, photometric solutions to the light curves are being obtained. This talk will present the results of this investigation.
Pigulski and Kolaczkowski (2002, A&A 388, 88) announced the first discovery of Beta Cephei (BCep) pulsators in the LMC. This was a remarkable discovery, since theoretical analyses of pulsational stability had previously predicted that early B main-sequence stars with metallicities lower than Z = 0.01 should not pulsate at all (e.g. Pamyatnykh 1999, Acta Astron 49, 199). Following this announcement, and announcements of 92 BCep candidates in the LMC by Kolaczkowski and Pigulski (2006, MemSAIt. 77, 336), more detailed studies adopting a variety of opacity calculations and metal mixtures indicated that BCep pulsations could be explained in low-metallicity environments after all (Miglio et al. 2007, MNRAS 375, L21; Miglio et al. 2007, Com.Ast. 151, 48; Zdravkov and Pamyatnykh 2008, J.Phys.Conf.Ser. 118, 012079). In order to ascertain the nature of these pulsations, multi-colour photometry of sufficient precision is required. We have obtained 4 weeks of UBVI photometry on two fields in the LMC that surround stars which have been identified as strong Beta Cephei candidates from OGLE data. We report on the results of this photometric campaign.
I adapt a recent work by Zahn et al. on the shape of rapidly rotating stars to explore the degree of oblateness for uniform, differential and shellular rotation of the star. I also discuss the relation of these results to the classical Roche limit.
Applied and Industrial Physics
This paper presents results of long term temperature monitoring in an energy efficient solar house at the University of Fort Hare, South Africa. Measured data was stored by a datalogger every 10 minutes. Formulas for predicting the daily indoor maximum, average and minimum temperatures were developed on the basis of outdoor climatic parameters. Passive solar housing aims to raise and lower indoor temperatures in winter and summer respectively. As a result, analysis of the data and development of predictive formulas of indoor temperature were done separately on part of the winter and summer seasons. The models were then validated against measurements taken in different time periods. Results indicated that indoor maximum, average and minimum temperatures can be predicted on the basis of outdoor temperature. Prediction of maximum indoor temperature was improved by incorporating daily solar irradiance in the formula. It was also revealed that indoor temperatures are affected by outdoor temperatures of the previous two days. The different ways at which the house is cooling and heating were also investigated.
Few options are apparent for achieving a benign, sustainable energy future except those relying on the utilization of solar energy in one way or another. Experience with fossil and nuclear sources has revealed an array of unsolved problems of increasing concern as more evidence and understanding emerges. Africa has the world’s best solar resources. Several countries have exploited solar energy for water heating, crop drying, medical applications, and telecommunications, among other things. Solar energy can contribute to supply heat energy in households of Sub-Saharan African countries. Having this in mind a small scale dish concentrating solar energy system is being developed at the Eduardo Mondlane University in Mozambique. The system comprises of the collector, the heat storage and the oven. The main structure of the system has been constructed, consisting of the dish reflector, the sun tracker and the piping mechanism. The reflector consists of 6 petals which together resemble nearly a parabolic surface with the following parameters: Diameter................ Dp = 2400mm Focal length............ f = 915mm F/D ratio..................... n=0.135 Half opening angle..............66.50o Half complementary angle........23.49o Angle of incidence..............33.26 The process of gluing trapezoidal mirrors was done on individual petals using the laser beam radiation. Using reflection laws on planar surfaces which states that rays of light parallel to the parabola axis are reflected to a focal point, each trapezoidal mirror tile gluing was preceded by scanning its reflection close to the theoretical focal point. An individual petal was covered by 163 mirror tiles. The reflector surface is comprised by 978 mirror tiles. An infrared camera was used to scan the temperature of the concentrated heat. The maximum temperature reached so far is 350oC. The experimental focal area was found by mapping the reflections of the whole reflective surface. The shape of the receiver/absorber obtained by the scanning process is semi-spherical. The future work is the study of efficiency of the collector as a whole. This will be done by measuring the ambient air temperature, the inlet and outlet temperature of the absorber.
Electrostatic ion propulsion systems resist miniaturization due to constraints imposed by the size of the discharge chamber. We introduce a thruster concept where the same field is responsible for both ionization of the neutrals and acceleration of the ions, by letting the neutral propellant gas escape into a high field region through a thin, hollow needle at high electric potential. The ionization mechanism is thus reminiscent of corona ionization. Although the thruster only ionizes a small fraction of the neutral gas, the ions nevertheless impart a great deal of momentum to the plume, creating an ion wind. We propose a model to estimate the electric behavior of the system, and two further models for the obtained thrust. A comparison with experimental data shows that the models capture the dominant physical effects and give a reasonable description of the system. Apart from being about a thousand times less massive than conventional systems, the thruster, which is at the proof-of-concept stage, performed quite well yielding around 0.3 mN/Watt during initial tests. The thruster small size and simplicity are advantageous in many situations, such as for satellite station keeping and deep space probes.
Miniaturization of space prolusion systems has many advantages, both from an economic and performance perspective. One of the most promising propulsion technologies, the gridded electrostatic ion engine, resists miniaturization due to the poor down-scaling characteristics of the ion producing discharge chamber. We investigate a system where the ionization and acceleration mechanisms are coupled, thereby making the same electric field responsible for both. This sidesteps the need of a discharge chamber and allows for design of a much smaller engine. We introduce the concept and present some initial results, commenting on the feasibility of such a design.
High Voltage Direct Current (HVDC) overhead power transmission trends are currently advancing toward high system voltages over very long distances in a bid to viably tap from remotely located renewable energy sources. Corona effect plays a very crucial role in the design of overhead power transmission systems. Through corona, part of the energy carried on the transmission line is expended through ionization and movement of charges in the air dielectric. Corona limitations influence selection of key line parameters such as diameter of phase conductors, the number of conductors per phase and conductor clearances to the ground. Since overhead transmission lines are installed in open air, the generated electric fields are non-uniform. As a result, the accompanying ionization in the surrounding air is non-uniform and does not occur throughout the inter-electrode gap. Instead, the ionization is confined to a very small region around the high voltage fitting referred to as the Effective Ionization Region (EIR). As such, corona power loss is proportional to the size of the EIR. This paper discusses the concept of effective ionization region from a theoretical perspective. Computer modeling was used to investigate the effect of geometrical as well as electrical line parameters on the size of the EIR. A comparison is made between single and bundled conductor configurations. Results show that the radius of the EIR of a single conductor that is energized at 800 kVDC drops by about 45% if the conductor were a sub-conductor in a four-bundle configuration.
In the last decade the quantum communication community has witnessed great advances in photonic quantum cryptography technology with the research, development and commercialization of automated Quantum Key Distribution (QKD) devices. These first generation devices are however bottle necked by the achievable spacial coverage. This is due to the intrinsic absorption of the quantum particle into the communication line. As QKD is of paramount importance in the future ICT landscape, various innovative solutions have been developed and tested to expand the spatial coverage of these networks [R. Alleaume, 2007, C. Elliott, 2004, F. Xu, 2009] as in the QuantumCity initiative. One such technique that is being considered for the creation of a global QKD network is to couple quantum-secured Metropolitan Area Networks(MANs) with secured ground-to-satellite links as access points to the global network. This technique, however, is susceptible to atmospheric parameters and provides very limited contact times for the quantum key exchange. As parallel step to the development of satellite-based QKD networks, we intend to investigating the use of commercial aircrafts as secure transport mechanisms for a global QKD network. Commercial airliners create an ideal alternate global network for key distribution in terms of coverage, reliability and frequency. Further the airports at each connected city have the appropriate supporting infrastructure to serve as an access point to the global network. The quantum-secured global network will provide the infrastructure to create, distribute and manage encryption keys for use in secure global communication. The objective of this initial concept to create a proof of concept test network to investigate the aforementioned global QKD Network with the use of commercial airliners. The initial systems that are to be used will require the aircraft to be a trusted zone although certified tamper-proofing techniques will be used. The longterm objective is to upgrade the systems to contain quantum security through the use of quantum-powered memory solutions. A secure key management network layer will also be developed and implemented to control the flow and usage of keys within the global network. The secure key management network layer will further control the distribution of keys and encryption within the MAN.
Si, C and SiC are very well known technologically important materials. They have been extensively studied in their multitude of structures and polytypes, but there is a surprising dearth of information for off-50:50 compounds involving Si and C. What are the possible stable structures for such compounds? Using ab initio techniques, we investigate two proposed structures for crystalline SiC2: cubic pyrite and a tetragonal glitter structure. We find both structures to be metallic and mechanically stable. From their elastic properties, we make an assessment of the hardness of both structures. We find the tetragonal glitter phase to be lowest in energy with a pressure transition from the glitter to pyrite phase at 24.7 GPa.
Cerium-based ternary compounds demonstrate different and interesting phenomena depending on the valence of the Ce ion. The hybridization between Ce-4f electrons and the neighbouring conduction electrons is responsible for the anomolous properties observed in different systems. Depending on the strength of the f-ligand hybridization, one observes in these compounds phenomena such as magnetic ordering, heavy fermion behaviour, Kondo effect and valence fluctuations. The derived local magnetic moments associated with the state of Ce ions order antiferromagnetically in most cases. CeCuGe, CeCuSi and CeAuGe belong to a small family of equiatomic Ce based compounds that order ferromagnetically at low temperatures. The ordering temperatures are TC = 10 K, 15.5 K and 10 K for CeCuGe, CeCuSi and CeAuGe respectively. Here we report on the continuation of our studies into the thermodynamic and magnetic-field sensitivity of magnetic ordering, TC in CeCuSi compound, which has been identified to exhibit ferromagnetic ground state below TC = 15 K. CeCuSi crystallizes in an ordered hexagonal ZrBeSi-type structure (space group P63/mmc No. 194). Magnetization and susceptibility measurements were performed and the fit to the Curie-Weiss law was done, calculating paramagnetic temperature, and effective moment. The ferromagnetic nature of the transition in this compound is indicated by a lamda-type anomaly at low temperatures from specific heat (Cp) data. At higher temperatures, the magnetic contribution to the specific heat, C4f (T) (which is accessed by subtracting an isostructural non-magnetic reference data, LaCuSi) is characterized by a broad Schottky anomaly resulting from electronic excitations to crystal electric field (CEF) levels. Least-squares Schottky fit for a two-level system yielded energy separation value of about 110 K. The presence of CEF in this compound was confirmed by inelastic neutron scattering (INS) data.
Highly crystalline nano-spherical Fe-Pt systems were produced by 248 nm laser irradiation of liquid precursor at different laser fluence ranging from 100 – 375 mJ/cm2. The influence of laser intensity on the particle size, iron composition and vibrational properties was systematically investigated. The study reveals that the larger particles have higher iron content. The prepared precursor solution through Fe (III) acetyacetonate and Pt (II) acetyacetonate underwent a deep photolysis to polycrystalline of nano Fe-Pt alloys. Fe (II) and Pt (I) acetyacetone decompose into Fe0 and Pt0 nanoparticles (NPs). The material is shown to contain only iron and platinum by EDS. We have observed hysteretic loop in structural phase transition of Fe60Pt40 NPs. The material shows high resistance (~2,700 Ohms) at lower temperatures and low resistance (~2,200 Ohms) at higher temperatures. Pan theory was used to exemplify the phase transition.
Alq3 is widely used in organic light emitting diodes (OLEDs) as emission and electron transport layer. During the fabrication of OLEDs the current trend is to use solution-processing; however, Alq3 must be vacuum deposited. One possible way to overcome this problem is to use different polymers containing the Alq3. In this study blended thin films of Alq3 embedded in PMMA and PS matrices at different molar concentration ranging from 0.5% to 15% was investigated. The photoluminescence (PL) characteristics of the blended films of Alq3:PMMA and Alq3:PS was obtained. This was done by exposure of the films to 335 nm light. Emission peaks were obtained at 520 nm and 517 nm for the Alq3:PMMA and Alq3:PS films, respectively. The measurements were repeated for a period of one month, with one week intervals between measurements, to confirm the results and also to see if there is any decrease in luminescence intensity over time. Degradation in emission was observed during this period. Absorption measurements were performed on the blended and unblended films to investigate the effect of the polymer matrix. The x-ray diffraction (XRD) spectra confirmed the amorphous nature of the blended films. Atomic force microscopy (AFM) measurements showed that smooth uniform films had formed.
The discovery of the giant magnetocaloric effect (MCE) in Gd5Si2Ge2, a member of the substitution series Gd5Si4-xGex has generated significant interest into MCE phenomena in rare earth intermetalic compounds. Much recent effort has gone into determining how the MCE in such systems is influenced by the many salient features associated with rare earth magnetism. Here the MCE in polycrystalline ferromagnetic PrSi is determined from specific heat measurements. While the magnitude of the MCE in PrSi is found to be modest when compared to other binary systems, the power of MCE analyses in probing electronic and ground-state properties in magnetic systems is emphasized in this work. We forward a description in terms of a novel magnetic ground state for the 4f -electrons associated with the Pr3+ -ions in this particular compound. It is found that the MCE in this system can be accurately reproduced by modeling the system as a modified Ising-type ferromagnet with the addition of a significant higher order exchange term and assuming that the ground state of the 4f-electrons is the unperturbed 9-fold degenerate free-ion spin-orbit coupled ground state multiplet. Such a ground state would imply that the usual splitting of the degenerate energy levels of the free ion multiplet by the crystalline electric field is absent in this system, which in turn implies that significant multipolar interactions may be responsible for the higher order exchange terms present in the magnetic Hamiltonian.
The investigation of low voltage electron induced cathodoluminescence (CL) degradation of Zn3(PO4)2:Tb phosphor has been extended for possible application in low voltage field emission displays (FED). There are two important reasons for studying phosphate based phosphors. These are their chemical, thermal stability and variation in emission colour, which is influenced by the site occupied by the light emitting cations. The CL degradation phenomenon under low voltage electron beam irradiation of Zn3(PO4)2 has however not been reported so far. The primary objective of this study was to investigate the correlation between the CL emission intensity, CL degradation behaviour and the changes on the surface chemistry from the Zn3(PO4)2:Tb phosphor after prolonged electron beam exposure. The Zn3(PO4)2:Tb3+ phosphors was synthesized by a sol-gel combustion method. The surface chemical reactions and influence on the CL intensity induced by a prolonged electron beam bombardment were monitored using in situ Auger electron spectroscopy (AES) combined with CL spectroscopy. The chemical state of the surface before and after prolonged electron exposure of Zn3(PO4)2:Tb3+ phosphor to electron beam was determined using X-ray photoelectron spectroscopy (XPS). The bluish green CL emission that were obtained with minor emission peaks at 380, 420, 440 nm were due to the transition from excited level 5D3 → 7F6, 7F5, and 7F4 levels of Tb3+ ions, while the peaks at 490, 542, 584 and 620 nm were due to 5D4 → 7F6,5,4,3 transitions when the powders were irradiated with a 2 keV and 10 uA electron beam. AES data suggested that the Tb3+ CL intensity decreases with an increase in the electron dose rate. The XPS data suggested that a thermodynamically stable ZnO and P2O5 layer formed on the surface as a result of the electron stimulated surface chemical reactions (ESSCRs). The continues growth of this layer is contributing to the continues CL degradation of the Zn3(PO4)2:Tb phosphor.
Nanocomposite materials have wide range of applications in solar energy conversion. In this work, C-NiO nanocomposite coatings are prepared using sol-gel synthesis and deposited on aluminium substrates using a spin coater. The coatings are prepared from alcoholic sols based on Ni-acetate using diethalonamine as a chelating agent and polyethylene glycol (PEG) as organic template. Sucrose is used as a carbon source. Sols with different PEG and sucrose concentrations are prepared and coated on aluminium substrates. The effect of heat temperature on the properties is also investigated. The optical and structural changes of the nanocomposite coatings are characterized by UV-Vis, FTIR, thermal emissometer, Raman, and SEM techniques. It has been shown that the solar absorption increases with increasing the heating temperature in the temperature range 300 – 550 °C due to the increase in carbon content in the composite material. Preliminary durability studies on these samples will also be presented
Copper based interconnect are widely used in the electronic industry. They provide the conductive path required to achieve connection from one circuit element to another. However, the application of copper based interconnect is often limited by the oxidation behaviour of copper. At elevated temperature and humidity, the bonding strength of copper interconnect deteriorates due to copper oxide formation on the surface which could result at device failure. Effort is made in this study to investigate the effect of ruthenium incorporation on the oxidation and electronic interconnection characteristic of Cu-CNT nanocomposite synthesized through the powder metallurgical technique. The compacted samples were sintered by conventional sintering method. Oxidation test was performed in tube furnace. The samples were oxidized isothermally at different temperature. Electronics weighed balance with the precision of 0.01mg was employed to investigate the weight gain of the sample due to oxidation. The morphology structure, oxide layer thickness and possible crack on the sintered sample were examined using scanning electron microscope with energy dispersive X-ray spectroscopy (SEM/EDS). Phase identification was performed using an X-ray diffraction spectroscopy (XRD). It could be concluded from the result obtained that minor addition of Ru has a positive effect on the oxidation resistance of Cu-CNT nanocomposite due to formation of a well protected oxide.
We report on the electrical transport properties of graphene oxide (GO) and GO reduced using an organic acid (RGO). Two terminal GO and RGO devices were assembled via dielectrophoresis. The current-voltage characteristics of these devices were studied at low temperatures (77 K to 300 K) using a micromanipulated probe station. The I-V characteristics for RGO devices were found to be almost linear, indicating metallic behavior. This is in contrast to reports by other groups who have suggested variable range hopping (both Mott and Efros-Shklovskii) or space charge limited conduction. The density of states at the Fermi level in RGO was found to be an order of magnitude greater than that of GO. This was confirmed by four probe low temperature RT data (3 K to 300 K) which showed a T^2 dependence. Further support for the metallic nature of RGO was confirmed with magnetoresistance data at low temperatures (from 3 K) and fields up to 12 T. We believe reduction with organic acid does not introduce a high defect density, unlike the conventional reducing agent hydrazine. The superb electronic properties of RGO synthesized with this new method could find use in high-speed spintronic and magnetic memory devices.
Nanostructured materials are used to develop lithium-ion batteries with high energy, high-rate capability and excellent cycling stability due to their huge surface area, short distance for mass and charge transport and freedom for volume change. They are capable of reversibly accommodating large amount of Li, on intercalation mesoporous β-MnO2 can accommodate Li/Mn = 0.92:1 with 81% capacity retention. This is contrast to a very limited amount of Li, that could be intercalated electrochemically into the bulk crystalline MnO2. The electrochemical properties of MnO2 are governed by the rich and complex microstructure it accommodates and characterisation at the atomistic level is difficult experimentally.Atomistic computer simulations offer a unique platform of exploring structural features at the nanoscale. Simulated amorphisation and recrystallisation technique, involving tens of thousands of atoms, has been successfully used to generate models of various nano-forms of the complex manganese dioxides, which include microstructural details. In the current study, we apply this method to study lithium insertion into the nanospheres, nanosheets, nanorods and nanoporous structures of the binary manganese dioxides. Different Li concentrations (up to Li:Mn = 0.73:1) were inserted into the different nanostructures. Molecular dynamics simulation under the NPT ensemble was performed, in order to allow the system to expand. The variation of mechanical properties and changes of microstructural features with low and high lithium concentration are investigated. The resulting microstructure provides valuable insights into the origins of electrochemical activity which could make it a suitable battery electrode.
Crystals of LiFePO4 and related materials have recently received a lot of attention due to their promising use as cathodes in rechargeable lithium ion batteries. These compounds have been known for good stability and low cost. The Co based compound LiCoPO4, has rapidly become of particular interest as recent measurements found a potential of 4.8 V, while the Mn based compound structure has an equilibrium voltage of 4.1 V, which is compatible with the electrolyte presently used in Li-ion batteries. Before the introduction of amorphisation recrystallisation methods to quaternary LiMPO4 (M=Mn and Co), it will be important to commence with ternaries (CoPO4 and MnPO4), since ternaries are less complex. Hence we investigate the mechanical, optical and structural stabilities of the different polymorphs of CoPO4 and MnPO4, before atomistic simulations, by employing the pseudo-potential planewave calculations within the Local Density Approximation (LDA) and Pardew-Wang Generalized Gradient Approximation (GGA).
After thirty-five years the empirical pseudopotential electronic band structures that were fitted by James R. Chelikowsky and Marvin L. Cohen (Phys. Rev. B 14, 556, 1976) are still used widely and are regarded by some workers as being among the best. Although the original paper by Chelikowsky and Cohen has received more than 600 citations, not all subsequent workers have been able to reproduce the band structures accurately. In a few of the citing papers, significant errors were introduced by, for example, incorrectly adding the effects of non-locality in the core potentials and spin-orbit coupling. In the original work both of these effects were added as perturbations. In the present work it is shown that the original calculations of Chelikowsky and Cohen can be reproduced accurately, even without the use of perturbation theory. This reproduction allows the original band structures to be used with great ease. As an example, the original band structures are used as input to an optimization calculation which produces equivalent fourteen-band and thirty-band (full-zone) k•p parameters for the original eleven diamond and zinc-blende semiconductors. The original band structures are also compared to those obtained through state-of-the-art ab-initio density functional calculations.
The preparedness of matriculants for university studies has been a subject of intense debate and interrogation at various levels of public discourse. In addition, the standardization of Grade 12 assessment marks in various subjects by the Quality Assurance Body (Umalusi) has of late become a highly contentious issue. It is in recognition of these considerations that we undertook to investigate the extent to which Grade 12 achievement levels in Mathematics, Physical Science and English can serve as a measure of science students’ preparedness for university study. Some of the underlying critical factors that may have a significant bearing on the aforementioned scenario were also investigated.
Science, Engineering and Technology (SET) have become essential in the development of the country in all its facets. It has been observed over the past few years that students entering university for degree studies in their respective fields experience considerable level of difficulty. Inadequate understanding of basic mathematics and physics concepts has been identified as a critical deficiency characterizing the performance of SET first year students. As a norm, the intake is usually based on Grade 12 results. Admission to mainstream (three year) programme or a four year extended programme is determined on the basis of M-Scores by various respective institutions. Analysis of students’ performance in the four year programme at the University of Johannesburg painted a bleak picture and this led to the introduction of the Foundation Provision Programme (PFP) as a precursor to the normal first year curriculum. This programme serves to adequately develop the basic or foundational competencies necessary for students to embark on successful first year physics studies and beyond. The sample in this study comprised a group of freshmen from Grade 12 registered for various degrees in Science , Engineering and Optometry. All these groups receive physics tuition in one lecture class. The Foundation Provision Programme was implemented for the first time at the University of Johannesburg in 2010 and the results obtained were compared to the results of students’ performance obtained from previous years.
Active learning techniques were used for the first time in the Statistical Physics module presented to third year students at the University of the Witwatersrand in the first semester of 2009. The reaction of the students, recorded in a formal Student Assessment of Lecturer Performance, administered by the Centre for Teaching and Learning Development at the University, was overwhelmingly positive. In addition, the final results of the students were excellent. This paper reports on the methods employed, the results of the student survey and the course results. These results are compared with the results of the same group of students in the second year Thermal Physics module, taught by the same lecturer using more traditional methods of instruction, during the second semester of 2008. The employment of innovative teaching strategies makes a significant difference to the confidence and attitude of the students, and an increase in the level of performance of the majority of students is apparent. Active learning is now the standard method of instruction in both the second and third year Thermal and Statistical Physics courses, and details of the materials developed over the last three years will be provided.
This study examines the effect on the learning experiences of students of changes to the running of laboratory and tutorial sessions of the first year auxiliary physics course at the University of the Witwatersrand, between 2001 and 2010. This research is underpinned by the principles of indirect interactive instructional skills and experiential learning. Two concerns drove this study: the first being a mismatch between the marks awarded to students for their laboratory reports, and their subsequent performance in formal practical tests. The second was the students’ poor engagement – and hence performance in problem solving - during tutorial sessions. Several possible contributing factors were identified and changes were implemented in an attempt to improve the learning experienced by the students. Three sources of data suggest that the changes have had a positive effect on the learning of physics by the students in this course.
Lasers, Optics and Spectroscopy
The transient absorption of Zinc phthalocyanine (ZnPc) has been studied by femtosecond pump-probe spectroscopy. We present results on the ultrafast spectroscopy of ZnPc, specifically identifying three ultrafast components observed. These are the inertial solvation dynamics (~1.8 ps), the dielectric solvation dynamics (~39 ps) and vibrational relaxation (~2.7 ps) in the electronic excited state. Results using different pump wavelengths, different sample concentrations and different solvents help to clarify the mechanisms behind the observed signals. The red shift of the ground state (Q band) spectra is also discussed.
Understanding molecular dynamics on potential energy surfaces has been at the heart of ultrafast transient absorption in the recent years [Philipp K, etal., 2005],[Karel G, etal., 2008]. Based on the on-going experiment at our ultrafast laboratory, we ran density functional calculations on metal dithizonates, starting with their kernel molecule, formaldimine, to validate what we observe experimentally. This we treated in terms of ground state energy, absorption spectrum, vibrational frequency and potential energy surface using two different softwares: Amsterdam Density Functional (ADF) and Gausian(09). The overall results show that B3LYP functional combined with CEP-31G basis set gave the closest results to the experimentally observed data.
The Nobel prize winning technology involving a mode-locked femtosecond laser frequency comb (FLFC) has been at the epi-centre of a scientific revolution over the last two or so decades. Even though originally designed to count cycles in optical clocks, it has had a profound impact on other research areas, with a range of applications including spectroscopy, trace gas detection, signal processing, astrophysics and many others. In the near future most of these applications may be taken over by the compact monolithic micro-resonator generated frequency combs, there is, however, no doubt that mode-locked FLFC will continue growing their range of applicability over the next few decades. We present a brief overview of two simple high-resolution high-precision spectroscopy experiments. One experiment is of caesium-133 atoms contained in a vapour cell, and the other molecular spectroscopy where multi-channels are simultaneously detected by a charge coupled device (CCD) camera with a specimen imprinting its unique fingerprint. In both experiments excitation is through radiation from a mode-locked FLFC. In the caesium experiment the laser beam is set counter-propagating after the beamsplitting cube, focussed to a reasonable waist in the interaction region in the vapour cell, thereby exciting a multitude of low lying magnetic dipole and electric quadrupole transitions allowing the measurement of transition energies and hyperfine coupling coefficients for the 8S, 9S and 7D3/2,5/2 states. In the second experiment the comb is filtered by a Fabry-Perot cavity changing the repetition rate of the laser from 1 GHz to repetition rates in the range 3 - 15 GHz to match the resolution of the spectrometre. The spectrometre comprises of the Virtually Image Phased Array, the 2400 lines/mm reflection grating and the CCD camera. We discuss, the spectrometre, the results of both experiments, and briefly some other possible applications of laser comb.
Nuclear, Particle and Radiation Physics
Saldanha Bay on the West Coast of South Africa is surrounded by various nature reserves and protected areas which amongst other includes heritage sites like the West Coast Fossil Park. Large parts of these sensitive areas were however exposed to anthropogenic activities, which includes farming and mining as well as training and preparation for war. This article investigates the prospect of determining the impact that anthropogenic activities had on areas via mapping of primordial radioactive isotopes. For this purpose In Situ gamma ray data were obtained from selected locations around Saldanha Bay. The variation in radioactive isotope concentrations is compared to historical data of anthropogenic activities.
Gold mine tailings dams contain considerable amounts of Ra-226 and have therefore been identified as large sources of radon. In this contribution we will discuss measurements at a disused Kloof mine dump near Carletonville to investigate the radon exhalation from the dump as well as the gamma radiation from the dump. We have also taken soil samples in order to find the activity concentrations as well as to study the diffusion and emanation of radon.
In this work a 7.5 cm x 7.5 cm NaI (Tl) detector was used to study activity concentrations of primordial radionuclides in soil and sand samples. The detector and the sample were placed inside a lead castle to reduce background measurements from the surroundings such as the wall and the floor. The samples were placed inside a 1L Marinelli beaker which surrounds the detector for better relative. Additional lead bricks were placed below the detector to further reduce the background by 10%. A Full Spectrum Analysis (FSA) method was used to extract the activity concentrations of 238U, 232Th and 40K from the measured data. The FSA method uses standard spectra corresponding to the radionuclides being investigated, which are multiplied by the activity concentrations and then added to fit the measured spectrum. Accurate concentrations are then extracted using a chi-squared (χ2) minimization procedure. Eight samples were measured using the NaI detector and analyzed using the FSA method. The samples were measured for about 24 hours for good statistics. The 238U activity concentration values varied from 10 ± 2 Bq/kg (iThemba soil, HS6) to 256 ± 10 Bq/kg (Kloof sample). The 232Th activity concentration values varied from 7 ± 1 Bq/kg (Anstip beach sand) to 45 ± 5 Bq/kg (Rawsonille soil #B31). The 40K activity concentration values varied from 45 ± 27 Bq/kg (iThemba soil, HS6) to 120 ± 9 Bq/kg (Rawsonille soil, #B28). The χ2 values also varied from sample to sample with the lowest being 0.13 (Anstip beach sand) and the highest being 6820 (iThemba soil, HS1). A high χ2 value usually represents incomplete gain drift corrections, improper set of fitting functions, proper inclusion of coincidence summing or the presence of anthropogenic radionuclei.
The goal of any measurement and data-analysis technique should always be to minimize uncertainties, whether it is statistical or systematic. Although uncertainties are unavoidable, one can find ways to reduce them depending on the circumstances. High-precision measurements play a crucial role in constraining various quantities. Recently a new inorganic scintillation detector has become available in large cylindrical sizes (e.g. 7.6 cm in diameter and with length over 15 cm). It consists of LaBr3 and has an energy resolution which is not as good as HPGe, but superior to the energy resolution of NaI and CsI, and does not need to be cooled to LN2 temperatures. Since La has a naturally radioactive isotope, 138La, which emits γ-rays, the detector has an internal calibration source for energy and dead-time corrections. Moreover it produces pulses with fast rise time, which allows setting up the electronics such that measurements can be made with a high count rate and a low dead time. Gamma-ray spectra were collected with a 3"×3" LaBr3 detector for LaBr3 intrinsic background, 22Na and 137Cs which exhibit some gain drifts. A set of off-line analysis methods of these spectra is presented to select the procedure that yields the optimal precision and accuracy.
Radiopharmaceuticals is compounds labelled with a radioactive isotope that are used for studying different organs in the human body. Technetium-99m (99mTc) labelled iminodiacetic acid (IDA) derivatives are commonly used as hepatobiliary imaging agents. Radiopharmaceuticals used for hepatobiliary imaging are divided into two groups based on the physiologic function of the liver they are designed to evaluate.The IDA agent of choice for NTP is DISIDA (N-2,6-diisopropyl-phenylcarbamoylmethyliminodicetic acid).A cold kit is a pre prepared vial consisting of the compound to be labelled with the radioactive isotope and a suitable reducing agent. The radioactive isotope of choice for IDA labelling is 99mTc. Kits for DISIDA is commercially available and usually contain the IDA derivative and stannous chloride dihydrate as the reducing agent. Twenty gram DISIDA is needed for each production batch of DISIDA kits Labelling is accomplished by adding 99mTcO4- to the kit and mixing well. Approximately 3 to 5 mCi (111-185 MBq) 99mTc-IDA derivative is injected intravenously into patients who have fasted for 4 to 6 hours prior to administration. The biodistribution DISIDA was confirmed by performing a biodistribution study on a Chacma baboon.The yield of the DISIDA synthesis was improved from 26 - 34 g to 76 g (53%).The labelling with technetium give > 95% radiochemical purity.The improved synthesis resulted in increased cost effectiveness of the commercial DISIDA kits.
Theoretical and Computational Physics
GEANT 4 is a C++ library developed by CERN to simulate particle physics experiments. However, GEANT 4 can be also be used for other applications that need not have anything to do with particle or high-energy physics. Here we discuss an application of GEANT 4 that simulates the EMU Spectrometer for Muon Spin Relaxation measurements at the ISIS facility in the Rutherford Appleton Laboratory. Both the incident muon and decay positron trajectories are affected. This changes the stopping location of muons for small samples and also the efficiency of the positron detection, in a strongly field dependent way. This leads to a systematic field dependent false asymmetry. The simulation has been performed in support of the interpretation of experiments todetermine the effect on diamond samples doped with muonium, which is a known analogue of the hydrogen atom.
The spectrum of a carbon nanotube in a strong enough magnetic field (>50T for a 3nm nanotube) revealed an almost dispersionless band at the Fermi energy. The formation of Landau levels has been theoretically and experimentally investigated. In experimental studies the existence of Landau levels is indirectly derived from longitudinal conductance measurements. We will show that a more direct indication of Landau Levels in the system exists, namely a quantized Hall effect. We present numerical results that show the application of an electric field along the length of the tube lead to a current circulating the circumference and, furthermore, that the conductance is exactly e2/h. This is interesting because the magnetic flux averages to zero around the tube’s circumference. This phenomenon could be explained in terms of the two-dimensional topological theory for the quantum Hall effect even though this system is quasi-one dimensional.
Ab initio density functional calculations within the generalised gradient approximation (GGA) have been carried out on a wide range of phases and stoichiometries for the platinum-titanium (Pt-Ti) and iridium-titanium (Ir-Ti) alloy systems, using the Vienna Ab Initio Simulation Package (VASP). The elastic constants and elastic moduli are calculated and the electronic structure and density of states (DOS) are considered to understand the hardness and stability properties of the alloys.
Reactions involving four particles, either in the entrance or final channel, are quite involved when computing observables in comparison to three-body reactions. Yet these reactions are of interest in studying reactions of astrophysical interest, such as hep process, which is essential for describing the quantitative solar model. At lower solar energies, it is difficult to measure the cross-section for the hep reaction. Moreover, there are discrepancies in calculations of the S-factor using different models and compared to the value predicted by the Standard Solar model. This problem in turn implies problems in estimating the correct value for the reaction cross-section which is crucial for the understanding of the Standard Solar model. In order to address this discrepancy and other observables, a reliable numerical technique is necessary. In this work we propose a new spectral method capable of calculating low-energy phase shifts for scattering of the nucleon off a light nucleus. We consider reactions of the type (3+1) --> (3+1), within the Faddeev-Yakubovsky framework. We show that these equations can be transformed into spectral-type set of equations that are numerical less expensive to solve in comparison to competing methods.
One-Way Quantum computing is based on the preparation of certain entangled states of several particles, which are subsequently individually measured. The measurements serve to process information as well as to read out the final result of the computation. The implementation with OAM carrying photons is based on the usage of qubits (only two OAM values are relevant) but is a first step towards the implementation of a generalized form of One-Way Quantum computing by means of qudits (involving more then two OAM values).
Quantum Cryptography, one aspect of which is Quantum Key Distri- bution (QKD), provides the only physically secure and proven method for the transmission of a secret key between two distant parties, Alice and Bob. The goal of QKD is to guarantee that a possible eavesdropper (Eve), with access to the communication channel is unable to obtain useful information about the message. The Coherent-One-Way (COW) protocol is one of the most recent practical QKD protocols. However, its security proof still remains unrealized. We therefore present a necessary condition for the security of the COW protocol. In the proof, we describe bob's measurements by non-commuting POVM elements which satisfies this proof.
Ground-state properties of three-nucleon systems consisting of one and two hyperons are studied with the integro-differential equations approach. The Hamiltonian of the systems is constructed with semi-realistic nucleon-nucleon interactions and phenomenological nucleon-hyperon interactions. The results obtained for the ground-state energies and root-mean-square radii are compared with the findings of other theoretical methods.
Over the past dozen years we have produced some of the most widely used interactive computer-based curricular materials for the teaching of introductory and advanced physics courses. These materials are now hosted on and distributed from the Open Source Physics (OSP) Collection of the ComPADRE National Science Digital Library. This talk outlines the tripartite integration of ComPADRE with the Easy Java Simulations modeling and authoring tool and the OSP code library. The pedagogical and technical features of this learning platform and our current efforts to align this material with United States national and state standards for science teaching are described. The Open Source Physics Collection is available on the: http://www.compadre.org/osp/ Partial funding for this work was obtained through NSF grants DUE-0442581 and DUE-0937731.
Ab-intio Density Fuctional Theory has been employed to investigate the stabilities of Li< sub >2< /sub >O, Li< sub >2< /sub >S, Li< sub >2< /sub >O< sub >2< /sub > and Li< sub >2< /sub >S< sub >2< /sub > systems. Calculations were carried out utilizing Plane-wave Pseudopotential method within GGA-PBE using VASP CODE. We found good agreement between predicted data and experimental data of lattice parameters . The elastic constant for Li< sub >2< /sub >O, Li< sub >2< /sub >S accord well with experimental results and those of Li< sub >2< /sub >O< sub >2< /sub > and Li< sub >2< /sub >S< sub >2< /sub > are reasonably predicted, and satisfied stability conditions. Phonon dispersion of Li< sub >2< /sub >O and Li< sub >2< /sub >S compare well with those obtained from neutron scattering experiments. We predicted phonon dispersion of Li< sub >2< /sub >O< sub >2< /sub > suggest that the structure is stable, while those of Li< sub >2< /sub >S< sub >2< /sub >< sub >2< /sub > display soft modes along along Г direction hence suggesting structural instabilities. Furthermore phonon density of states attribute the instability to the vibrations of the sulphur atoms in the ab plane.
Planewave pseudopotential calculations were conducted to predict the phase stability of Pt-Cr binary alloys. The heats of formation are determined for five different phases, L12, A15, DOC, DO'C and tP16 of Pt3Cr and PtCr3. We observed that the cubic L12 Pt3Cr is the most stable structure in agreement with the experiments. The results for PtCr3 indicate the negative heat of formation for the A15 phase whereas all the remaining studied phases have positive heats of formation. In addition the phase stability study was performed on two phases, B2 and L10 of the PtCr alloy. L10 phase was found to be more stable compared with the PtCr B2 phase. Elastic constants and moduli were investigated to determine the strength of the Pt-Cr systems. The strength of PtCr L10 is greater than that of B2 phase. The ratio of shear to bulk modulus (G/B) has been used to predict the ductility or the brittleness of the material.
The system ZrO2-xSxx is limited by the cases ZrO2 and ZrS2 (x = 0, 2). Physical properties of such systems for 0 < x < 2 are of great interest, but difficult to obtain from first principles calculation. A recently suggested approach, i.e virtual crystal approximation, allows simulation for changes in composition, while retaining a small unit cell. In this paper we employ Density Functional Theory (DFT) within the Generalized Gradient Approximation (GGA)to study the zirconium dioxide – sulfide system ZrO2-xSxx, using the CASTEP code. We investigated geometric cell size effects, mechanical properties and electronic structure for these systems at various amounts of sulfur concentrations. For the treatment of solid solutions in the recently introduced VCA approach, CASTEP allows to define partial occupancies for atomic sites; in our case, the anionic lattice sites of ZrO2-xSxx crystal are defined to be less than fully oxygen occupied: (2-x); and consequently similar sites can then be attributed a partial sulfur character: (x). The lattice parameters and cell size increases for zirconia as the sulfur content is increased. Furthermore, the trend of Fermi level and the electron distributions differ, giving rise to reduced band gap as S is increased. The cubic structure ZrO2-xSxx is stable for x<1.2, however, the structure gives negative tetragonal shear modulus at x≥1.2, condition of mechanical instability.
In order to investigate the diffusion of indium (In) when an In thin film is coated with a copper (Cu) thin film, thin In/Cu films were grown on a silicon dioxide (SiO2) substrate. Both the In and the Cu layers were grown by electron beam evaporation. The temperature during evaporation of the films ranged between 27 and 38 oC. The In films (500 Å) were coated with Cu films (500 Å and 1000 Å). The films were characterized with X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The In/Cu layers interdiffused during evaporation and formed intermetallic CuIn2 and Cu11In9 phases. The In layer completely reacted with the Cu layers during the deposition process. The In layer was effectively coated with a Cu layer (1000 Å). The In (500 Å)/Cu (1000 Å) films were annealed at temperatures ranging between 150 and 300 oC for times varying between 25 and 121 min. For annealed films the diffraction data demonstrated peaks only for Cu and Cu11In9 and AES depth profiles pointed out the stability of the Cu11In9 and In diffusion to the surface of the film. The results of this work provide insight towards the In doping of Cu crystals at temperatures higher than the melting point of In (156.6 oC).
The behaviour of iodine implanted in highly oriented pyrolytic graphite (HOPG) has been investigated using Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and x-ray diffraction (XRD). Iodine ions were implanted into HOPG using an energy of 360 keV and a dose of 1×1015 atoms cm-2 at room temperature. The implanted samples were annealed in vacuum at 900 0C, 1000 0C, 1100 0C and 1200 0C, all for 9 hours. The results revealed that iodine was released from the HOPG at the above annealing temperatures. RBS evaluation of the full width at half maximum (FWHM) and the number of iodine atoms before and after annealing did not reveal Fickian diffusion as the mechanism by which the iodine atoms were released from the HOPG. Evaluation of (002) peak intensities using XRD revealed an increase in preferred orientation of the graphitic layers after heat treatment of 1200 0C. The high resolution SEM micrographs of the HOPG samples before and after heat treatment showed no evidence of alterations on the polished surface.
Preparation of Er3+, Ho3+ and Tm3+ ions co-doped SiO2 nanoparticle phosphor powders by sol gel method is reported. The morphology and the particle size of the SiO2 host matrix were confirmed by field emission scanning electron microscopy (FESEM). Ultraviolet, visible (UV/VIS) and cathodoluminescence measurements were carried out in order to investigate the optical properties of our powder phosphors. Green emissions at 520 nm from Er3+ and 544 nm from Ho3+, and red emissions at 665 nm from both Er3+ and Ho3+ ions are reported. Another emission peak in the near infra-red (NIR) region at 875 nm from Er3+ was also measured. Blue emission at 460 nm, red at 705 nm and a NIR peak in the region of 865 nm from Tm3+ were observed. Red, green and blue (RGB) colours were measured from both SiO2:Er3+,Tm3+ and SiO2:Ho3+,Tm3+ systems. The change in the intensities of the emission peaks in both the SiO2:Ho3+,Tm3+ and SiO2:Er3+,Tm3+ systems with the change in accelerating beam voltage is shown. Energy transfer from Tm3+ ions to Er3+ and Ho3+ ions was observed. A mechanism explaining the increase and decrease behaviour of the emission with accelerating beam voltage from both systems is reported.
We have investigated the electronic, structural and optical properties of the PtAs< sub >< /sub>2 and PtAsS structures using the density functional theory in the framework of Vienna Ab initio Simulation Package (VASP) code within the generalized gradient approximations of Perdew, Burke and Erzenhof (GGA-PBE) . The total energy calculation of PtAs < sub >< /sub>2 and PtAsS have been computed and predict equilibrium lattice parameters that are in good agreement with the experiment. Elastic constants of these structures compare well with experimental measurements, both PtAs< sub >< /sub>2 and PtAsS gave positive independent elastic moduli, condition of mechanical stability. We found good correlation between heats of formation, elastic constants and phonon dispersion curves, all satisfying stability conditions. In particular the phonon dispersion display real frequencies along high symmetry direction of the Brillion zone.
We investigate the transformation from hematite to ilmenite structure by solid solution approach using ab initio quantum-mechanical simulation technique. The method uses the Density functional theory with the plane-wave (PW) pseudopotential method within the CASTEP code. We performed full structural relaxation allowing lattice parameters and cell violume to change to their mechanical equilibrium. We found that the transformation process does not change the structure (space group) as observed during geometry optimization. Our calculated properties i.e bond length, cell volume, lattice parameters, bulk modulus and density of states shows that hematite and ilmenite structures have similar property behavior with the lattice parameters corresponding to the experimental value, being large by 0.12%. The bond length from 20% to 50% shows the dominant of the Ti-O bond, in agreement with the experimental value. This bond length behavior is consistent with the density of state. However, we observe that the states overlap from VB to CB in VCA which is not expected from the experimental observations of the hematite and ilmenite crystal structures. The transformation from hematite to ilmenite structure due to doping was observed from 20% atomic titanium.
Rutile and anatase structured tin dioxide ceramics have been intensively studied in recent years because of their potential in sensing and fuel cells.The present work uses classical molecular dynamics simulations focused on the structure and possible transformation from anatase to rutile tin dioxide.The empirical Buckingham potential has been used to describe the interatomic interactions in tin dioxide.The total energy of the NVE ensemble of the two structures at various temperatures has been calculated in order to determine the transition temperature and pressure.The results obtained showed an energy increase with temperature which was constantly compared with experiments.The radial distribution functions for the two structures suggest the transformations at temperature above 900 degrees celcious in agreement with the experiments.
Rutile and anatase structured tin dioxide ceramics have been intensively studied in recent years because of their potential in sensing and fuel cells. The present work uses classical molecular dynamics simulations focused on the structure and possible transformation from anatase to rutile tin dioxide. The empirical Buckingham potential has been used to describe the interatomic interactions in tin dioxide. The total energy of the NVE ensemble of the two structures at various temperatures has been calculated in order to determine the transition temperature and pressure. The results obtained showed an energy increase with temperature which was constantly compared with experiments. The radial distribution functions for the two structures suggest the transformations at temperature above 900 ºC in agreement with the experiments.
The effects of side chain size and spacing on the mobility of the PEO backbone have been studied using molecular dynamics (MD) in Forcite code of Materials studio. The systems investigated contain a PEO chain of 186 ethylene oxide (EO) monomers to which side chains of 5, 6, 7, 8 and 9 PDMS monomers have been attached. The separations of the side chains used are 5, 10, 15, 20 and 50 EO monomers. The overall mobility of the polymer host system is found to have minima at side chain size of 6 and 7. A maximum is seen at side chain spacing of 8 and 9. No evident was found to suggest that side chain spacing has an effect on the mobility of the PEO polymer backbone. Furthermore, the point of attachment of the side chain has no effect on the polymer mobility.
Cobalt pentlandite (Co9S8)is the major source of cobalt. Of the several cobalt minerals, cobalt pentlandite is the most abundant and has the widest geographical distribution. This presentation looks at the computer simulation of Co9S8 structure using different computer simulation techniques, i.e. METADISE, VASP. These techniques have been employed to investigate the effect of molecular adsorption of water on the low-index surfaces of Co9S8
CeCuGe is one of the few compounds to order ferromagnetically at low temperatures. The compound has been observed to exhibit paramagnetic-ferromagnetic (PM-FM) magnetic transition, TC = 10 K This magnetic ordering has been observed from specific heat (Cp), magnetization, (M(T,B)) and resistivity data. Isothermal magnetization of a hexagonal, highly ordered CeCuGe were measured in order to study the critical scaling behavior in the vicinity of magnetic ordering in this polycrystalline compound. From the analyses of the magnetization data, TC was confirmed using modified Arrott plot technique. It was also observed from the critical exponents, beta, gamma and sigma corresponding to the spontaneous magnetization, initial susceptibility and isothermal magnetization were obtained, respectively. The resulting critical exponents obtained reveals that the system falls within Heisenberg and 3-D Ising model.
Within the confines of the standard model of rare earth magnetism, the high temperature magnetic properties of rare earth ions in a crystalline environment are identical to the magnetic behaviour of free tri-positive rare earth ions. At low temperatures deviations from free-ion behaviour occur due to the action of an anisotropic crystalline electric field (CEF). For Pr3+-ions in a low symmetry crystalline environment it is expected that the 9-fold degeneracy of the spin orbit coupled ground state multiplet associated with the free tri-positive ion should be completely uplifted by the CEF yielding a non-magnetic singlet ground state for the Pr3+ 4f-electrons. PrNiGe2 is known to order ferromagnetically despite the fact that the Pr3+ ions occupy the low-symmetry m2m sites in the CeNiSi2-type structure. This has prompted the current study into the ground state properties of PrNiGe2. The structure of the CEF-split energy levels in this system could be determined from specific heat measurements. The analyses point to the formation of a pseudo-doublet ground state in the system, and we forward a conceptual explanation of the observed magnetic order in PrNiGe2 in terms a fortuitous merging of electronic singlet levels into a local level dispensation of higher degeneracy.
Graphene characterization and device fabrication often require extensive use of the scanning electron microscopy (SEM) and tunneling electron microscopy (TEM). These techniques involve electron beam irradiation that usually causes defects in materials under investigation. The radiation induced defects may lead to significant damage to the material which may alter its properties, normally an undesirable phenomenon. However, recent experiments show that electron radiation can have beneficial effects in materials. Graphenic materials have the ability to reorganize their structures under irradiation like no other materials. This property can find application in radiation hard electronics owing to the technological importance of graphenic materials. Here we report the Raman and scanning tunneling microscopy (STM) results of pristine and heat treated electron -beam irradiated graphene sheets (mono-layer and double layers) to demonstrate graphenic reconstructions. STM creates images of the charge density of electrons at the Fermi level such that all surface atoms can be visible.
Short range biaxial and hydrostatic strains in quantum dots caused by spacer layers affect the aspect ratio to a great extent, leading to variations in the conduction and valence band energy levels. This paper aims to study the effect of the indium mole fraction and thickness of the spacer layer on the size and shape of multi-stack quantum dots grown by metalorganic vapour phase epitaxy. Quantum dots of varying aspect ratios (height:width) have been grown. Structures containing two stacks of quantum dots have been grown on etched GaSb (001) substrate, with different thickness and indium content of the GaInSb spacer layers. Experiments are currently under way to study the effect of the thickness of the spacer and the composition on the size distribution, shape and density of three stacks of InSb quantum dots. The structures are characterized by X-ray diffraction, photoluminescence, scanning and transmission electron microscopy.
Amorphous and nanocrystalline silicon carbide thin films were deposited on corning glass 7059 and c-Si (100) substrates using the Hot Wire Chemical Vapor Deposition (HWCVD) and Magnetron Sputtering (MS) techniques. Samples were prepared at low substrate temperatures below 300 degrees celsius and a gas mixture of SiH4/CH4/H2 was used in case of HWCVD. For MS, the temperature of the substrate was varied between 300-600 degrees celsius and Ar plasma was used to sputter a SiC target in a H2 plume environment. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the phase changes and crystallinity in the films. Plan view and cross section specimen were prepared by the TRIPOD polishing technique for the TEM structural investigation. Fourier Transform Infrared Spectroscopy (FTIR) was used to investigate the hydrogen content and the SiC vibrational bonds in the samples.
Silicon carbide (SiC) materials have attracted a lot of attention because of their good optical and electrical properties. In recent years there has been an intense focus in research on controlling the optical and electrical properties of amorphous SiC thin films deposited at low temperature of the substrate. SiC has a wide bandgap which can be tuned by varying the carbon content in the thin film. The SiC thin films also have a high electron mobility, high electron saturation velocity, optical transparency which make them candidates for use as window layer in solar cells. The low deposition rate issue in SiC thin films processing has not yet been resolved. This contribution will focus on optimizing the deposition rate by studying the influence of the deposition parameters on the deposition rate. The thin films ranging from amorphous to microcrystalline SiC has been deposited by magnetron sputtering and hot wire chemical vapour deposition at low substrate temperature of the substrate below 300 oC. The optical properties of the films have been investigated transmission/reflectance spectrum using the OJL model in SCOUT and by computation using the iterative method of Swanepoel. The electrical properties such as conductivity, bulk concentration and mobility will be investigated by Hall Effect measurements.
Hydroxyapatite coating (HAp, Ca10(PO4)6OH2), produced by air plasma spraying, was investigated for depth profiling behavior of phase composition, crystallinity and residual stress using diffractometry techniques. Probing was carried out using conventional 8 keV laboratory X-rays, 11 keV and 100 keV radiations from 2nd and 3rd synchrotron facilities, respectively. The latter employed measurements in transmission geometry. Results showed HAp together with its thermal products tetracalcium phosphate (TTCP), tricalcium phosphate (TCP) and calcium oxide to be present throughout the coating thickness. Quantitative refinement results employing Rietveld refinement showed HAp and TTCP to be the two main phases, with the former decreasing with depth whilst the latter increases. The largest changes occurred in the former showed a significant decrease in the first 45 m. Crystallinity investigation indicated the coating to be more crystalline at the near-surface region decreasing with depth below the surface. Both the normal stresses 11 and 33 show a change in stress state i.e. from compressive in first 135m to tensile towards the coating-substrate interface.
An Auger electron spectroscopy study was performed in order to determine the bulk concentration of S in Fe-S. A number of studies have been performed that employs AES to study diffusion and segregation in metals. The most common application is the use of the technique to determine the diffusion parameters i.e. activation energy (Q) and the pre-exponential factor (D0). This approach makes use of a known bulk concentration and uses Fick`s semi-infinite solution to extract the diffusion parameters. Our research employs AES to determine the bulk S concentration in Fe-S. With the diffusion parameters obtained from literature and the use of the adapted t1/2 equation, derived from Fick`s semi-infinite solution, the bulk concentration of S was determined. AES measurements were performed at various temperatures for a fixed period of time in order to observe the segregation of S from the bulk of Fe to the surface. These constant temperature measurements were performed at temperatures in the range 200-800 °C where the temperature was increased in increments of 50 °C. A non-linear least square software program was developed to fit the adapted t1/2 equation to the data, in order to extract values for the bulk concentration of S. An average concentration value was calculated for the range of temperatures investigated and compared to concentration values obtain by linear programmed heating performed over the same temperature range with a linear increase in temperature.
Titania is a cheap and nontoxic polymorphic material of current interest for a variety of technological applications like in gas sensing and photovoltaic cells. Generally, TiO2, with a band gap of 3.2 eV, can only be excited by a small UV fraction of solar light, which accounts for only 3-5% of the solar energy. Various strategies have been pursued including doping with metallic elements (e.g. Fe) or nonmetallic elements (e.g. N) with the aim of shifting the absorption into the visible range. In the present work TiO2 was doped with platinum (Pt), palladium (Pd) and gold (Au) at doping levels of 5% weight, following the standard sol-gel methods. Structural characterization was carried out using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Optical properties were studied using the Diffuse Reflectance Spectroscopy (DRS). The results of the lattice parameter calculations from the XRD patterns of metal loaded TiO2 are almost the same as for the undoped. Doping with Pt and Pd resulted in a lower anatase to rutile phase transformation temperature while doping with Au did not affect the transformation temperature. SEM micrographs show that the surface contains irregular shaped particles which are the aggregation of tiny crystals for lower temperatures. At higher temperatures (900 °C), we observe spheroids of metallic nanocrystals on the surface of TiO2.The reflectance spectra of the metal loaded TiO2 reveal substantial reduction in reflection (i.e. they show enhanced absorption)from 400nm to the entire visible region.
Ce doped silica has potential applications for a luminescent material as phosphors for cathodoluminescence, scintillators and detectors. Ce ions can occur in a trivalent or a tetravalent state: only the trivalent Ce3+ state with a single 4f electron is optically active, while the tetravalent Ce4+ ion is non-luminescent. X-ray photoelectron spectroscopy (XPS) is a suitable technique to investigate the oxidation states of Ce in cerium oxides and such studies have been carried out because of the importance of CeO2/Ce2O3 conversion in automotive exhaust catalysts. However, the XPS Ce(3d) spectrum of cerium oxide is rather complex as it contains ten closely spaced and overlapping peaks on a strong background. The main challenge is to obtain accurate fits to experimental data while still maintaining a good physical basis for the fitting parameters. The analysis of Ce in SiO2:Ce is even more challenging since the Ce concentration for luminescent samples is only in the region of 1%. Although it has been experimentally shown that to improve the luminescence efficiency of Ce doped silica it can be useful to anneal the glass in a reducing atmosphere, with the implication that this increases the concentration of Ce3+ luminescent ions, we are not aware of XPS measurements that correlate the relative concentrations of the Ce3+ and Ce4+ ions to the luminescent properties. In this work, cerium doped silica was prepared by the sol-gel method. The effect of annealing temperature and atmosphere on the luminescent properties are correlated to XPS measurements of the oxidation state of Ce in the samples.
MgxZn1-xO films were grown on c-plane sapphire substrates by metal organic chemical vapour deposition. The effects of growth rate and low temperature (280ºC) ZnO buffer layer thickness on the incorporation efficiency of Mg as well as the optical and structural properties of the MgxZn1-xO films were investigated. Low growth rates enhance the segregation of Mg, leading to a magnesium rich region near the interface with the sapphire substrate. Hence, a shoulder is observed on the higher energy side of the main donor bound exciton in the photoluminescence spectrum. The effect of a ZnO buffer layer grown for 5 min and 10 min at 280ºC for MgxZn1-xO films with low Mg content (xv = 0.2) and for MgxZn1-xO with high Mg content (xv = 0.5) was also investigated. The ZnO buffer layer causes an improvement in the Mg incorporation but deteriorates the structural and optical properties for all the films. The MgxZn1-xO thin films grown on a thinner ZnO buffer layer showed the best optical and structural properties. Furthermore, the deposition of thin films on a ZnO buffer layer leads to an increase in lateral growth rate, which enhances the width of columnar grains. A ZnO buffer layer does not prevent the formation of a Mg rich interfacial layer.
Fe2O3 nanostructures were synthesized for various pH values on Corning glass substrates by aqueous chemical growth (ACG) using a solution of ferric chloride (FeCl3). It was found that increasing the pH from 1.16 to 5.0 of the solution significantly leads to a modification of the Fe2O3 morphology or orientation from randomly parallel needle shaped rod-like to randomly perpendicular rectangular structure on to a substrate.
Palladium (Pd) and ruthenium (Ru) Schottky barrier diodes were fabricated on (100) Sb-doped n-type germanium using resistive evaporation and electron beam deposition systems, respectively. Electrical characterization of these diodes using current-voltage (I-V) measurements was performed under various annealing conditions. The morphological evolution of the surface was analysed using the scanning electron microscopy. The variation of the electrical and structural properties of these Schottky diodes can be attributed to combined effects of interfacial reaction and phase transformation during the annealing process. Thermal stability of both the Pd/n-Ge (100) and Ru/n-Ge (100) Schottky diodes is maintained up to annealing temperature of 550°C. Results have also indicated that the onset temperature for agglomeration in Pd/n-Ge (100) system occurs between 500 – 600°C, and in Ru/n-Ge (100) system occurs between 600 – 700°C.
The AlxGa1-xN materials system is important for realising tuneable LED and laser sources and short wavelength photodetectors. While AlxGa1-xN (x = 0) is fairly known, there is interest in studying the effects of an increasing Al fraction on properties of devices. In particular, on the properties of a bi-metal Ni\ Au Schottky contact. We utilise Current-Voltage (I-V) and the forward Capacitance-Voltage (C-V) characteristics; and, Admittance (Capacitance-Frequency (C-f) and Conductance-Frequency (G-f) techniques to study the Ni\ Au Schottky barrier contact by monitoring the diode ideality (n), the Schottky barrier height and the density of interface states (Dit). The as deposited Ni\ Au Schottky contact is non- ideal with an ideality factor, n = 1.6 eV. We also observe for low forward voltages, a Recombination-Generation (R-G) component to current conduction. The forward C-V measurements reveal an anomalous peak at 0.45 V confirming the observed non-ideality. The as deposited Dit increases exponentially to about 4.5 x 1011 eV-1 cm-2 towards the mid band gap. G-f measurements reveal a broad peak around 300 kHz at room temperature, which below 240 K evolves into two peaks centred around 40 kHz and 1 MHz. Preliminary isochronal annealing in Oxygen between room temperature and 623 K indicate that the I-V characteristics improve with idealities around 1 eV and the Dit is reduced from 4.5 x 1011 eV-1 cm-2 to about 3.5 x 1010 eV-1 cm-2 in the annealed samples. Our results are consistent with a metal-oxide-semiconductor model for the annealed Ni\ Au Scottky contact.
We investigate the electrical parameters of n- GaN irradiated at room temperature with 5.4 MeV alpha-particles from an americium-241 (Am-241) radio-nuclide source under dark conditions. The radiation damage was studied using current–voltage (I–V) and capacitance –voltage (C–V) characteristics for a total irradiation time of 14 hours at a dose rate of and the corresponding dose range of . Diode parameters such as the Schottky barrier height, ideality factor, saturation current, free carrier concentration and reverse leakage current were monitored from the I–V and C–V measurements that were recorded by a computer programme developed using National Instrument’s LabView software. The irradiation results reveal an increase in the ideality factor, series resistance, saturation current and reverse leakage current over the dose range investigated. The free carrier density and the Schottky barrier height were found to decrease with increasing dose. Our results suggest that the observed effects are as a result of the radiation damage to our sample.
Measurements of X-rays diffraction (XRD), electrical resistivity (T), magnetic susceptibility (T), and magnetization (µ0H) are repeated for the pseudo-ternary alloy (Ce1xTbx) Pt2Si2. XRD results for all the compositions of (Ce1xTbx) Pt2Si2 system indicate a tetragonal CaBe2Ge2-type structure. (T) results indicate evolution from coherent Kondo lattice to incoherent single ion Kondo scattering with increase in Tb content up to x=0.8, and followed by a metallic behavior above x = 0.8. (T) data at high temperatures follow the Currie-Weiss relation for all alloy compositions and give effective moment value µeff which increases gradually from value of 2.54 µB for Ce3+- ion to the expected values of 9.72 µB for Tb3+ -ion. The alloys compositions in the concentration range of 0.7 x 1 exhibit antiferromagnetism and the low temperature (T) data were used to find the Neel temperature TN as a function of x. (µ0H) data are presented for all investigated compositions.
The Cr-Re alloy system was chosen in order to investigate a previous observation of the co-existence of superconductivity and antiferromagnetism, as well as the possibility of a quantum critical point (QCP) in this alloy system. For the initial studies a Cr86Re14 mother alloy was doped with V in order to decrease the electron to atom ratio, resulting in a decrease in the Néel temperature (TN). The preliminary study focused on first obtaining a pure body centered cubic (BCC) Cr86Re14 mother alloy, as in this alloy system, a secondary A15 phase may co-exist. X-ray diffraction and electron microprobe analyses of as-cast and annealed samples revealed only BCC structure with good homogeneity and actual Re concentration of 16 ± 1 atomic percent. In order to initiate investigations into the magnetic properties and possible quantum criticality in this system, electrical resistivity and magnetic susceptibility measurements as a function of temperature are reported for (Cr86Re14)100-yVy alloys with y = 0, 5, 7 and 9. Electrical resistivity as a function of temperature for these samples reveal an anomaly on cooling through TN. The anomaly observed shifts down to lower temperatures as the V concentration is increased. Magnetic susceptibility data supports these observations. These promising results warrant further investigation into this alloy system with the possibility of contributing towards the current interest in quantum criticality in chromium alloy systems.
The main aim of this project is to synthesize, isolate, identify and characterize graphene, defined as a one-atom layer of hexagonally bonded carbon atoms using mechanical exfoliation method. Moreover, this technique is based on pulling apart the layers of a piece of highly oriented pyrolytic graphite(HOPG) and transfer layers from the graphite onto a SiO2 substrate. Graphene is the first truly 2D dimensional material and has a number of remarkable mechanical and electrical properties. It is substantially stronger than steel, and it is very stretchable. The thermal and electrical conductivity is very high and it can be used as a flexible conductor. Graphene is a zero-gap semiconductor and has a novel electron structure, with its conduction and valence bands meeting at the Dirac point. This characteristic enables the electron conduction by shifting the Fermi level with applied field.
The effects of annealing temperature and time on glassy carbon implanted with 360 keV strontium ions at room and high temperatures are reported. The samples were implanted with strontium ions to a fluence of 2 x 1016 ions/cm2 at room temperatures, 350ºC and 600ºC. The room temperature implanted samples were isochronally vacuum annealed at temperatures from 200ºC to 700ºC for 1 hour. The depth profiles of the implanted samples before and after annealing were obtained by ion beam technique, viz. Rutherford Backscattering Scattering (RBS). Scanning Electron Microscopy (SEM) was employed to investigate the effect of implanted ions and annealing temperatures on the microstructure of the substrate.
Hydrogen is the lightest element and it has the highest energy content per unit of weight of any known element and thus, it can be used as a source of clean energy. Previous research studies on hydrogen storage show that storing hydrogen as a solid hydride is preferred compared to liquid and gas because of the very high pressures and low temperatures needed to maintain the system. Hydrogen storage in solid form offers the safest alternative for transportation and storage of hydrogen. Metal hydrides are known to possess high volumetric hydrogen densities (of the order of 3 to 8 wt. %) along with the ability to store hydrogen at atmospheric pressure and room temperature. Moreover, many metals and alloys are capable of reversibly absorbing large amounts of hydrogen. The unique properties of hydrogen storage alloys are used in numerous applications such as rechargeable batteries, cooling devices, hydrogen storage systems for fuel cells and in gas sensors applications. The research interest of this project is placed on Ti6Al4V alloy as promising candidate for hydrogen storage. The changes in microstructure (volume fractions of alpha- and beta-phase) will be studied as a function of hydrogen concentration and pressure. X Ray Diffraction (XRD) will be used for phase analysis, while the hydrogen content and hydrogen depth profile will be determined by Electron Recoil Detection Analysis (ERDA) and Heavy Ions (ERDA) methods.
Optical selection rules for the intervalley scattering for direct and indirect transitions are investigated. The non-zero matrix elements of the scattering tensors are derived by the method of vector coupling coefficients. These coefficients are needed for the understanding of Gunn effect.
The diffusion of a dilute S solution in Fe was investigated in order to determine the diffusion path and the energy required, Q, for diffusion in a bulk Fe crystal structure. Different pseudototentials were investigated in order to determine the pseudototential that best describes the ground state of the bulk Fe structure. Pseudototentials that gave a good description of the ground state were subsequently tested to find the appropriate bulk cell size that will give the best description of S diffusion in Fe. This was done by calculating the binding energy of pure Fe and that of S in Fe for different cell sizes and to find the cell size were the binding energy converges. Using the appropriate cell size, an investigation was performed in order to determine the diffusion mechanism of S in Fe. The nudged elastic band method was employed to calculate the minimum energy path of S diffusion in Fe, which gives the energy required for diffusion of S in Fe, namely the activation energy, Q. Activation energies for both interstitial S and substitutional S in the Fe crystal was calculated by the nudged elastic band method in order to determine the most energetically favourable diffusion mechanism for S in bulk Fe.
Noble gas ion sputtering combined with Auger analysis has been applied extensively to determine the elemental composition as function of sputtering depth. Data from these depth profiles have been used to determine the sputter yield coefficients (S) of elements that are of fundamental interests in many fields, such as in: sample preparations particularly were the sputter ion source is applied, surface cleaning (the surface is usually cleaned by cyclic ion bombardment and annealing), and understanding the theoretical studies of ion sputtering, etc. Many published data on the Auger-ion (Ar+) sputtering of indium (In) focused on In compounds like indium nitride (InN), indium phosphide (InP) etc. In this study, the focus is on metal In films that were grown by electron beam evaporation on silica substrates. The APPH's (Auger peak to peak heights) were measured while the In films were subjected to different energy Ar+ ion sputtering. The Ar+ ion energy was varied from 0.5 to 4.0 keV. The normal to the film surfaces was kept at a tilt angle of 30o with respect to the direction of the incident electron beam. The sputter yield coefficients (S) of the In films were determined as 4.5, 6.4, 10.6, 11.5 and 10.7 atoms/ion for the Ar+ energies 0.5, 1.0, 2.0, 3.0 and 4.0 keV respectively.
ZnO and PVP capped ZnO nanoparticles were synthesised using the sol-gel method at low temperature with ethanol as a solvent, zinc acetate as a precursor and methanol as the cleaning agent. The influence of temperature on the morphology, structure and the optical properties of the ZnO nanoparticles were investigated. The effect of addition different molar masses of the PVP during the synthesis on the ZnO emission peaks was systematically monitored. Polyvinyl pyrrolidone (PVP) is utilized to cap ZnO nanorods from a zinc acetate precursor at low temperature, since the PVP can be easily removed by burning the solid products. The photoluminescence (PL) characterization of the ZnO nanostructures exhibited a broad emission in the visible range with maximum peaks at 449 and/or 530nm, this was influenced by the addition of different molar masses of the PVP. The scanning electron microscopy (SEM) images of ZnO and PVP capped ZnO has showed the presence of the agglomerated ZnO particles which could be due to the agglomeration of the smaller particles. The x-ray diffraction (XRD) spectra for ZnO nanoparticles show the entire peaks corresponding to the various planes of wurtzite ZnO, indicating a single phase. The absorption edges of these ZnO nanoparticles are shifted by additions of PVP polymer. The absorption spectra of the ZnO showed slight shifts with reference to the various molar masses of PVP.
A wet chemistry route is an essential method used today to synthesize high quality ZnO nanoparticles and nanorods. It is a novel low cost method to produce nanoparticles and nanorods with relatively high dispersivity. In this study optical properties and morphology of the nanoparticles and nanorods were investigated. The X-ray diffraction (XRD) patterns of both nanoparticles and nanorods revealed the hexagonal wurtzite structure, the diffraction peaks matched well with the JCPDS (card no 80-0075) standard data. The transmission electron microscopy (TEM) revealed the nanoparticles and nanorods with different diameters and lengths. The optical absorption and photoluminescence (PL) were also obtained and it was demonstrated that the PL exhibited a strong near-band-edge emission (UV) and weak visible emission bands. The PL intensity increased with increasing precursor concentration (0.1M-1.5M) and was quenched at higher concentrations, which may be attributed to concentration quenching effects. The fourier transform infrared spectroscopy (FTIR) was used to determine the distinct stretching mode frequencies. The ZnO nanoparticles and nanorods were investigated for possible applications in transparent electrodes in solar cells and gas sensors.
The existence of the ternary intermetallic compound CeRh2Sn2 has been known since the crystallographic report of Selsane et al on the CeM2Sn2 family of compounds in which M is a d–electron element. The crystal structure is well ordered and the sole magnetic species, Ce, occupies a unique symmetry site in the unit cell. Subsequent studies into these compounds revealed a general trend of magnetic ordering at very low temperatures. CeRh2Sn2 was found to order antiferromagnetic through a peculiar smeared out transition around TN=0.4 K. Most significantly though was the giant electronic specific heat witnessed in the Sommerfield coefficient Cp(T)/T which was found to develop in this compound even well above the magnetic ordering temperature. The behaviour of this system was explained in the framework of a heavy-electron quasiparticle state forming out of the many-body Kondo interaction between localized magnetic moments of Ce ions and the conduction electrons. This results in an exceedingly high electronic density of states at the Fermi energy EF. In this work we present a detailed study of specific heat, magnetic susceptibility, and electrical resistivity of CeRh2Sn2 in order to map the field stability of salient cooperative effects. The magnetic ordering is found to be instable to fields beyond about 0.5 T. At the same time, applied magnetic fields displace the huge 4f-electron entropy towards higher temperatures. Further evidence for the importance of the Kondo effect in CeRh2Sn2 will be discussed.
The novel ternary intermetallic compound Ce6Pd12In5 forms in a well-defined atomic stoichiometry with a hexagonal crystal structure (space group P63/mcm) and a unique atom in each of the crystallographic sites available in this space group. The bonding lengths are comparatively short and suggest strong interaction especially between Ce and Pd atoms. In this work we report on our findings of the first studies into the physical properties of Ce6Pd12In5. A key finding is the occurrence of long-range ferromagnetic-like order below TC =1.6 K, where the specific turns into a lambda-like anomaly peaking at CP = 8 J/(mol Ce K). The electrical resistivity hints at a low-lying Kondo scale in this compound. The magnetic susceptibility shows well-defined Curie-Weiss behaviour over an extended temperature range with an effective magnetic moment value that is indicative of conduction-electron hybridization effects on the Ce localized moment. The 4f–electron derived magnetic contributions to the entropy and to the electrical resistivity in Ce6Pd12In5 are assessed by means of the nonmagnetic counterpart La6Pd12In5.
We present specific heat, electrical resistivity, susceptibility, and thermopower measurements on the two novel intermetallic compounds CeFe2Al8 and CeCo2Al8. They form in an orthorhombic crystal structure of space group Pbam. In the unit cell representation Ce atom occupies only one lattice site,having Ce-Ce interatomic distance of 4.03Å. Magnetic susceptibility of CeFe2Al8 in the range of 1.9K - 400K yields an effective paramagnetic moment value of 3.89μB together with prevailing antiferromagnetic interaction through Weiss temperature Θ=-745.8K. The overall temperature dependence suggests valence instability in this compound and we model the appearance of broad peak around T=230K in the susceptibility to a T2lnT dependence attributed to an intermediate valent state. On the other hand for CeCo2Al8 the local moment state is depicted through an effective moment close to the free Ce3+ ion value. No long-range magnetic ordering is found in either of the two compounds down to 1.9K. The magnetic contribution of electrical resistivity on CeFe2Al8 and CeCo2Al8 compounds follows –lnT behavior at intermediate temperatures which is typical of incoherent Kondo interactions between conduction electrons and magnetic Ce ions. A Fermi liquid behavior in resistivity measurement is observed in CeFe2Al8 compound towards the ground state,whereas clear deviations from standard Fermi liquid behavior are indicative of strong electronic correlation effects in CeCo2Al8. At 2K the electronic specific heat of this compound reaches γ =0.106J/mol-K2,and exhibits a –lnT divergence towards T→0. We explain this behavior in terms of quantum criticality that stems from low-lying magnetic ordering effects. In studies of the thermoelectric power,a maximum is reached at T=140K(S=24μV/K) and T=30K(S=23μV/K) for CeFe2Al8 and CeCo2Al8 compounds respectively. We propose a description for this behavior in terms of formation of fine structure in the electronic density of states near the Fermi energy(EF).
Recent measurements on an antiferromagnetic (AFM) Cr0.978Al0.022 alloy single crystal suggest that the triple point (TP) on the magnetic phase diagram (MPD) might be a special type of critical point. This suggestion is further investigated here through electrical resistivity and specific heat measurements on a series of Cr1-xAlx alloy single crystals with x around the triple point concentration (xc). Néel temperatures (TN), obtained from the resistivity measurements, indicate that xc ≈ 0.02 is situated at the position of a rather sharp and deep minimum of the AFM-paramagnetic phase transition line on the T - x MPD. The minimum is deeper than that generally accepted for this alloy system. The Sommerfeld electronic specific heat coefficient, obtained from the specific heat measurements, shows an interesting aspect. The Sommerfeld electronic specific heat coefficient as function of Al concentration peaks relatively sharply at xc, an aspect for which the explanation is two-fold: either the phase boundary line separating the incommensurate (I) and commensurate (C) spin-density-wave (SDW) phases on the MPD, starts at the TP and reaches T = 0 K vertically below this point, or the TP itself is situated close to T = 0 K. It is reasoned that the latter seems a more likely possibility, making this system unique in exhibiting a critical point at 0 K where ISDW, CSDW and paramagnetic (P) phases coexist.
Traditionally magnetic forces are supposed to act on moving electric scalar charges and the hitherto undetected Dirac’s magnetic (scalar) charges, and that work done by such forces is always zero. Any possible evidence to the contrary is vehemently denied in order to justify this long held myth. Following our recently developed and validated concept of magnetic vector charges as sources of magnetic fields, we show that magnetic forces, like gravitational and electric forces, act on objects with corresponding physical attributes, and do nonzero work. This is illustrated here by depicting the mechanical generation of electricity and operation of simple electric motors in terms of magnetic vector charges. Other supporting examples and technological applications that can be cited include the jumping ring experiment, Gouy magnetic balance, Hall magnetic field probe, vibrating sample magnetometer, magnetic levitation and magnetic separation of materials. This simple realization may have far-reaching implications on our overall understanding of magnetism and its ultimate effective utilization.
Metallurgical products play an important role in everyday life. The search for alloys with better material properties such as strength, wear and corrosion resistance continues to this day. In addition to these desirable properties, the search for ways to reduce production costs and time has led to a large amount of research being conducted on the processes which determines the material properties of metals and alloys. One of these processes is known as segregation. To improve segregation studies the influence of surface evaporation should be considered. As experimental segregation studies are performed under high vacuum conditions, certain elements are prone to this neglected phenomenon. Although some attempts have been made to develop segregations models that take surface evaporation into account, these models can only predict segregation in either the kinetic region or when equilibrium has been reached. In addition the effect of evaporation has received little attention in previous experimental studies. In this study surface evaporation during segregation of a Sb/Cu system was measured. These results together with the modified Darken model will be used to simultaneously predict kinetic and equilibrium segregation including the effects of surface evaporation. A first approximation will be discussed as well as modifications made to an Auger system in order to measure the actual surface evaporation that takes place during segregation.
The optical and structural properties of ZnO grown with a mixture of oxygen and nitric oxide as oxidants are investigated. With R defined as the ratio of oxygen to nitric oxide molar flow rates, samples grown with R ≥ 1 (i.e. a higher low rate of oxygen compared to nitric oxide) reveal a 3D growth mode with columns perpendicular to the substrate, while for R < 1 a preferential 2D growth mode is observed. The low temperature photoluminescence studies show that the columnar growth observed for R ≥ 1 coincides with an increase in intensity of the transitions around 3.31 eV arising from structural defects whereas this emission is quenched for R < 1. The disappearance of the transition at 3.31 eV is followed by the emergence of a transition at ~3.356 eV, which is assigned to an acceptor bound exciton related to nitrogen. Furthermore, this transition is not visible in the oxygen-rich regime, due to the high density of structural defects in the samples. This work aims to show the importance in minimizing structural defects in order to achieve acceptor bound exciton emission in ZnO doped with nitrogen and possibly stable p-type ZnO.
Non-resonant microwave absorption measurements at 9.4 GHz(X-band)using electron spin resonance spectrometer (ESR), were carried out on pulsed laser deposited(PLD)polycrystalline FeSi thin films grown on Si(111)substrate. The low-field microwave absorption(LFA)properties of the films were investigated as a function of DC modulation field, temperature, microwave power and angle. The DC field and AC field were orientated parallel to the film surface. The DC field was orientated normal to the AC field. The anisotropy field was observed to have a central influence on the LFA shape on all the measurements made, which makes LFA very similar to giant magneto-impedance(GMI). Thin films of FeSi could be potential candidates for magnetic field sensors(based on their GMI). Magnetic anisotropy could be a signature of ferromagnetic state of a material and hence thin films of FeSi are promising candidates for Spintronics applications.
SrGa2S4:Ce3+ thin films are promising for full colour thin-film electroluminescent (EL) and field emission displays (FED’s) because of its good optical properties. These films were previously prepared using several different techniques such as sputtering (RF), molecular beam epitaxy (MBE), reactive multi-source deposition (MSD), metal-organic chemical vapour deposition (MOCVD), deposition from binary vapours (DBV) and Pulsed Laser Deposition (PLD). In the present study, Ce doped SrGa2S4 thin films were prepared for the first time using pulsed reactive cross laser ablation (PRCLA) technique. Characterization of the films was carried out with scanning electron microscopy (SEM), atomic force microscopy (AFM) and x-ray diffraction (XRD). Cathodoluminescence (CL) and photoluminescence (PL) measurements were taken with a S2000 Ocean Optics Spectrometer and a Varian Cary Eclipse Fluorescence Spectrophotometer respectively. The substrate temperature, number of pulses and the working pressure are the parameters that were varied during the preparation of the thin films. A single-phase SrGa2S4 layer with high crystallinity was obtained at the growth temperature of 400oC. XRD patterns also showed that the properties of the films are relatively sensitive to substrate temperature. PL showed one broad band that can fit two Gaussian peaks according to the two Ce3+ emission peaks which are known to originate from radiative transitions from 5d (T2g) → 4f (2F5/2) and from 5d (T2g) → 4f ((2F7/2) respectively. CL showed two broad emission peaks around 441nm and 478nm which are due to Ce3+ transitions. The AFM images of the films prepared had a rough surface, which became smooth after annealing in vacuum at 700oC temperature. Non-uniformity in particles of the films and rough surface were observed from the SEM images.
Surface mapping of quantum well structures using photoluminescence measurements is ideal for determining the overall behaviour of such samples with regard to their chemical composition and well-width fluctuations. In some areas where the structural variations are large over a sample, photoluminescence mapping can yield information which would otherwise require several samples to be grown. Well thickness and strain variations across a sample will show as a small spatially-dependent change in the peak energy and the full wave half maximum, so that variations in confinement energies in the QW due to non-uniformity in the film growth can be observed with this method. In this work, spatial variations in the exciton band of photoluminescence spectra were measured using surface mapping of the emission by scanning the sample laterally in the plane of the quantum well across the sample area. The luminescence intensity was then mapped as a function of the emission peak energy (and emission linewidth) and the spatial coordinates x and y, resulting in contour plots. The results show that knowledge about whether the structures are alloy-disordered or rough (characterised by thickness fluctuations) can be derived through this simple photoluminescence mapping. In rough structures, there is strong correlation between variations in peak energy position and linewidth of the main exciton band. On the other hand, in alloy-disordered structures, there is virtually no correlation between the peak energy and linewidth in comparison to the rough structures. Surface mapping has also revealed variations in peak energy across the rough samples do not only correspond to monolayer fluctuations in well-width, but fluctuations in steps of submonolayers are very common. This explains why thinner QWs give narrower lines compared to thicker ones in these CdSe-ZnSe systems studied here. Further, this analysis shows alloy-disordered structures give good quality samples in terms of optical properties.
Nano-pahsed structures based on metal-dielectric composites also called cermets (Ceramic-Metal) are considered among the most effective spectrally selective solar absorbers. For high temperature applications (stable up to 650°C) noble metals nanoparticles and refractory oxide host matrices are ideal as per their high temperature chemical inertness and stability: Pt/Al2O3 cermet nano-composites are a representative family. This contribution reports on the optical properties of Pt/Al2O3 cermet nano-composites deposited in a multilayered tandem structure. The radio-frequency sputtering optimized Pt/Al2O3 solar absorbers consist of stainless steel substrate/ Mo coating layer/ Pt-Al2O3/ protective Al2O3 layer and stainless steel substrate/ Mo coating layer /Pt-Al2O3 for different composition and thickness of the Pt-Al2O3 cermet coatings. The coatings microstructure, morphology, composition, optical properties were analyzed by x-ray diffraction, atomic force microscopy, infrared attenuated total reflection and UV-VI-NIR specular reflectances.
ZrN films were deposited on Si<100> substrates, Al and brass strips by DC magnetron sputtering under varying conditions of power, pressure, argon and nitrogen gas flow rates as well as temperature and characterized by SEM, AFM, RBS, resonant RBS, and XRD. The films are transparent and semiconducting. They tend to absorb oxygen. The films were found to adhere well to the substrates. The colour of the films varied depending on deposition conditions and have been expressed in the Lab* colour system. Potential uses of such films are as protective hard coatings as well as decorative layers.
CaTiO3 exhibits a Perovskite structures that has an ABO3 stochiometry where A and B represent metal cations with a combined charge of 6+ valency and O with 6- valency, and thus bring about charge neutrality to the compound. Upon introducing Pr3+ ions in the site of Ca2+ ions, charge imbalance occurs by introducing a 7+ valency, and this force the material to compensate for this effect by introducing negative Ca & Ti vacancies, and positive O vacancies. These act by hampering the material luminescence. Thus substitution of Ti4+ by co-doping with Ba2+ and In3+ ions result into a 6+ valency and thus an electrically neutral compound. This reinforces the red emission of Pr3+ ions inside CaTiO3. CaTiO3: Pr3+ exhibiting red emission is synthesized via solid state reaction, and its luminescence is enhanced by co-doping with Ba2+ and In3+ that act as charge compensators. This optimal emission of CaTiO3:Pr3+,Ba2+,In2+ is achieved by solid state reaction at 1300 oC for 4h, by direct firing, according to 1:1 molar ratio of TiO2 Anatase and CaCO3 compounds, doping with 0.3 mol%Pr3+ (PrCl3) and charge compensating by adding In3+ (In(NO3)3) and Ba2+ (Ba(NO3)2).The observed 613 nm photoluminescence (PL) peak, when exciting the material with 330 nm ultraviolet light to promote (2p(O) → 3d (Ti)) absorption using the PL spectrometer, is attributed to the 1D2 → 3H4 transition of the Pr3+ ions. The glow curves were measured using Thermo luminescence (TL) spectroscopy. Addition of Ba2+ and In3+ ions into CaTiO3:Pr3+ phosphor enhances the optical performance of the Pr3+ red-emission. Additionally the phase was identified using the X-ray diffraction (XRD) and the surface morphology identified using Scanning Electron Microscopy (SEM).
There has been a growing demand for quantum cutting and upconverting efficient nano luminescent materials during the last decades. Wide band gap luminescent materials, specially the fluoride, are one of the major role players. Due to a number of advantages such as the excellent transparency in theinfrared (IR) to far ultraviolet (UV) range and low-energy phonons as well as low toxicity they have recently gathered considerable attention for their potential applications in bioimaging, disease detection and diagnostics. The same group of materials having downconversion characteristics can be applied for the improvement of solar cell efficiency in the quantum cutting processes. Here, we report on the synthesis and luminescence properties of colloidal double fluorides which belong to the general composition MF-LnF3 co-doped with optically active rare earth ions. During the thermal decomposition of the alkaline metals trifluoroacetate and lanthanide trifluoroacetate precursors NaYF4, KYF4, and KY3F10 samples have been obtained. The noncoordinating, high-boiling solvent octadecene was selected as the growth medium for the nanoparticles. Oleiamine and oleic acid were employed as the surfactant, which attached to the particle surface during the crystal growth and kept the nanoparticles well-separated from each other. Nanocrystals of RE3+ doped in the range of 40 nm were obtained. High Resolution Transmission Electron Microscopy (HRTEM) was used to evaluate the morphology and particle size distribution (PSD) of the prepared nanoparticles. The structural (XRD) and spectroscopic properties of optically active lanthanides doped fluorides have been determined based on the excitation spectra, emission spectra and luminescence decay curves recorded in the UV-Vis spectral regions.
Previous studies in our laboratory on the (Cr100-xAlx)95Mo5 alloy system through electrical resistivity and magnetoelastic measurements suggest that spin-density-wave (SDW) antiferromagnetism is fully suppressed down to 4 K in the concentration range 2.0 ≤ x ≤ 5.0 at.% Al. The current interest in quantum criticality in Cr and Cr alloy systems warrants further detailed investigation into this aspect. In this regard we report results of electrical resistivity, Seebeck coefficient and Sommerfeld electronic specific heat coefficient (obtained from specific heat measurements) for an extended alloy range 0 ≤ x ≤ 8.1 at.% Al. The Seebeck coefficient turns out to be a more sensitive parameter than electrical resistivity in obtaining magnetic phase transition temperatures for this alloy system, particularly for the commensurate (C) SDW phase region x ≥ 5.0 at.% Al. The electrical resistivity and Seebeck coefficient data depicts total suppression of AFM down to 2 K for 1.5 ≤ x ≤ 5.3 at.% Al. The results of Sommerfeld electronic specific heat coefficient confirms this finding, showing a sharp increase in the incommensurate (I) SDW phase up to x ≈ 1.5 at.% Al, roughly leveling off in the paramagnetic range 2.0 ≤ x ≤ 4.6 at.% Al, followed by a slow decreasing trend in the CSDW phase x ≥ 5.0 at.% Al. From the present study it appears that both points (x ≈ 1.5 at.% Al, T = 0 K) and (x ≈ 5.0 at.% Al, T = 0 K) are possible quantum critical points on the magnetic phase diagram of the (Cr100-xAlx)95Mo5 alloy system. It is then rather unique to have both ISDW-P and P-CSDW quantum critical points existing in the same alloy system.
Zinc-oxide nanoparticles are well known for their novel optical and electronic properties for applications in various fields such as solar cells, ultra violet shielding, gas sensors, paint and heat mirrors. We report on the relation between the structure and optical properties of ZnO nanoparticles synthesized via the sol-gel technique, with specific emphasis on the effect of growth and reaction temperatures. High-resolution microscopy techniques, complemented by Raman spectroscopy and x-ray diffraction, confirm that the crystallinity and particle size of ZnO nanoparticles is directly related to the synthesis conditions. Optical absorption and emission spectroscopy show that optical band gap and photoluminescence of the ZnO nanoparticles are intimately related to its structural properties, ascribed to the quantum confinement effect. Photoluminescence spectroscopy confirm the emission peaks in the ultraviolet (380 nm) and visible (500 nm) region; the latter attributed to the presence of the singly ionized oxygen vacancies in the nanoparticle.
Porphyrins are macro-cycles often organized into nano-scale structures which perform many of the essential light-harvesting-, electron- and energy-transfer functions in various natural and synthetic systems. In addition to the sub-picosecond charge generation and transfer, they exhibit various shapes and functional properties that make them useful for the construction of ultrafast nanodevices and more specifically solar cells. This latter case would require to expand their specific J, S and Q absorption bands; a task of this research work. They are related to chlorophyll molecules found in natural systems that carry out light harvesting, charge separation and energy conversion. Using the free base and diacid forms of tetrakis (4-sulfonatophenyl) porphyrin and by varying the ionic strength of aqueous solutions used, Schwab et al were able to form single and bundled nanorods, whereas Wang et al used the mixture of tetrakis (4-sulfonatophenyl) porphyrin and Sn(IV) tetrakis(4-pyridyl) porphyrin to form a mixture of nanotubes and nanorods. Once synthesized, the incorporation of such nanostructures into a functional device presents its own set of unique problems, but one promising approach is to incorporate the nanotubes/rods onto a support to obtain an array that can be directly used as a device. Properties investigated after synthesis included the optical (UV-visible spectroscopy), physical (Transmission electron microscopy) and the growth mechanism of the nanotubes/rods. Understanding the sizes and growth mechanism of nanorods is essential for the successful implementation in PV-like solar cells hybrid systems.
Presently Ca AlxOy:Tb green phosphors are mainly prepared by the solid-state process which is more feasible than others in terms of operation and large-scale production. However, the raw materials are usually not mixed well and very high temperature is needed to synthesize the final powder using the solid-state process. In order to solve these problems, Terbium ion doped calcium aluminate (Ca AlxOy:Tb3+ green phosphors were obtained at low temperatures (500ºC) by the solution - combustion of corresponding metal nitrate–urea solution mixtures, over a time of 5 min. The morphology and structural properties were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), SEM shows that the particles are irregular shape and are aggregated and the structural analysis revealed the presence of monoclinic CaAl4O7 at optimized fuel to nitrate molar ratio. The characteristic luminescence properties were investigated using emission spectra. It was found that the oxidizer: fuel molar ratios showed greatly influence not only on morphology, but also on their PL spectra. The photoluminescence (PL) excitation spectrum was characterized by a dominant broad band centered on 239 nm. Intense emission bands were observed at 489 nm, 544 nm, 587 nm and 622 nm originating from the 5 D4 to 7 FJ transition and other bands were observed at 379, 417 and 438nm originating from 5 D3 to 7 FJ transition, which correspond to the crystal field splitting components of the Tb3+ 4f states in Ca Al4 O7. The effects of doping concentration of Tb3+ on luminescence lifetimes and intensities were also investigated. The samples with Tb concentration of 1.0 mole % exhibited the highest luminescence enhancement and longer luminescence lifetimes.
Crystalline lead sulfide (PbS) nanorods were synthesized by chemical bath deposition (CBD) method using the mixed aqueous solutions of lead acetate, thiourea and ammonium solution. The lead acetate concentrations were varied in the deposition process to determine the effect thereof. The temperature of the bath was maintained at 80ºC. The final yields were characterized for structural, morphology and optical properties. The X-ray diffraction (XRD) spectra of the PbS nanorods correspond to the various planes of cubic PbS. A single phase was obtained. The average particle sizes calculated using the XRD spectra were found to be 14±0.5 nm for particles stirred for 10 minutes and 16±0.5 nm for samples stirred for 5 minutes respectively. When the concentration of lead acetate was increased the particle size also increased. Scanning electron microscopy (SEM) micrographs depict nanorod structures for a high mol concentration of lead acetate and a spherical shape for a low mol percentage. Energy dispersive X-ray (EDS) analyses confirm the presence of all the expected elements. The solid powder nanorods show good optical properties with high absorptions in the UV and visible regions. The band gap energies were estimated to be 1.69 to 1.84 eV which are higher than the bulk PbS band gap of 0.37 eV. The absorption edge and the band gap energies of these PbS nanorods have shifted depending on the ionic strength of the precursors.
Cobalt sulphide nanoparticles are one of the most complex and challenging particles to synthesize due to strongly reducibility of the cobalt ion and the oxidizable nature of the sulphide ion. Cobalt sulphide therefore exist in various phases with different compositions including Co4S3, Co9S8, CoS, Co1-xS, Co3S4, Co2S3 and CoS2. Herein, we describe the synthesis of CoxSy nanoparticles; furthermore we interrogate the influence of the reaction conditions on the structure and optical properties of the nanoparticles. CoxSy nanoparticles were synthesized by thermolysis of N, N, N´, N´-tetramethylthiuram disulphide cobalt (II) complex in the presence of stabilizing agent Hexadecylamine (HDA). The advantage of this method is that the bond between the cobalt and sulphur atoms already exist, therefore decomposition of the complex at write conditions will result in the formation of CoxSy nanoparticles. The cobalt sulfide nanoparticles were prepared by varying concentration from the range 5.0 mg to 5.0 g of the CoCl2[(CH3)2NCS2S2CN(CH3)2]2 complex at 130 °C temperature. The effect of temperature was also investigated by varying the temperature from 80 to 250 °C. The nanoparticles were characterised by a combination of absorption spectroscopy, photoluminescence (PL), X-ray diffractometry (XRD) and transmission electron microscopy (TEM).
Rare earth oxysulfide materials exhibit high luminescence efficiency as well as excellent chemical stability. These materials are widely used as luminescent host materials of several commercially available red emitting phosphors. However, there still remain issues affecting the operational parameters such as luminescence efficiency, stability against temperature, high color purity and long afterglow. Y2O2S:Eu nano powders were prepared by low-temperature synthesis method using sulfur-contained organic fuel in an ethanol-aqueous solution. The prepared nano powders were characterized by Scanning electron microscope, X-ray diffraction, photoluminescence spectra and X-ray photoelectron spectroscopy. It is shown that the assistant fuel ethanol has the effect of decreasing the water needed, simplifying the experiment procedure by dissolving rare earth nitrate and sulfur-contained organic fuel into an even solution, and prompting the formation of rare earth oxysulfide by igniting first during heating that leads to combustion decomposition reaction. Y2O2S:Eu nano crystallines with strong photoluminescence and X-ray luminescence are obtained using thiourea as organic fuel. Mixtures of Y2O3:Eu and Y2O2S:Eu are acquired using thiourea as fuel, and the content of Y2O2S:Eu increases until reaches to about half of the Y2O3:Eu with the increasing amount of thiourea. Y2O2SO4:Eu emerges when S/Y=6 and increases with increasing thiourea content.
Tb3+-activated calcium silicate (CaxSyyOz:Tb3+) nanophosphors have been synthesized by means of simple and low temperatures (500°C) using the solution-combustion of corresponding metal nitrate, TEOS and urea solution mixtures. The structural evolution of the phosphor was studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and the luminescence properties of the phosphor powders investigated as a function of terbium concentration. The XRD study indicates that new peaks appear as the terbium concentration increase results in phase changes from CaSiO3 to Ca3Si2O7. The later phase may be favored as a result of some Si sites substituted by the Tb ions as the concentration of Tb was changed from 1 to 4 mol. %. The CaxSiyOz:Tb phosphor powders show the characteristic emission of Tb3+ under UV excitation, with the emission transitions of (5D4→7F3,4,5,6) with excitation 257nm have been measured. Particularly, the green emission transition at 545nm has been found to be more prominent and intense which is due to an electronic transition of 5D4-7F5. The decay curves implied that these phosphors contain fast, medium and slow-decay process. The oxygen vacancies trap levels may be considered to be responsible for the long afterglow phosphorescence at room temperature. Such green strong emission displaying powder phosphor will find applications in the development of coated screens in certain electronic systems.
Tb3+-doped SrZnAl2O4 nanocrystalline phosphor with good crystallinity were successfully prepared by a combustion method at a relatively low temperature (500o-C), using urea as fuel and metal nitrates as precursors. The effects of Sr and Zn concentrations on the structure and luminescent properties of the material were investigated. In addition, different concentrations of Tb3+- were also used to determine the concentration that gives maximum intensity. The samples obtained were characterized by X-ray diffraction, scanning electron microscopy (SEM) and transmission electron micrograph (TEM), and photoluminescence (PL) properties of doped samples were investigated. The green emission obtained is associated with 5-D4→7F5 transitions of Tb3+- at 543 nm. This emission was shown to increase with the concentration of Tb3+- and it quenched at high concentrations. This was attributed to concentration quenching effects. The SrZnAl2O4:Tb3+- phosphor was evaluated for possible application in different types of light emitting devices.
Barium-substituted phosphor powders of (Ba1-xSrx)Al2O4:Eu2+ ;Nd3+ composition were prepared by combustion method at an initiating temperature of 5000C, using urea as reducer. The powders were then annealed at higher temperatures of 8000C and 10000C for 3 hours. Their crystallinity and phase were characterized by x-ray diffraction (XRD). The x-ray diffraction shows that the crystallinity of (Ba1-xSrx)Al2O4:Eu2+;Nd3+ at (x=0) greatly improved after annealing at the higher temperature of 10000C and shows the hexagonal structure that closely correspond with the JCPDS data (PDF#17-0306). The morphology of the (Ba1-xSrx)Al2O4:Eu2+;Nd3+ composition were investigated using Scanning Electron Microscopy (SEM). The effect of varying the Ba/Sr concentration on the thermoluminescence (TL) glow curves was investigated using the Thermoluminescence Reader (Integral-Pc Based) Nucleonix TL 1009I. Photoluminescence (PL) properties of all phosphor samples were investigated by measuring their emission spectra using a 325nm He-Cd laser. For the as-prepared phosphor samples, the sample with x=0 (BaAl2O4:Eu2+;Nd3+showed a much higher intensity compared to both the sample with x=1 (SrAl2O4:Eu2+;Nd3+and the mixed composition of the two. (BaAl2O4:Eu2+;Nd3+(x=0) and SrAl2O4:Eu2+;Nd3+ (x=1)showed a broad emission at 505nm and 522nm respectively while the mixed composition showed two peaks at 447nm and 517nm. The broad emission peaks attributed to 4f65d1-4f 7 transitions of Eu2+ were obtained. Their phosphorescence was investigated by using the fluorescence Cary eclipse spectrophotometer coupled with a xenon lamp. Phosphorescence show higher luminescence for (Ba1-xSrx)Al2O4:Eu2+;Nd3+at (x=0).
The study of annealing effects on platinum binary systems has both the fundamental and applied aspects. It is due to the fact that the formation of ordered domains increases the strength and surface hardness and could also have an impact on surface activity and chemical properties. The changes of mechanical, physical and chemical properties caused by annealing could be of significant importance for application of platinum systems as catalysis, gas sensors, fuel cells, glass industry, electronics and biomedicine. The Pt-Mo coatings used in this study were deposited by electron beam and sputtering methods. In order to study annealing effects the Pt-Mo coated systems were subjected to heat treatment at elevated temperatures for different annealing periods. Several complimentary techniques such as X-ray Diffraction (XRD), Rutherford Backscattering Spectroscopy (RBS), Particle Induced X-ray Emission (PIXE), and Scanning Electron Microscopy (SEM) were used for coatings characterisation.
Pr3+-Tb3+, and Pr3+-Eu3+ ion pairs co-doped in SiO2 matrix were prepared by a sol-gel method. The photoluminescence (PL) measurements revealed the red and green emissions centred at 614 nm (5D0-7F2) and 541 nm (5D4→7F5) for single doped Eu3+ and Tb3+ ions in SiO2, respectively. Co-doping of Eu3+ and Tb3+ ions with Pr3+ in SiO2 showed that the energy transfer between Pr3+ and nearest Eu3+ and Tb3+ ions takes place. At the same time, however, there was evidence of luminescence quenching of Eu3+ and Tb3+ emissions at certain concentrations of Pr3+. The quenching was also confirmed by cathodoluminescence (CL) measurements recorded from the same powders. . Possible mechanism of energy transfer from Pr3+ to Eu3+ and Tb3+ and its quenching effects are discussed.
The concentration of any dopant in a semiconductor can usually easily and readily be established using the plasma resonance minimum. Infrared reflectance spectroscopy was used to assess the doping of SiC wafers by phosphor implantation. However, results obtained did not match the theoretical predictions. The problem was investigated, including an assessment of the applicable theory. Results will be presented and discussed.
The magnetocaloric effect (MCE) may become strongly enhanced in systems exhibiting a second order magnetic phase transition due to critical behaviour of the order parameter in close proximity of the transition region. Recently it has been shown that, analogous to the order parameter, the MCE shows scaling behaviour with the applied magnetic field along the critical isotherm. It has been argued that this behaviour should be indicative of the universality class of the system. Using ferromagnetic CeAuGe as a model system with high crystallographic and atomic order, we have determined the MCE from specific heat measurements in various applied fields. In the low field limit we observe scaling behaviour reminiscent of a mean field ferromagnet. However, in progressively larger applied fields the MCE in CeAuGe significantly exceeds that of the mean field reference system, indicating that the refrigeration capacity of CeAuGe is significantly larger than that of a model mean field ferromagnet. Our results are contextualized in terms of a more general amenability of local-moment magnetic compositions as model systems for magnetic refrigeration.
The importance of gold for scientific use is of fundamental importance to research and technology developments. The bulk gold shows reluctance to participate in chemical reactions, the effect which has been corrected by the change in the size towards nanoclusters and, this makes the study of temperature effects on gold very important. We have performed molecular dynamics simulations on bulk and nanomaterials of gold at various temperatures to study the effect of size on the melting temperature. Melting temperature of bulk gold was determined to be 1320 K and this in good agreement with the experimental value of 1337 K. Different gold nanoclusters and nanotubes melt at temperatures lower than 1200 K which is a significant decrease in the melting temperature from the bulk. The face centered cubic (fcc) bulk gold structure remains intact at high temperatures while spherical nanoclusters and cylindrical nanotubes respectively transform into tetragonal structures and patches of spherical clusters.
The interaction of Pd and SiC samples during thermal annealing were investigated using x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) as well as Scanning Electron Microscopy (SEM). Two sample configuration were used (1) the palladium (50nm) was deposited onto a degreased 6H-SiC substrate and (2) palladium ions were implanted into the 6H-SiC substrate at room temperature and at 200°C with a fluency of 1 x 1015cm-2. Both sets of the samples were annealed between temperatures of 200°C-800°C for a period of 60 min. The study showed that the thermally annealed Pd on SiC showed no detectable reaction after annealing at 200°C-400°C. At 500°C the RBS spectra shows that the Pd had reacted with SiC. Using XRD it was observed that metal rich silicides Pd3Si tend to form first after annealing at 500°C followed by the formation of Pd2Si after annealing at 600°C, 700°C and 800°C. No Carbon compounds were detected by XRD in these annealing temperature ranges. The implanted samples showed no detectable phase formation after annealing at temperature ranges of 200°C-800°C for a period of 60 min and at 1000°C for 10 hours. The RBS / Channeling results for the implanted samples are also shown.
There is a general misunderstanding regarding the creation of Frenkel pairs in the vicinity of the observed critical temperature Tc in superionic materials. The measured conductivity increases sharply at a temperature coinciding with the deviation of temperature-dependent lattice constant from 6.2086 A. Frenkel pairs responsible for superionic conduction are gradually generated well below the critical temperature and this is informed by the calculation of S-parameter through the measurements of Doppler Broadening at various temperature points. It is interesting to note that the lattice constant plays a pivotal role in the superionic conductivity threshold. Positron annihilation spectroscopy, through the determination of positron lifetime and Doppler broadening, reveals that the generation rate of Frenkel pairs becomes prominent at 100 K below the critical temperature of 693 K. This is also a clear indication of continuous disordering of fluorine sublattice noticeable at a temperature of 593 K. The fact that the defect positron lifetime is constant in the temperature range (300 – 900) K confirms a non-distortion of Ba-sublattice.
Previous experimental investigations have only shown, without explanation, that the pre-exponential factor (D0), in the diffusion coefficient of Sb segregating in Cu, is dependent on the surface orientation of a crystal. In this study, the surface concentration of Sb in a Sb doped Cu(111/110) bi-crystal was measured using a method combining AES and linear temperature heating. Segregation parameters, including the D0’s are extracted from the experimental data of surface concentration versus temperature using the modified Darken model. The difference in the two pre-exponential factors D0 (Sb in Cu(111)) and D0 (Sb in Cu(110)) is explained thermodynamically in terms of entropy change ΔS that is calculated, for the first time, for a Cu(111/110) bi-crystal.
In 2003, photoluminescence emission in tin dioxide nanoribbons was reported and it was later demonstrated that the absorption of nitrogen dioxide by these structures quenches the intensity of visible light emission in SnO2 nanobelts which suggested he possibility of developing a new class of contactless devices based on gas sensitive optical devices. The motivation behind this project is to clarify the fundamental aspects of light emission mechanisms of nanorods instead of nanoribbons and to characterise their response to gas species in view of their possible applications. The benefits of this project may include enhancing the sensitivity of conductometric gas sensors through the design and synthesis of porous three-dimensional tin oxide nanostructures. Several methods have been used to prepare SnO2 nanorods including thermal evaporation, thermal decomposition, solution phase growth and hydrothermal methods. Amongst these, the thermal evaporation approach has been used to synthesize a wide variety of 1-D materials. This often has involved the use of a catalyst in which nanowire growth proceeds by vapour-liquid-solid (VLS) mechanism. However metal catalysts can serve as impurities and contaminate the nanowires, possibly forming defect states that limit their applications in devices. The experiental procedure that will be used to deposit the tin dioxide (SnO2) nanorods is called pulsed laser deposition (PLD). These structures were characterized using X-ray powder diffraction, scanning electron microscope, transmission electron microscope and photoluminescence spectroscopy. The XRD patterns of the SnO2 nanorods showed peaks with 2θ values of 26.97 °, 34.34 °, 38.26 °, 52.01 °, 54.90 °, 71.28 °, and 78.40 °, corresponding to SnO2 tetragonal rutile crystal planes of (110), (101), (200), (211), (220), (202) and (321) respectively. Raman spectra taken at room temperature for Sno2 nanorods which shows bands at 576 and 359 cm-1 in addition to the Ag1 vibrational mode at 635 cm-1. TEM image of the SnO2 nanorods indicates a relatively uniform rod-like morphology. These rods are of 15-20nm in length and 2.5-5nm in diameter. The room temperature photoluminescence spectra for SnO2 nanorods showed a red emission at 580nm was observed from the SnO2 nanorods using the He-Cd laser (~325nm) as the excitation source.
The binary compound CeCu5 is an archetypal heavy-fermion Kondo lattice with an antiferromagnetic ground state. Strong hybridization effects between the localized 4f-electron moment and conduction electrons result in incoherent spin scattering in the electronic transport at high temperatures, but with an effective magnetic moment nearly equal to that of the free-ion Ce3+ state. Long-range magnetic ordering at TN = 3.9 K and the Kondo temperature TK=2.2 K are of similar energy scales, making this system ideally suited to studies of competing magnetic interactions in the strongly correlated electron class of systems. In this work we synthesized a series of pseudo-binary compounds CeCu5-xAlx. The dilution of Cu by Al results in augmenting the electronic density of states close to the Fermi energy, which is also the location of the magnetic and hybridized 4f electron levels resonance. Our studies of the temperature and magnetic field dependencies of thermoelectric power and electrical resistivity in the CeCu5-xAlx series of compounds will be discussed in terms of comparitive calculations of the Kondo energy scale from data of magnetoresistivity (the single-ion Bethe Ansatz theory) and from the thermoelectric power (phenomenological description in presence of intermediate valency) by means of two different theoretical models.
Since the first report of the anomalous magnetic and electronic phenomena in CeRu2Al10 this intermetallic class of compounds, incorporating also members with elements Os and Fe in the place of Ru, have attracted considerable interest. N spite of large interatomic distances antiferromagnetic order sets in at TN =27 K in the ru and Os derivatives, with simultaneous energy gap formation in the electronic energy levels. In this work we studied the electronic and thermal transport in two novel members of the 1:2:10 series, YFe2Al10 and YbFe2Al10. These two compounds were synthesized with the purpose of studying the effects of a unit cell volume that is compressed well beyond that of CeRu2Al10 in order to expose the role of Fe-based magnetism in the observed electronic correlations. Here we discuss our results of thermal and electronic transport in the pair of compounds YFe2Al10 and YbFe2Al10. Both exhibit anomalous thermopower behavior and a peak that develops at low temperatures. This is attributed to an enhanced density of states that is achieved through hybridization between conduction electrons and magnetic moments with a localized nature. The thermal conductivity of both compounds shows a weak temperature dependence and especially in the case of YFe2Al10 is the observed behavior amenable to a description in terms of glass-like behavior that most likely results from optical phonon modes which effectively scatter heat-carrying quasiparticles. The electrical resistivity suggests low-lying magnetic cooperative behavior.
It is evident that the Eu2+ ion acts as a luminescent centre emitting in the blue (λmax ~ 440 nm) spectral region for CaAl2O4:Eu2+. The R3+ ion is believed to act as a trap or somehow modify the trap properties in these phosphors. Despite a large number of research on the phenomenon the mechanism of the persistent luminescence of the CaAl2O4:Eu2+,R3+ materials has not been well presented. The theories that have so far been put forward are generally contradictory therefore much less agreement exists on the role of the R3+ co-dopant. New emerging applications for the long phosphorescent materials such as radiation detection and sensors for structural damage, fracture of materials and temperature, require the exact luminescence mechanisms and the identification of the trap levels/locations. Analysis of the thermoluminescence (TL) glow curves is one of the most significant ways to measure the number and also the activation energy of the trapping levels in these materials. In the present study the TL properties of the Eu2+, R3+ doped CaAl2O4:Eu2+, Nd3+/Dy3+ were investigated above room temperature. The trap depths were estimated with the aid of the peak shape method. The glow curve of CaAl2O4:Eu2+ with a first peak at 50 °C was found to correspond to several traps. The Nd3+ and Dy3+ ions were observed to greatly enhance the intensity of the high-temperature TL peaks and also form most of the traps suitable for intense and long-lasting persistent luminescence. The trap- depths and the R3+ or R2+ level positions did not exhibit any well defined relationship. The traps may thus involve more complex mechanisms than the simple charge transfer to (or from) the R3+ ions.
Polycrystalline diamond (PCD) cylindrical cutters used in oil well drilling are susceptible to fracture due to the hostile environment of randomly occurring high impact loads. These cutters commonly consist of a PCD layer sintered onto a cobalt-tungsten carbide (Co-WC) substrate. The fact that the cutters fail after repeated use gives rise to the possibility of fatigue type processes in diamond. The study of stress fields and their relative geometries thus become crucial in the quest to have extended lives for these cutters. Since the diamond Raman line reveals both the nature and magnitude of the stress present in the material this technique was employed in this work. Room temperature measurements show a general compressive stress field on the PCD before being fatigued using a ball on three balls test rig and it then gradually deteriorates to a residual tensile stress field. Whereas a general compressive stress is desirable in the PCD layer as it inhibits the propagation of cracks, on the contrary tensile stresses facilitate formation of cracks leading to catastrophic failure of the cutters.
Vanadium dioxide falls into the category of smart thermochromic materials, one where the property of the material changes due to the effect of temperature. High quality pure thermochromic thin films of vanadium dioxide were prepared by using two physical methods, Rf-reactive sputtering and pulsed laser deposition methods. These deposition methods were used to study the effect of deposition temperature on vanadium dioxide thin films microstructure, electrical and optical properties. The deposition conditions (oxygen pressure, substrate-target distance, substrate temperature and etc) on both methods were carefully optimized for the quality of VO2 thin films on a glass substrate, with a substrate temperature ranging from 350-600 degrees celcius. All the vo2 thin films prepared by Pulse laser deposition technique and Rf- reactive sputtering were characterized by X-ray diffraction technique, observed to be crystallized showing the first peak at 27.87 degrees celcius which corresponds to the (011) plane and the monoclinic structure of VO2. This peak indicates the existence of Vanadium dioxide. Thermochromic properties of VO2 thin films were investigated by measuring the optical and electrical properties below and above the critical temperature using the UV/VIS technique. T he atomic force microscopy was also used to investigate the surface roughness of all the VO2 thin films. RBS was used for quantitative analysis of composition, thickness and depth profiles of VO2 thin films and it was conducted using a 2 MeV beam of alpha particles, other techniques as well as TEM, SEM, FTIR were also employed to investigate the morphology, composition, and crystallographic information on VO2 thin films.
Different compositions of Ti-Pt were varied within the B2 region i.e. 45-55 at% Pt using solid solution method employed in CASTEP code. The Ti50Pt50 structure was found to be more stable as compared with other different compositions with the heats of formation of -0.8584 eV. We also observed that the more we increase the Ti and decrease the Pt composition, the change of the heats of formation becomes minimal and this was observed on the plotted curves of the heats of formation against the composition range. The elastic properties of the compositions were also calculated.
We report on the characterization of commercially terbium doped gadolinium oxysulfide (Gd2O2S:Tb) phosphor and the degradation of the Gd2O2S:Tb green phosphor for its application in CRT screens. As a result, degradation of the cathodoluminescence (CL) intensity during irradiation of the powder with 2 keV electrons in an oxygen pressure of 1 x 10-6 Torr was studied. The ESSCR mechanisms was used to explain the effects of sulphur desorption and the formation of a non-luminescent oxide layer. A dead layer of Gd2O3 and Gd2S3 are responsible for the degradation of the CL intensity with an increase in electron dose. The XPS results have proved the presence of Gd2O3 and Gd2S3 on the degraded powder spots.
Recent studies have employed manganese dioxide (MnO2) catalysts to improve the efficiency of Li-air batteries, while other studies have revealed that the Li/O2 cell can be recharged with high efficiency without a catalyst using an appropriate porous carbon electrode. Interestingly, charge voltages of these cells are similar to those of the MnO2, while both of these exhibits higher charge voltages than the cobalt-catalyzed cells. However, it has been observed that in the absence of a catalyst the recharge occurs at about high voltage, a large hysteresis between charge and discharge voltages. Ab initio planewave pseudopotential method will be used to investigate the interaction of the oxygen molecule with the (110) surfaces of MnO2, in particular tendencies of oxygen reduction. Their surface and adsorption energies were determined together with the distances between O-O of the oxygen molecule, in the vertical and horizontal orientation, the metal and oxygen molecule before and after relaxation. We found that manganese dioxide is more effective and gave lowest adsorption energies, and reduces oxygen molecule better than titanium dioxide. However the horizontal orientation of the oxygen molecule is more favourable in terms of oxygen reduction than the vertical orientation.
Astrophysics and Space Science
The cosmic microwave background (CMB) carries information from the last scattering surface that puts constraints on the multitude of proposed cosmological models and the gravity theories they are based on. Amongst such theories are the f(R) theories of gravity which have become an interesting endeavour to correct for the degeneracies of the concordance model. We present a description of CMB anisotropies generated by tensor perturbations in f(R) theories of gravity. The power spectra of the observables TT and EE in the special case of f(R)=Rn are computed using a modified version of CAMB.
The direct evidence for the acceleration of hadronic cosmic rays at supernova remnants underlined the need for a 3D time dependent treatment of the propagation of Galactic Cosmic Rays (CRs). Full 3D time dependent calculations of the propagation of CRs have shown that if CRs indeed originate from supernova remnants, transient point-like sources, the flux of the CR primary component measured at Earth depends strongly on the local source history, whereas the secondary component shows only little or no variations due to nearby sources. The most widely used steady state, rotational symmetric models (2D) of CR propagation cannot take into account the local source history, but rather mimic source histories that result in the same local CR flux as the smeared-out sources assumed in 2D models and do not necessarily coincide with the real local source history. Using a steady state, rotational symmetric model for a parameter study, one may expect different best fit values looking at the primary and secondary CR components separately, as it is unlikely that the source history mimicked by the 2D models coincide with the real local source history. We adapted the 2D version of the GALPROP code to a cluster environment and perform parameter studies comparing CR spectra with mainly primary and secondary CR data separately. First results of these studies will be presented and recommendations for further such studies will be given.
A two-dimensional hydrodynamic numerical model is extended and applied to simulate the interaction between stellar winds and the interstellar medium (ISM). In particular, the stellar wind evolution of O-and B-type stars is calculated. First, the evolution of a stellar wind into the ISM and also a more dense molecular cloud are considered for the case of no relative motion between the star and the ISM. This interaction results in a cavity being blown into the ISM. Of importance is the boundary radius (astropause) and the location where the outflow speed decreases from supersonic to subsonic speeds, the termination shock. Parameters like ISM density, outflow speed and mass-loss rate were varied to study the difference in the computed astropause (AP) and termination shock (TS) radii. The evolution of these structures is presented to a simulation time of 1 My. The evolution of stellar winds into the ISM including relative motion is also considered. It is shown that the positions of the TS and AP are dependent on the mass-loss rate and stellar wind outflow speed of the star and the interstellar medium density and relative speed. When these massive stars reach the end of their life, they end their life in a supernova explosion. The explosion results in a blast wave moving outward, called the forward shock (FS) and a reverse shock (RS) also forms which moves inward. Ferreira and de Jager 2008 simulated supernova remnant (SNR) evolution for the case of evolution into the undisturbed ISM (no cavity). The evolution of SNR is simulated taking also into account the pre-existing cavity blown out by the stellar winds of these massive stars. The results of this study showed that the evolution of the SNR is definitely influenced by the presence of a stellar wind cavity even if the cavity is only a few pc in extent.
It is well known that neutron stars have very strong magnetic fields and that these fields impact the behaviour of the star. The focus of our research is on the interaction between the magnetic field and the constituent matter in the neutron star’s interior. In particular we investigate the possibility of a ferromagnetic phase in the neutron star interior being the source of the star’s ultra-strong magnetic field. Considering such a phase provides us with a feedback mechanism between the constituent matter and the magnetic field and vice versa, which we believe is a necessary component in understanding the evolution of the neutron star’s magnetic field. In this talk we will shortly introduce a description of ferromagnetism in the neutron star interior. We will also discuss the impact of such a phase as well as allude to possible observational consequences.
In his work on superfluid helium, Feynman gave both qualitative and quantitative descriptions of the excitation spectrum of superfluid helium. I briefly review Feynman's approach to superfluid helium and assess the applicability of this theory to neutron star interiors.
The advent of the MeerKAT and SKA radio telescope projects will allow for vastly improved precision in the measurement of pulsar radio frequency emissions. Thus, the propagation of a pulsar’s conical radio beam through the strong gravitational field region in the vicinity of a compact object, i.e. black-hole or another neutron star, will probe the metric of spacetime in the high-field regime. This talk outlines the theoretical and associated numerical predictions of General Relativistic effects, such as the Lense-Thirring effect, on such a pulsar signal.
Applied and Industrial Physics
In the past 50 years lasers has found numerous applications in medicine. One of their advantages is their use for minimalistic or non-invasive diagnosis and treatment. Often that means light penetration through skin and the correct dose required relies on accurate information regarding the skin’s optical properties. Light absorption in the epidermal layer is a major factor in determining the laser light fluence that reaches the deeper skin levels. Darker skin has an epidermal melanin volume fraction about twice that of lighter skin. Due to melanin absorption, less laser light reaches the deeper skin layers in dark skin tones. Laser-tissue interaction modelling software can correct for this by adapting the dose applied to the skin. To correctly apply such software it is important to characterise the skin in terms of skin tone with an easy and reliable method. Measuring the melanin content of the skin is the best method, but it needs to be done non-invasively. However, access to samples of all skin types is often limited and skin-like phantoms are used instead. The objective of this study is to compare experimentally measured absorption features of liquid skin-like phantoms representing Skin Types I to VI with computational simulated skin from the Realistic Skin Model (RSM) part of the ASAP® software from Breault Research. Skin-like phantoms were prepared by adding Intralipid (20% fat emulsion) to samples of increasing melanin concentration at pH ~ 7. UV-VIS transmittance spectra of the samples were measured over the wavelength range 370 to 900 nm and compared to simulated results from ASAP® using the same optical parameters. Experimental and computational results indicated that at shorter wavelengths melanin absorption displayed non-monotonic features that may allow for more accurate ways of determining melanin concentration and therefore the expected absorption through the epidermal layer. This suggests that the phantoms may be able to represent optical characteristics of real skin.
Optical Coherence tomography (OCT) is a non-invasive imaging technique that is known to be used to monitor changes and differences in different types of materials. In the field of Biophysics/Biophotonics OCT can be used to non-invasively detect changes occurring in tissue. This study will look at using OCT to detect skin cancer. Similar work has been done by other groups, however the influence of skin tone for such detection has not been evaluated to a great extent. The effect of skin tone on detection of skin cancer will form a core part of the investigations as well as determining the efficacy of the system for early cancer detection. Preliminary work will be done using phantoms that simulate the different skin tones. Melanin is the component found in the skin, largely responsible for the different skin tones and will incorporated into our phantoms as carbon black, black ink or synthetic melanin. The optical properties of such phantoms can be determined on the Integrating Sphere (IS) and OCT systems. A correlation will then be drawn between the optical properties and the OCT signals obtained. Once the preliminary tests are done to optimise the method for image analysis, further studies will be done on patients and compared against conventional histology results. This talk will thus look at some OCT applications, some of the preliminary work done using our OCT system and the steps to determining the effect of skin tone on cancer detection.
In order to apply light treatment to skin, the absorption through the outer layers of the skin needs to be considered. Human skin is a highly scattering medium and the melanin in the epidermal layer of the skin is a major absorber of light in the visible and near infrared wavelengths. Darker skin has a higher concentration of melanin in the epidermis and absorbs more light than fair skin. Ideally the effect of the skin treatment on the outer layers of the skin should be tested on in vitro multi layer skin models. This is not always feasible. For this work phantoms were used together with skin cancer cells to test the effect of outer layer absorption on the efficiency of Photo Dynamic Therapy treatment. Three solid phantoms were prepared from clear resin, TiO particles (scattering particles) and carbon black (absorption particles). Different carbon black concentrations were used to simulate different skin types. Cells were prepared and treated with the photosensitiser. The phantoms were placed inside the wells containing the photosensitised cells, just touching the media in the well. The attenuation of the different phantoms was calculated and the laser treatment times were adjusted to keep the light dose delivered to the cells at 4.5 J/cm2. After laser treatment cell viability was measured, using the Cell Titer Blue Viability Assay, for each of the wells. The phantoms attenuated the laser light by between 10 and 30%. Cell viability for the wells with the phantoms was less than without the phantoms and the differences are attributed to the reduced oxygen in the presence of the phantom. The initial results of the experiment indicate that the phantoms can be used to optically mimic the effect of the outer skin layers. For future work the experimental set-up will be refined.
Semiconductors are important because of their numerous technological applications. As such, there have been a number of investigations on the recrystallization of amorphous GaAs. We present results of the isochronal annealing of an ̴ 140nm amorphous layer of GaAs on a crystalline substrate produced by bombarding with 100 KeV argon ions with a fluence of 5×1015 ions/cm2. Raman measurements were taken at room temperature which was the base temperature. Sharp peaks characteristic of polycrystalline GaAs were observed at 600o C. Similar experiments have also been carried out with the technique of Brillouin scattering (SBS). SBS is a laser based technique used to study the acoustic phonons and elastic properties of materials. SBS spectra were collected using a 514.5 nm laser in a backscattering arrangement and analysed using a Fabry-P`erot interferometer supplied by Sandercock. Changes in the elastic properties during the isochronal anneal were observed at temperatures below 400o C, in agreement with results previously obtained. The differences in the Raman spectroscopy and SBS results may be explained by the differences in the two techniques.
Ever since the inception of the quantum computing idea, many technologies for its realization have been tried. The fundamental intention is to implement and manipulate a two-dimensional quantum mechanical unit of information that is termed a quantum bit (qubit). In this context during the past decade, solid-state fluorescing defects have drawn huge interest. The promising candidates for solid-state qubits are nitrogen-vacancy (NV) centers in diamonds due to their individual addressability, optical spin polarization and spin coherence time of milliseconds at room-temperature. Here, an on-going study on enhancing NV centres creation and NV electronic spin coherence times is reported. The vacancies in diamond crystal samples are created using Van der Graaf accelerator and then the samples are annealed at elevated temperatures to form NV centres. Sample measurements in terms of detecting the defects and their concentrations and electron spin coherence times are performed using techniques such as Raman spectroscopy, absorption spectrum, photoluminescence and confocal microscopy equipped with Electronic spin resonance (ESR).
The dispersion relation of surface acoustic waves in TiC thin hard films of various thicknesses on silicon and silicon carbide substrate is studied using Surface Brillouin scattering. Simulations of surface Brillouin scattering (SBS) spectra of TiC thin hard films on silicon and SiC substrates have been carried out over a range of film thickness from 5nm to 1000nm. The simulations are based on the elastodynamic Green's function method that predicts the surface displacement amplitudes of acoustic phonons. These simulations provide information essential to understand and analyze experimental data emerging from SBS experiments. There are striking differences in both the simulated and experimental SBS spectra depending on the respective elastic properties of the film and the substrate. In fast on slow situations (TiC on Silicon), the Rayleigh mode is accompanied by broadened resonances; in slow on fast situations (TiC on SiC), several orders of Sezawa modes are observed together with the Rayleigh mode and sharper resonances. The velocity dispersion of the modes as a function of the product of surface wavevector and thickness (k//d) has also been simulated and obtained experimentally, allowing the elastic constants of the films to be determined.
In this work, we report on the fabrication of multiwall carbon nanotubes (MWCNT)/NiO nanocomposite thin films by the pulsed laser deposition technique on to aluminium substrates. Samples were prepared with different MWCNT content, and varying substrate temperatures. Structural and optical properties were investigated by scanning electron microscopy, x-ray diffraction, Raman spectroscopy, UV-Vis spectroscopy and the thermal emissometer. The morphological and structural changes induced by laser beam will also be presented.
Rare–earths (Tb3+ or Ce3+) doped zinc aluminate (ZnAl2O4) nanocrystals were successfully prepared by a modified combustion method using urea (CH4N2O, 99–99.5%) as fuel and characterized to investigate down-converted emission by energy transfer from Ce3+ to Tb3+ . The post-preparation annealing of the samples was carried out in a reducing atmosphere of 4%H2/96%N2 mixture at an elevated temperature of 7000C for 4 hours. This was meant to reduce non-emitting Ce4+ to Ce3+ and also to improve crystallinity. As confirmed by X-ray diffraction (XRD) data, the materials were highly crystalline and the XRD patterns were consistent with the closed-packed face centered cubic spinel structure of ZnAl2O4. The photoluminescence excitation and photoluminescence data collected when ZnAl2O4:Ce3+,Tb3+ samples, with different concentrations of Ce3+ and Tb3+ ions, were excited at different wavelengths (230―325nm) showed that green emission of Tb3+ was sensitized by Ce3+, i.e. there was energy transfer from Ce3+ to Tb3+ resulting in improvement of green emission due to the 5D4 → 7F5 transitions of Tb3+ ions. This study therefore, sets out to discuss the sensitizing effect of Ce3+ and the effect of post annealing on the structure of ZnAl2O4:Ce3+,Tb3+. The results indicate the potential of the phosphor powders for applications in luminescent display panels and further development as UV down-converters for improved performance of conventional photovoltaic cells.
Lightning has in recent times wreaked havoc during the rainy season in several parts of South Africa and this prompted the physics community under the auspices of the South African Institute of Physics (SAIP) to devise meaningful strategies for promoting public awareness. In response to this call, first year physics students at the University of Johannesburg responded to a carefully designed questionnaire that seeks to probe students’ level of understanding of lightning. Analysis of responses reveals lack of scientific understanding of lightning as a natural phenomenon. Amongst others, this can to some degree be attributed to superstitious or cultural beliefs.
In recent years student satisfaction surveys have become increasingly popular in universities. Whether this gain in popularity is because students themselves know best when they are learning, or whether it is because universities merely wish to ensure that they have satisfied customers, is still a matter of debate. Nevertheless, the importance of student satisfaction surveys in universities is undeniable, and consequently it is of interest to understand the most important contributing factors to student satisfaction, or equivalently to student disappointment. By induction, based on our own teaching experience, we have made the following bold postulate: Students will be least disappointed in courses where they perform equally well in all aspects of the course that are assessed. Conversely, students will be most disappointed in courses where they perform very differently in two or more assessments of the course. In science courses, for example, there are typically four different assessments: a practical, three or four assignments, a midterm exam, and a final exam. Although the above hypothesis remains untested in this preliminary work, some of its consequences will be explored through the visualisation of assessment data gathered at the University of Victoria, Canada. In this talk a new way of visualizing and understanding the various components of the assessment data will be presented. The visualisation is constructed to be consistent with the above hypothesis. A concrete example of how the visualization may be used in a modern university setting, the problem of adjusting marks is considered. Such mark adjustments are apparently necessary to increase throughput rates. In this case it is shown that the visualization may be used to achieve an optimized adjustment which both increases the throughput rate and at the same time minimizes student disappointment, assuming the hypothesis is correct.
This is the second part of our study, where we are analysing the reasons and explanations behind each answer of each student. In the first part of our study (2010) using an open circuit (battery, single wire, resistive element) in which the resistive element (resistor, heating element, light bulb) and the words (“current’, “charge flow”) are inter-changed in an electrically equivalent circuit and the students answers were found to be influenced by the different elements and words.
The Faculty of Military Science of Stellenbosch University presents an Algebra-based Physics course to first year students. The practical component comprises of a range of experiments that differ significantly in characteristics. Questionnaires are yearly completed by the students to evaluate the theoretical and practical components of this course. This paper will explore some of the results of these surveys, focusing especially on the experience of the student with regards to the features of experiments that are presented.
Traditional lecturing of Physics in a preaching pattern to a multitude stduent masses in a large lecture theatre, has not succeeded in attracting more number into Physics further than second year. University of Pretoria Physics Education Research Group (UPPERG) has introduced "Interactive-Inquiry Learning" Teaching where Masters and Doctoral students are guided to lecture groups of 40 BSc4Y program students, in smaller laboratories: discussing Physics theories and concepts while supervising practiacls and encouraging these youger students into excercises and tutorials of Physics. Regular observation is done where eight senior students of Physics Department of the University of Pretoria lecture the twelve groups of students in the BSc4Y program. "Lecturers" use the IIL Teaching method. Some results of this teaching are shown. Further observation show that contact and consultation are more spontaneous to younger senior students lecturers than to older folk lecturers. Bitter interacions in cases of "accommodation in conceptual change" ensue often leading to successful learning, which otherwise would seldonm occur when a "matured proffessor" lectures the youngsters.
Lasers, Optics and Spectroscopy
Laguerre-Gaussian (LG) beams possess many interesting properties, one of which is that they carry orbital angular momentum, an extrinsic component of angular momentum, when the electric field or mode has an azimuthal angular dependence of exp(il), where l is the azimuthal mode index. These beams are easily generated using spatial light modulators (SLMs), where a Gaussian beam incident on an SLM is shaped according to a predetermined phase pattern. For LG beams, this phase pattern has typically only included the azimuthally dependent phase component of the field, ignoring the amplitude and radial components present in the full LG expression. Although this approximation has yielded azimuthal modes which compare well with theory, it excites higher order transverse modes. We show both theoretically and experimentally, using a method of modal decomposition, that higher order transverse modes are excited. The results were compared to the full LG expression, where no higher order modes were seen. This is of particular interest regarding the orbital angular momentum carried by these beams.
In this work we propose an implementation of a new intra–cavity beam shaping technique to vary the intensity distribution of the fundamental mode in a resonator cavity while maintaining a constant intensity distribution at the output. This method can be useful for fitting a transversal intensity profile of the required mode with a pump beam profile in the region of the active medium to increase mode discrimination.
We outline an approach for the calculation of the mean focal length of an aberrated lens, and provide closed form solutions that show that the focal length of the lens is dependent on the presence of defocus, x-astigmatism and spherical aberration. The results are applicable to Gaussian beams in the presence of arbitrary sized apertures. The theoretical results are confirmed experimentally, showing excellent agreement. As the final results are in algebraic form, the theory may readily be applied in the laboratory if the aberration coefficients of the lens are known.
Due to their self-renewal and pluripotency characteristics, stem cells possess the potential to dramatically advance current therapies in tissue regeneration and engineering. Nonetheless, there still remains a pressing necessity to answer the biological questions concerning how stem cell renewal and differentiation programs are operated and regulated at the genetic level. Genetic manipulation such as delivery of exogenous gene expression or knockout with small interfering RNA (siRNA) is relatively rare in mouse embryonic stem (ES) cells. However, genetic species can be selectively introduced and subsequently expressed in live mammalian cells via optical systems, a technique normally referred to as photo-transfection. During photo-transfection, localized application of femtosecond (fs) laser pulses onto the cell plasma membrane induces transient submicrometer holes, thereby facilitating cytosolic uptake of extracellular exogenous materials. This novel optical cell transfection technique allows targeted treatment of cells promoting limited use of reagents or chemicals that can cause spontaneous differentiation and also interfere with the physiological properties of ES cells. In this work, we report for the first time that fs laser pulses can be utilized as an optical syringe for successful transient photo-transfection and induced differentiation of mouse ES cell colonies. This was achieved by using a tightly focused titanium sapphire laser beam spot (~1.1 µm diameter spot size, 790nm, 80MHz, 200fs and 50 mW average power output), where E14g2a cells were differentiated into the extraembryonic endoderm via photo-transfection with the Gata-6 transcription factor.
Since its inception, optical trapping has found its place in the field of physics, biology and recently in micro-fluidics. To a physicist, this technique opens up avenues to carry out force measurement with utmost accuracy and precision on the micro-scale level. This is attained by measuring the magnitude of the trapping force required to restrain a trapped particle to its equilibrium position. Here three such methods will be discussed in light of their relevance to the current optical system in the laboratory. The most direct method carried out was the drag force method, by keeping the particle fixed while accurately controlling the movement of the automated stage we were able to measure the magnitude of the force applied to a particle in order to release it from the trap. Alternatively, since trapped particles are submerged in liquid medium, any thermal fluctuation within the medium would lead to the trapped particle displacing from its equilibrium position. Under this condition, the particle is said to behave as a simple harmonic oscillator. By measuring the variation of the trapped particle with respect to its equilibrium position we were able determine the trap stiffness using the Equipartition theorem. Lastly, we discuss the Allan variance method. Since most systems contain some form of noise, the Allan variance method was used to eliminate any form of white noise that could exist to accurately and precisely determine the trapping force using CMOS technology. Most scientific programs can be used to determine the Allan variance; we used Labview as a platform for the analysis. An overview of all the methods mentioned will be discussed in detail to give an understanding of the most accurate and precise method of calibrating an optical trap.
Nuclear, Particle and Radiation Physics
This work is part of a program based on the electro-production of baryonic resonances. In part, the physics goals are to assess the relevant degrees of freedom appropriate to describe high momentum transfer exclusive reactions. In particular, the interest is in the region where the four – momentum transfer squared, t ~ Q2 varies beyond the validity of the constituent quark model (CQM), towards the region where one may search for evidence of pQCD. In this work, the differential cross section for p(e,e’ω)p has been studied at a Q2 ≈ 5.5 GeV. We present further analysis including comparison with other data. A significant divergence from an extrapolation of previous data is obvious.
The measurement of the isolated di-photon cross-section at the LHC is crucial as these events constitute an irreducible background to new physics processes, such as a Higgs boson or graviton decaying to two photons. These events also provide important information for the understanding of QCD processes. The di-photon cross-section has been measured in ATLAS using the full 2010 data sample, corresponding to an integrated luminosity of 37 pb-1. Results as a function of the di-photon invariant mass, transverse momentum and azimuthal separation are presented and compared with NLO theoretical predictions. Focusing on the invariant mass region between 100 and 150 GeV where a light Higgs boson is searched for in the di-photon channel, exclusion limits are set on the Standard Model prediction. The results obtained are already at the level of the results from TeVatron experiments in this channel.
The H -> ZZ(*) -> 4leptons channel is one of the most promising channels for a low-mass Higgs boson, as it has both a clean signature and a large branching fraction for Higgs mass below a few hundred GeV. The ATLAS experiment has explored this channel using the LHC collisions data at a center-of-mass energy of 7 TeV, using data-driven background estimation techniques. Some Abelian Hidden Sector models predict the existence of new Higgs and gauge (Z') bosons, weakly coupled to the Standard Model (SM) sector. The decay of the SM Higgs to a pair of Z' bosons is allowed, possibly with a large branching fraction, by the mixing of the Higgs sectors. The decay of the Z' bosons to lepton pairs with a large branching fraction can then be explored in the same way as the standard H -> ZZ(*) -> 4l channel, by relaxing the constraints on the di-lepton invariant mass. Perspectives on this new approach with the ATLAS experiment are presented.
Experiments at the Large Hadron Collider (LHC) aim to address fundamental questions in particle physics such as the origin of mass for elementary particles, the matter/anti-matter asymmetry in the universe, the nature of Dark Matter, the state of matter in the early universe, etc. The ATLAS detector is one of the four major detectors being exploited at the LHC to shed some light on the questions. The Higgs boson is a hypothesized particle responsible for the elementary particle masses. In this talk, we will discuss the search strategies of the Higgs boson with the ATLAS detector at the LHC. In particular, we will present results based on the ATLAS data collected in 2010.
Theoretical and Computational Physics
Models with extra spacial dimensions offer a new way to address outstanding problems in and beyond the standard model. In such models the Planck scale in the bulk can be of the order of the electro-weak symmetry breaking scale. This allows the coupling strength of gravity to increase to a size similar to the other interactions, opening the way to the unification of gravity and the gauge interactions. The increased strength of gravity in the bulk space-time means quantum gravity effects would be observable in the TeV energy range - an energy range now attainable in modern particle colliders. If the Planck scale is low enough, black-holes could form during collisions at particle colliders. These black-holes will emit radiation, losing mass, energy, momentum, etc. This radiation should be detectable making it, possible to "see" black-holes in particle colliders.
N=4 SYM theory has been extensively studied in the planar limit. A very significant result of this study is a map from the planar dilatation operator to the Hamiltonian of an integrable spin chain. In this talk we consider a large N (but not planar) limit of the theory. This is a considerably more complicated problem since non-planar corrections need to be summed. This summation is accomplished by employing the restricted Schur polynomials. We give an analytic formula for the action of the dilatation operator on the resticted Schur polynomials and then proceed to calculate the one loop anomalous dimensions. Our result shows that the dilatation operator reduces to a set of decoupled harmonic oscillators, generalizing results known from the sector of theory constructed using two complex Higgs fields (two flavors) to the sector with three complex Higgs fields (three flavors).
ABJM theory is an N=6 Superconformal Chern-Simons theory with gauge group U(N)XU(N). This gauge theory has been extensively studied in the planar limit. Motivated by similar results for N=4 SYM, the planar dilatation operator of ABJM theory has been mapped to the Hamiltonian of an integrable system. Recently, it has been argued that there are large N (but not planar) limits of N=4 SYM theory for which the dilatation operator remains integrable. Indeed, it reduces to a set of decoupled harmonic oscillators. Motivated by this result, in this talk we study large N but non-planar limits of the ABJM theory. This is accomplished by constructing a complete set of gauge invariant operators for the theory. The free two point functions of these operators is computed and the action of the dilatation operator on these gauge invariant operators is constructed.
A new correspondence between type IIA string theory on AdS4×CP3 and an N=6 Super Chern-Simons-matter theory was proposed by Aharony, Bergman, Jafferis and Maldacena (ABJM) in 2008. We construct the D4-brane giant graviton, extended and moving in the complex projective space, which is dual to a subdeterminant operator in ABJM theory. This dynamically stable configuration factorizes at maximum size into two topologically stable D4-branes (each wrapped on a different CP2 cycle) dual to ABJM dibaryons. We also mention our more recent results obtained from an analysis of small fluctuations around this CP3 giant graviton.
N=4 SYM theory has been extensively studied in the planar limit. An important result is that the planar dilatation operator can be mapped to the Hamiltonian of an integrable system. In this talk we study certain large N (but not planar) limits of the theory. We argue that the dilatation operator remains integrable: it reduces to a set of decoupled harmonic oscillators. This challenges claims that integrability is spoiled by non-planar corrections. We argue that the decoupled oscillators that arise describe the vibration modes of a quantum membrane.
Astrophysics and Space Science
Applying the quantisation of action variables to a theory of gravitation one derives discrete values for planetary distances from the sun. The calculated values agree with the observed values of all the planets. The derived equation also applies to the exoplanets. The application is similar to that of the Wilson-Sommerfeld rule for atomic orbits.
Perturbation theory uses Lagrangian techniques that require vector fields to be compared at finitely separated points. This method can be generalised to the strong gravitational field regime in one of two ways, using either covariant or Lie derivatives. In this paper, I argue that those methods based on the Lie derivative are more useful. The Lie derivative provides a clear picture of how the deformation of the fluid flow takes place. It also provides a natural way to discuss large perturbations. I apply this method to some elementary stability problems in the study of stellar structure.
Genetic algorithms form a class of search heuristics that incorporate, in a computational setting, the biological notion of evolution by means of mutation and natural selection. Compared to more conventional search and optimisation techniques, genetic algorithms are very easy to implement and they tend to be extremely robust and versatile. Although already ubiquitous in fields such as computer science, engineering and artificial intelligence, genetic algorithms have not yet been widely adopted in the physical sciences. I provide a very brief introduction to genetic algorithms and outline their relevance to a number of diverse problems in astronomy and astrophysics, from stellar structure modelling and astroseismological analyses to robotic telescope scheduling. In particular I discuss a difficult optimisation problem in gravitational microlensing analysis for which it is hoped that genetic algorithms might facilitate an efficient exploration of an enormous parameter space.
To understand the formation of stars one must be able to identify young objects. RCW 34 is a star forming region in the constellation of Vela. A study was conducted in the near-infrared where it was found that a large number of stars surrounding the nebula shows NIR characteristics of lower mass pre-main sequence stars. To prove the suspicions a follow-up study was conducted in the optical. The results of a photometric and spectroscopic study on the the nebula and stars within a 7 arcmin x 7 arcmin region around the nebula will be presented.
We use the HI velocity profiles of The HI Nearby Galaxy Survey (THINGS) to study the phase structure of the ISM and its relation to galaxy properties and morphology. To construct high S/N profiles, we use a method analogous to the stacking method sometimes used in high redshift HI observations. We call these high S/N profiles super profiles. By decomposing the super profiles into Gaussian components, we found broad and narrow components which are evidence of the presence of Cold Neutral Medium (CNM) and Warm Neutral Medium (WNM). We also derive radial super profiles of the THINGS galaxies and we found some correspondance between the shapes of the super profiles and their location within the galaxies. We found that the narrrow components dominate inside the optical radius r25. We also analyze the shapes of the super profiles in low, moderate and high star formation rate (SFR) regions and we found that the narrow component dominates in high SFR regions. The profiles also tend to be more asymmetric and broader in high SFR regions.
In this project data from The HI Nearby Galaxy Survey (THINGS) and from the HERA CO Extragalactic Line Survey (HERACLES) is being used to compare the HI (neutral hydrogen) and CO dispersions in nearby galaxies in order to see if they are related. This is important in improving star formation laws (as they currently only use HI dispersions, but stars form from molecular gas) and to better understand turbulence in galaxies. Preliminary analysis indicates that the HI has higher velocity dispersions than the CO.
The aim of this project is to study the stellar populations, and thereby evolution and star formation histories of brightest cluster galaxies (BCGs). In particular, I will determine if a Single Stellar Populations (SSP) or Composite Stellar Populations (CSP) provides the most significant fit for the BCGs using high signal-to-noise ratio (S/N), long-slit spectra, obtained on the Gemini and WHT telescopes. By using the ULySS software package, the data will be fitted against the Pegase. HR and Vazdekis/Miles stellar population models to simultaneously derive the SSP equivalent ages and metallicities of the BCGs. Furthermore the stellar populations will be decomposed into two or more components, and the chi square (χ2) value for each component is used to determine whether a SSP or CSP represents the BCGs most accurately. We find that both young and old stars are present in the stellar populations of these BCGs, and those BCGs, therefore show surprisingly diverse star formation histories.
Applied and Industrial Physics
Diabetes mellitus (DM) is a chronic disease characterized by impeded glucose metabolism and preceded by diabetic ulcers which are chronic due to deteriorated healing process. Hypoxia, decreased fibroblast proliferation and impaired growth factors are amongst root factors that contribute to impaired healing. Photostimulation is a non-invasive treatment that utilizes low intensity laser irradiation (LILI) to provide healing or stimulate appropriate cellular functions. Human skin fibroblast cells (WS1) were used in this study that consisted of four groups viz. normal, normal wounded, diabetic wounded and hypoxic, each with a non-irradiated control. Wounding was simulated by creating a central scratch using a pipette. A diabetic state was induced by growing cells in media that contained excess glucose to a final concentration of 22.56 mM, and for hypoxic insult cells were incubated under anaerobic conditions (0% O2 and 20% CO2) for 4 h. Cells were then irradiated at a wavelength of 830 nm with 5 J/cm2 and incubated for 1 or 24 h. Morphological changes were observed by light microscopy; ELISA and flow cytometry were used to determine interleukin (IL)-1β, IL-6 and tumour necrosis factor (TNF)-α as inflammatory markers; and caspase 3/7 for apoptosis was determined by luminescence. After a 24 h incubation period the wounded area appeared decreased and hypoxic cells had regained normal morphologic features when irradiated, TNF-α and IL-1β had decreased in irradiated samples, whereas IL-6 was increased. Caspase 3/7 had decreased in irradiated samples at both 1 and 24 h. This study demonstrated the beneficial effects of LILI since the results showed significantly reduced inflammatory response in vitro and hastened wound healing particularly under diabetic and hypoxic conditions.
The study investigated the effects of low intensity laser radiation (LILI) and epidermal growth factor (EGF) on adult adipose derived stem cells (hADSCs) isolated from human adipose tissue. Isolated cells were cultured in complete Dulbeco’s Modified Eagle Medium (DMEM) containing 10% featal bovine serum (FBS), EGF and antibiotics and incubated at 37°C in 85% humidified atmosphere of 5% CO2. Semi-confluent monolayers of ADSCs were exposed to low intensity laser at 5 J/cm2 using 636 nm diode laser. Cell viability and proliferation were monitored using adenosine triphosphate (ATP) luminescence and optical density at 0, 24 and 48 h post irradiation. Application of low intensity laser irradiation and EGF on human ADSCs at 5 J/cm2 increased the viability and proliferation of these cells. Therefore low intensity laser therapy would probably enhance differentiation and proliferation of stem cells in vitro and these cells could be used in reconstructive surgery and tissue engineering.
Since the invention of the laser, its application in the health sector has been studied and in an attempt to discover effective alternative treatments, Low Level Laser Therapy (LLLT), commonly known as biostimulation or photo-biostimulation, has emerged. This therapy has been successfully used both in in vitro and in vivo studies in wound healing. Although this therapy is in use worldwide, the full mechanisms of action are not fully understood. Various cell culture models, such as wounded, diabetic wounded and hypoxic, have been exposed to visible and infra-red laser light and the effect on cell migration, cell survival, proliferation, cytotoxicity, mitochondrial responses, nitric oxide, secondary messengers, DNA damage and pro-inflammatory cytokines have been studied. Laser irradiation at the correct wavelength and fluence has shown to have a positive effect on stressed cells in vitro. There is an increase in migration, survival and proliferation, mitochondrial activity, nitric oxide and secondary messengers. A decrease in cytotoxicity, DNA damage and pro-inflammatory cytokines is also seen. LLLT offers an alternative wound healing therapy. At a biochemical level there is a positive effect on cells, with stressed cells being pushed into cell survival pathways.
Many nascent proteins, including nearly all membrane and secreted proteins, must traverse a membrane-bound protein-conducting channel prior to their full maturation. This channel, the Sec translocon, is found in all domains of life and possesses the novel ability to direct nascent proteins to the membrane or to the extracellular space, depending on their sequences, often concomitant with their synthesis by the ribosome. By combining atomic structures with cryo-electron microscopy data using the molecular dynamics flexible fitting method, we have developed some of the first views of inactive and active translocons in complex with the ribosome. These views reveal a conserved mode of interaction between translocon and ribosome as well as the roles of specific elements of both in protein localization. We also carried out two-dimensional potential-of-mean-force calculations to explore the structure of the nascent peptide within the translocon environment. The calculations revealed that the translocon exerts a small bias on the peptide towards a helical state. This bias can serve to facilitate, e.g., the insertion of nascent membrane proteins into the otherwise inhospitable lipid bilayer.
Some results from applying nuclear and related analytical techniques in medical, environmental and industrial biotechnologies are presented. In the biomedical experiments biomass from the blue-green alga Spirulina platensis has been used as a matrix for the development of pharmaceutical substances containing such essential trace elements as selenium, chromium and iodine. The feasibility of target-oriented introduction of these elements into Spirulina platensis biocomplexes retaining its protein composition and natural beneficial properties was shown. The negative influence of mercury on growth dynamics of Spirulina platensis was observed. Detoxification of Cr and Hg by Arthrobacter globiformis 151B was demonstrated. Microbial synthesis of technologically important silver nanoparticles by the novel actinomycete strain Streptomyces glaucus 71 MD was characterized by a combined use of Transmission Electron Microscopy (TEM) and ENAA. It was established that the tested actinomycete Streptomyces glaucus 71 MD produces silver nanoparticles extracellularly when acted upon by the silver nitrate solution, which offers a great advantage over an intracellular process of synthesis from the point of view of applications. The synthesis of silver nanoparticles by Spirulina platensis proceeded differently under the short-term and long-term silver action. Our studies will help to develop a rational microbial nanoparticle synthesis procedure.
Nitrilase and amidase enzymes catalyse the conversion of nitriles and amides to their corresponding acid and ammonia - chemistry which is useful in synthesis of drug compounds and fine chemicals. The conversion of cyanide, a nitrile, to ammonia and formic acid is useful in detoxification of contaminated wastewater. The cyanide dihydratase from Bacillus pumilus (CynDpum) catalyses this reaction and is thus a potentially useful biocatalyst. CynDpum and related nitrilase enzymes become activated by formation of spiral-shaped multimers. Evidence suggests that multimer formation regulates activity, by mechanisms which are still not clear. We have used a combination of transmission electron microscopy (EM) with 3-D image reconstruction, molecular modelling, and mutagenesis to investigate the mechanism of helix formation in CynDpum. We have implemented a new routine in the molecular dynamics package, NAMD, that allows helical symmetry to be used as a constraint, together with the EM volume. This method improves on asymmetrical modelling procedures and has aided in our atomic-level interpretation of low-resolution 3-D maps from negative-stain EM. This in turn has allowed us to make testable predictions about the importance of specific amino acids for the mechanical stability of the spirals. We show that salt bridges in one interface are not required for complex formation, but that mutations in this area can enhance the mechanical stability of the enzyme.
The current-voltage (I-V) characteristics of sulphur passivated n-GaSb have been studied in the temperature range 80 K - 330 K using Au Schottky barrier diodes (SBDs). The forward I-V characteristics have been analysed assuming that thermionic emission is the dominant current transport mechanism and that the barrier height patchiness takes on a Gaussian distribution. It has been found that sulphur treatment of the GaSb accompanied by prior etching steps lead to a significant improvement in the quality of the SBDs as is evidenced by the improvement in the ideality factor (n), rectification ratio, series resistance (Rs) and the barrier height (Φb) of the device, These improvements were attributed to removal of predominantly Sb-O from the GaSb surface together with passivation of surface states acting as recombination centres. Typical diode parameters obtained at room temperature (300K) were found to be to be 1.12 (n), 19.4Ω (Rs) and 0.52eV (Φb)) The ideality factor was found to diverge from unity whereas the barrier height increased with decrease in temperature. The increase in the ideality factor is attributed to the dominance of field assisted tunnelling over thermionic emission at low temperatures and is particularly severe for high free carrier concentration material. Finally, the surface state densities for sulphurized material were quantified using I-V measurements and compared to that of untreated material.
Energetic stability of magnetically ordering dopants in diamond holds the prospect of achieving a diamond based diluted magnetic semiconductor which, in addition to diamond’s extreme properties may successfully be considered for spintronic device applications. However, one of the problems to be addressed is that of predicting the most energetically stable lattice configurations or charge states in which such dopants can remain electrically or optically active, and at the same time induce magnetic moments when incorporated into diamond. We report ab initio DFT calculations on the formation energies of isolated 3d transition metal impurities at various lattice sites and charge states in diamond, and show that these impurities introduce deep donor and acceptor levels in the band gap of diamond. We further show that the formation energies as well as the magnetic ordering properties are critically dependent on the position of the Fermi level in the diamond band gap, with formation energies across the 3d series being lower in n-type or p-type diamond compared to intrinsic diamond, suggesting that co-doping with impurities such as boron, nitrogen or phosphorus will considerably enhance their stability in diamond. The majority of the 3d transition metals are found to be energetically most stable at a divacancy site in any charge state, with the formation energy of transition metals in the middle of the 3d series (Cr, Mn, Fe, Ni, Co) being lowest at any lattice site.
Crystalline thin solid films of WO3 have been prepared on Corning glass, FTO and Si through the low temperature, wet chemistry method of Aqueous Chemical Growth. SEM images of the thin films produced show that the surface morphology of the films is influenced by the kind of substrate used for the synthesis. While nanoplatelet-containing flower-like structures were consistently obtained on the Corning glass substrates, nanorod-containing urchin-like structures were consistently produced on the F-doped SnO2-glass substrates. A review, using SEM, is done here of the different kind of nanostructures that make up the thin films produced on these different substrates. Structural characterization of the different thin films synthesized on all three substrates was carried out using XRD, TEM, HRTEM and SAED. These showed that the films produced were made up of WO3 in the monoclinic, triclinic, cubic and hexagonal phase depending on what heat-treatment procedures the different substrates were subjected to post-synthesis. The quantum confinement effect is clearly demonstrated in the thin films as the optical band gap calculated for these films is seen to undergo a blue shift from the theoretical values of 2,7eV in the bulk to values of 3.18 eV, 3.93eV and 4.12 eV at the nano/microscale. This can be explained as being a consequence of the reduction of the grain size in these films to values in the nano-range, values as small as 9nm. By varying the time of synthesis for these films, a growth mechanism for the nanostructures produced is proposed. A comparative study of the hydrogen sensing properties of the WO3 thin films produced on all three substrates is carried out. This is compared to hydrogen sensing carried out on WO3 thin films prepared on Corning Glass, FTO and Si via the method of electrospinning. The structure-property relationship for gas sensing is thus assessed.
Graphene is a two-dimensional crystal of carbon atoms arranged in a honeycomb lattice. It is a zero band gap semimetal with very unique electronic optical and mechanical properties which make it useful for many applications such as ultra-high-speed field-effect transistors, p-n junction diodes, terahertz oscillators, and low-noise electronic, NEMS and optical sensors. The high quality mass production of this nanomaterial is a big challenge, for this work we have used chemical method which helped to get this goal. Raman and FTIR vibrational spectroscopies were investigated to the examination of the production quality.
The successful incorporation of ZnO nanoparticles and Ce3+ ions in Pr3+ doped SiO2 using a sol-gel process is reported. SiO2:Pr3+ gels, with or without ZnO and Ce3+, were dried at room temperature and annealed at 600 oC. The Field Emission Scanning Electron Microscopy (FESEM) indicated clustered SiO2 nanoparticles with with particle size diameter ranging between 20 to 120 nm while the high resolution transmission electron microscopy (HRTEM) revealed and energy dispersive x-ray spectroscopy (EDS) the presence of Zn, Ce, and Pr nanoclustures enveloped in SiO2 matrix.. The emission intensity from SiO2:Pr3+-Ce3+ was slightly enhanced compared to single doped SiO2:Ce3+. For ZnO.SiO2:Pr3+, the ZnO green emission was suppressed and enhanced emission from Pr3+ was observed. Energy transfer mechanism between Pr3+and Ce3+ as well as between ZnO and Pr3+ ¬ is discussed in detail.
Zinc oxide is a direct, wide bandgap semiconductor material with many promising properties for blue/UV optoelectronics, transparent electronics, spintronic devices and sensor applications. ZnO has been commonly used in its polycrystalline form in a wide range of applications such as sunscreens, catalysts, piezoelectric transducers, varistors, and as transparent conducting electrodes. ZnO has numerous attractive characteristics for electronics and optoelectronics devices especially in solar cells. It has direct bandgap energy of 3.37 eV, comparable to of TiO2 which makes it transparent in visible light. The exciton binding energy is about 60 meV for ZnO. The room temperature electron Hall mobility in single crystal ZnO is of the order of 200 cm2 V-1. Unlike TiO2, ZnO can be grown easily in anisotropic shape, specifically in form of nanorods or nanotubes. This central geometry of nano-scaled ZnO, in addition to the high surface/volume ratio for the adsorption of light-harvesting molecules, would offer a larger free mean path for electronic charge transfer and minimizes the e-hole recombination in a ZnO nanorods/nanotubes based excitonic Gratzel dye solar photocells. Indeed, this tubular ZnO geometry would enhance the device efficiency through the direct electrical pathways provided by the nanorods/nanotubes, ensures the rapid collection of carriers generated throughout the device, in addition to an effective light trapping. This contribution reports the growth mechanism of the ZnO nanorods by the so called Vayssieres hydrothermal method. It is demonstrated for the first time that in a large pH range within the hydrothermal process, the growth mode of the ZnO nanorods is a pure Frank-van der Merwe driven process. More precisely, nano-platellets of ZnO are formed at the early growth stage growing towards nano-rods. From crystallographic view point, both the ZnO nano-plattelets and nanorods exhibit the standard hexagonal wurtzite crystal structure, with (002) main orientation and lattice parameters a=~3.25 Å and c=~5.12 Å. The room temperature luminescence and Raman investigations indicates that the defects are mainly O deficiency driven.
An introduction to the new development project on the implementation of a radioactive ion beam (RIB) facility at iThemba LABS will be presented. In particular with relation to the uses of the RIB for the application of research in condense matter and materials research. Some leading examples by international laboratories using radioactive nuclei as probes to enhance the understanding of structural and/or electronic/magnetic lattice environment in materials (metals, insulators or semiconductors) will be also presented. The main procedures of such investigations are implantation induced lattice damage and its annealing behavior, the lattice site of the implanted ion after annealing, the interaction between impurities or impurities and intrinsic defects, the electronic and optical properties of the implanted species and the identification of defects and impurities. The understanding and the control of diffusion profiles of intrinsic and extrinsic defects particularly in semiconductors is significantly enhanced using radioactive tracer diffusion.
We studied charge injection and transport mechanisms in blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methylester (PCBM), by analysing dark, temperature dependent current-voltage characteristics of the P3HT:PCBM blend thin films sandwiched between aluminium electrodes in a MIM configuration. We present a general method of interpreting Fowler - Nordheim plots of metal/semiconductor/metal devices with pronounced non-linear characteristics by dividing them into several regions based on physical origins. We show that by applying appropriate electric fields it is possible to switch from electron-only conduction to hole-only conduction in a single Al/P3HT:PCBM/Al device. We affirm that electrons can be selectively transported through the lowest unoccupied molecular orbital of PCBM at low applied voltages and low temperatures; and alternatively holes can be transported through the highest occupied molecular orbital of P3HT at higher applied voltages and high temperature, within a single device.
Silica is a basic material of technological importance for optics, microelectronics, photonics and fibre optics. Its high absorption edge energy makes its particularly useful for UV applications and it has been used as a host material for a variety of luminescent lanthanide ions due to its chemical stability and non-hygroscopic nature. For lanthanide ions the 4f electron energy levels are shielded from the host environment by the filled outer 5s and 5p orbitals, so that the transitions between these states and therefore the luminescent wavelengths are relatively insensitive to the host. For this reason little attention has been paid until recently to the location of the impurity levels of the lanthanide ions within the energy gaps of their hosts. However, luminescence from some lanthanide ions, e.g. cerium, occurs due to f-d transitions from the unshielded 5d state of which the energy relative to the f-states is therefore host dependent. The absolute positions of the 4f and 5d states relative to the energy gap of the host also affect quenching and charge trapping phenomena and so they are required for proper modeling of phosphor performance. Recently Dorenbos has suggested that the energy levels of any of the 13 divalent lanthanides relative to the band edges of the host can be found using only three parameters. However, obtaining this data for a particular host is not always straightforward e.g. use can be made of the f-d transition energy of cerium, but different values for this parameter in silica have been reported. In this paper a scheme for the energy levels of both the divalent and trivalent lanthanide ions in silica is proposed and compared to the experimental data.
Europium (Eu2+) and gadolinium (Gd3+) doped BaAl2O4 powder phosphors were prepared by combustion method at different initiating temperatures (400-1000 oC), using urea as a comburent. The powders were annealed at different temperatures in the range of 400-1000 oC for 3 hours. X-ray diffraction data show that the crystallinity of the BaAl2O4 structure greatly improved as the annealing temperature increased. The FT-IR absorption bands observed at 533, 629 and 798 cm-1 for the samples annealed at higher temperatures (1100-1200 oC) are consistent with the stretching mode frequencies of BaAl2O4. Blue-green photoluminescence, with persistent / long afterglow, was observed at 503 nm.This emission was attributed to the 4f65d1- 4f7 transitions of the Eu2+ ions. The phosphorescence decay curves were obtained by irradiating the samples with a monochromatized xenon lamp at an excitation wavelength of 393 nm.The glow curves and the decay curves of the samples irradiated by UV source (360 nm) were investigated using investigated using the Thermoluminescence Reader (Integral-Pc Based) Nucleonix TL 1009I.
Photovoltaic cell devices constructed from organic materials are becoming increasingly efficient because of the discovery of the bulk heterojunction concept. Today, organic photovoltaic cell (OPVC) devices fabricated from the blends of poly(3-hyxylthiopene) (P3HT) and [6,6] –phenyl C61-butyric acid methyl ester (PCBM) are the most widely studied bulk heterojunction systems because of their relatively good photovoltaic (PV) properties. Although the P3HT:PCBM based solar devices exhibit excellent PV properties compared to other bulk heterojunction polymer solar cells, their efficiency ( 8%) is still too low compared to that ( 20%) of conventional silicon PV cells. One way of improving the performance of these devices is by inserting a metal oxide buffer layer between the active layer and the anode. In addition, the PV properties of OPVC devices can also be improved by post-fabrication heat treatment (or annealing). This review provides an overview of OPVC devices focusing on: a brief history of the OPVC devices, device construction, and the effects of ZnO nanoparticles buffer layer and post-fabrication annealing on the general performance of P3HT:PCBM based solar devices.
Europium-doped barium magnesium aluminate (BaMgAlx0y:Eu) phosphors were obtained at low temperatures (500˚C) using the solution - combustion of corresponding metal nitrate–urea solution mixtures. The particle size and morphology and the structural and luminescent properties of the as-synthesized phosphors were examined by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), Auger spectroscopy, transmission electron spectroscopy (TEM) and photoluminescence (PL). It was found that the Ba:Mg molar ratios showed greatly influence not only on the particle size and morphology, but also on their PL spectra and crystalline structure. The BaMgAl2O4 nanophosphors display a monoclinic structure indicating a small change in peak position at high angles due to differences in size between Ba and Mg ions. The peak of the emission band occurs at longer wavelength (around 615nm) with increase in Mg concentration but display a broad band emission at 515 nm for lower Mg concentration. The green emission is probably due to the influence of 5d electron states of Eu2+ in the crystal field because of atomic size variation causing crystal defects while the red emission is due to f-f transitions. This finding clearly demonstrate the possibility of fine tuning the color emission and solid solubility limit in Ba1-xMgxAl204:Eu phosphors through the simple and cheap process. A green phosphor with almost pure phase with significantly enhanced luminescence and longer after glow can be easily prepared by solution combustion method by doping with slight magnesium concentration (1 mole % Mg).
Diamond tools are generally made of sintered polycrystalline diamond (PCD) on a hard metal tungsten carbide (WC-Co) substrate during the High Pressure High Temperature (HPHT) process. The diamond tools are used widely as cutting and drilling tools due to their unique combination of superior properties. However, there are several issues that need to be resolved about the diamond tools. At present it is known that diamond tools degrade with time as it is normally used at high temperatures, but what is not known is the damage mechanism and how the degradation varies as a function of temperature. In this study, we have made use of a laser based system to raise the temperature of the diamond tool and measure the diamond tool temperature. A study of the thermal damage on the diamond tool with respect to the laser heating power and temperature has been carried out using Raman spectroscopy; to determine the structure and composition of the resulting diamond tool. We have shown the radical changes on the diamond tool as the laser heating power and temperature increases.
Premature withdrawal from university due to academic failure has the potential to present problems to both students and educators. The demand from industry for a larger pool of science and engineering graduates in South Africa is in line with international norms. This state of affairs is compounded by a shrinking pool of good quality school leavers. For the desired growth in graduate numbers to occur, physics educators are required to respond effectively through curriculum reform to optimize success and retention of students at first year level. The research project in this regard aims to investigate the level of preparedness in relation to Physics I students and to identify key success factors in this course. This course is compulsory for most first year science students and is largely perceived as difficult in comparison with other first year courses. More students are pursuing Physical Science at school and consequently study science, engineering and technology at universities (DoE, 2010) and University of Johannesburg is no exception. For many students, the first year at university represents a transition during which a variety of academic and social challenges are encountered. To this end, students’ entry-level preparedness was investigated through analysis of their high school examination results, administration of a diagnostic test and first year university assessment results. The results reveal that high school examination results appear not to be the only independent factor characterising students’ readiness for first year physics studies at university.
During the past decades there has been an increasing interest in alternative conceptions in physics. An alternative conception in science refers to a conception which in some aspects is contradictory to or inconsistent with the concept as intended by the scientists. Research in this domain tries to answer questions such as which misconceptions occur, what are their origins, how extensive are they and what can we do about them? Investigations of conceptual change approaches to remedy learners’ alternative conceptions often showed that they are resistant to change. In this study we intend to follow a different approach that focuses more positively on productive intuitive concepts of learners and students, i.e. concepts that may serve as a potential base for further refinement towards the science concept. Students’ and learners’ initial conceptions are investigated and analyzed for productive resources. A teaching sequence will then be developed based on conceptual refinement of their resources rather than using a cognitive conflict strategy that intends to accomplish conceptual change of alternative conceptions. The study is still in progress and the presentation will discuss the results of interviews conducted with learners in grades 10, 11 and 12 and with prospective teachers enrolled in the first year university physical sciences programme. The learners’ and students’ intuitive concepts were probed in the field of magnetism. A clinical interview approach was followed with semi-structured open-ended questions. The data was transcribed and coded according to a conceptual development scheme. The results of the interviews will inform the compilation. The subjects in depth understanding of magnetic phenomena’s and their positive intuitive concepts were probed and documented. The results of completing the interviews lead to the compilation of a structured questionnaire to investigate the consistency of potential positive intuitive concepts in magnetism. The findings lead to the potential refinement of specific magnetic concepts to the scientifically correct. The results showed deficiencies in the subjects’ knowledge in magnetism as well as productive intuitive concepts that can be used for further refinement to a more scientifically correct understanding of magnetic concepts.
Educational technology in the form of classroom response systems (clickers)have become a tool in classroom instruction. In the classroom the clickers are used to promote interactive student engagement and immediate assessment of student understanding. The paper reports on a study using clickers as well as flash cards (a low tech version of the clicker) as a tool to enhance interactive engagement and formative assessment in the Extended Curriculum Programme (ECP) Physics at UWC. The results suggest that clickers may be a useful tool for the instructor to engage students in class discussions and to monitor their understanding and for students to have immediate feedback on their own progress. Overall the students’ attitudes towards clickers were positive. They enjoyed the interactive nature of the clickers because of their familiarity with mobile phones and digital media. They also felt the anonymity associated with the assessment results decreased peer pressure.
Every year over a hundred people die unnecessarily from lighting in South Africa. Recent reporting and correspondence in the media clearly indicated a total lack of knowledge on the part of the public, from Ministerial level down, about the causes and effects of lightning. Much work has been done by organizations such as the CSIR and SA Weather Bureau, and there are several commercial concerns addressing this problem in both urban and wealthy rural areas such as game lodges and other tourist resorts. Recently it has been reported that the Department of Science and Technology, DST, has initiated a progamme to install lightning conductors in rural areas. The SAIP can, and should be, playing a leading role in addressing this problem and this presentation will discuss an assortment of possible ways that the SAIP can assist in sustaining remedial efforts in rural areas.
Lasers, Optics and Spectroscopy
With modern liquid crystal devices it is possible to implement digital holograms for the modulation in amplitude and phase of light: so called 'structured light'. In this non-specialist talk, we will introduce the concept of complex amplitude modulation, and show how light can be tailored to have interesting properties: defying diffraction, twisted like a spring in intensity and phase, and made to bend around corners! The ability to create arbitrarily complex fields opens up a plethora of applications, from controlling the micro-world through optical traps, to probing the quantum world with twisted light.
In this work we investigate the behavior of an instantaneous Poynting vector for different types of cylindrical symmetric beams such Gaussian beams, Bessel beams and so on. In most investigations for energy flow in real laser beams, the oscillation component of the Poynting vector is not considered. However this component of the Poynting vector carries useful information regarding beam behavior such as beam divergence, self reconstruction and diffraction effects, which are difficult or nearly impossible to extract from the field of the beam or the time averaged component of the pointing vector. The behavior of the Poynting vector is different in the near and far fields and understanding such differences leads to useful interpretation of the different beam behavior in these regions which is also part of what is investigated in this study.
The transformation of a Gaussian beam into a symmetrical TEMp0 Laguerre Gaussian beam (LGB) intensity distribution which has transverse superresolution properties is very useful in improving the spatial resolution of optical imaging microscopes by making the central diffractive spot smaller than the Airy spot. The beam shaping is achieved by using an annular binary Diffractive Optical Element (DOE) whose geometry is in connection with the location of the Laguerre polynomial zeros. The DOE imposes positions of p zeros of intensity distributions on the Gaussian beam, resulting to a generation of TEMp0 beams where there are minimum losses. The LGBs are well-known family of exact orthogonal solutions of free-space paraxial wave equation in cylindrical coordinates. Theoretical and experimental results will be demonstration for higher order TEMp0 modes of LGBs.
The phase measurements of a Shack-Hartmann wavefront sensor are calibrated. The process involves creating holograms of known amounts of individual phase aberrations on a spatial light modulator and reflects an aberration free laser beam off of the modulator’s liquid crystal display thereby giving the beam that aberration. The beam is then relayed onto the sensor for direct measurement. The results show that the method is accurate to the extent that the results accurately replicate the response of the wavelength calibration of the spatial light modulator.
Nuclear, Particle and Radiation Physics
Recent theoretical work has suggested that some nuclei in the rare earth region might exhibit tetrahderal deformations. Several nuclei have been studied at iThemba LABS, resulting in evidence against the possibility of low-lying tetrahedrally deformed bands. In a continued study of the region, further gamma-ray spectroscopy experiments have been performed at iThemba LABS, populating low lying states of nuclei in the (Z,N)=(70,90) region. We present some of the results obtained for 156Er, 160Er and 158Dy, particularly referencing the negative parity bands, and their relative structure in relation to the other nuclei in the region. Calculations of the theoretically predicted observables, such as the quadrupole deformation and the in-band transition rates and branching ratios will be discussed.
The nuclei 158,15968Er are classic examples of many phenomena in the study of nuclei at high spin. However, although the multi-quasi particle structures of 158,15968Er are well established, there is not much spectroscopic information on the vibrational structures in 158Er or how these couple to the odd neutron in 159Er. Additionally, there are several bands where the spins and/or parities are not firmly assigned. The relative lack of information at low spins is probably due to a combination of the low spins of the ground states of the nuclei 158,15969Tm that β+/ec decay to 158,15968Er, 2- and 5/2+ respectively. Two experiments were performed in the AFRODITE spectrometer to populate 158Er and 159Er using 1mg/cm2 150Sm(12C,4n) and 150Sm(13C,4n) reactions respectively at 65MeV, The intention is to study their yrare states below spin 20ħ. Preliminary results will be presented. A Coulomb excitation of 155Gd with 86Kr ions experiment is scheduled for early May at iThemba LABS. The Physics interest is in looking for collective core excitations of 154Gd that are coupled to the ground state neutron in 155Gd. The outcome of this experiment will be presented as well.
Negative-parity side bands in nuclei localized in the vicinity of N = 90 are known to have strong electric dipole transitions from each level to the ground-state band, with weak or unseen E2 transitions (E2s) in the medium (~15h) to low-spin region. These structures have always been regarded as octupole vibrational bands. On the other hand, Dudek et al. recently proposed that these structures might be associated with tetrahedral symmetries, as they have very weak inband E2s. In this study, data from the 148Nd (12C, 4n) 156Dy reaction, taken with GAMMASPHERE is analysed. Out of 23 rotational bands that have been observed in this present study for 156Dy, there are eleven new bands and more than 266 new transitions. We also report an observation of a band with characteristics of that discussed on the paragraph above i.e. a negative parity band with missing (or weak) inband transitions at low spin. Furthermore, the crossing between this band (tetrahedral candidate) and band 6 around spin 13h has allowed the band mixing calculation to be carried out, so that relative quadrupole moments could be deduced for these bands. However, a non-zero quadrupole moment has been deduced for our tetrahedral candidate, and this is incompatible with the tetrahedral shape. We therefore seek for an alternative description of this band.
Nuclei near N=90 are considered transitional, since they lie between nuclei that display vibrational spectra and nuclei that display rotational spectra. The structure of these transitional nulei, in particular the nature of the low-lying first excited states |0+2> still pose a great challenge with various theoretical models competing. A new perspective to the understanding of the structure of the N=88 and N=90 nuclei is brought by the proposition that the low-lying first excited 0+ states in these nuclei are not β-vibrations but constitute a second vacuum |0+2> coexisting with the ground state vacuum |0+1>. As part of a campaign studying nuclei near N=90, the low and high spin states of the N=88 nucleus 150Sm have been populated via 148Nd(α, 2n)150Sm at a beam energy of25MeV and 136Xe(18O, 4n)150Sm at 75MeV respectively. These experiments have been conducted on world-class multi-detector gamma-ray spectrometer arrays, namely JUROGAM at Jyvakyla in Finland as well as the AFRODITE at iThemba LABS Cape town here in South Africa. The isotone 152Gd has been studied via 152Sm(α,4n)152Gd at 45MeV fusion evaporation reaction taken with AFRODITE. Preliminary results focusing on the observed band structres for the two isotones 150Sm and 152Gd will be presented.
The Coulomb excitation of light nuclei can provide unique information on the matrix elements for excited states. This contribution will discuss experiments to measure such excitation as well as ab initio large shell model calculations calculations that can be tested by these measurements.
The notion of mirror symmetry is well-established for nuclei on or near the valley of stability, and this has its roots in the (largely) charge independent nature of the nuclear force. We have applied this assumption to nuclei near or beyond the drip lines, in particular, the mass-15 system for 15F and 15C. Using information on the bound states in 15C we predicted states in 15F in 2006 which were subsequently found in 2009. We have extended our searches to other mirror pairs of exotic nuclei: in particular mass-17. In the case of mass-17 nuclei, we obtain interactions by considering the states in 17C, from which we may obtain information on other exotic mass-17 nuclei.
Theoretical and Computational Physics
An entangled bipartite Gaussian state is coupled to two thermal reservoirs, one for each particle. A harmonic oscillation is allowed between the two particles. The reservoirs are assumed to have different temperatures and to be coupled to the particles with different coupling strengths. This allows for a realistic situation where a bipartite state may be shared between two parties who ”keep” their part in different environments. A master equation, previously derived in the non-rotating wave approximation, is solved for the system. The effects of a variation in the bath temperature on the entanglement, as well as that of the variation in coupling strengths are shown. For high temperatures, the entanglement vanishes if the coupling strength is large, whereas for low temperatures, the effect is reversed and it survives longest for a stronger coupling strength. The stationary dynamics of the system is also examined.
Decoherence indicates the process that a quantum system undergoes through the interaction with an external environment. The central issue and one of the greatest challenges in Nanotechnology and Quantum Information Processing is the way to control or delay the destructive effect of the environment on qubit coherence. For this reason, a qubit interacting with a reservoir of bosons (external environment), described by a Jaynes-Cummings model, is considered. The decoherence process corresponding to Lorentzian type distributions of field modes results in exponential like relaxations. At this stage we adopt the engineering reservoir approach as the way of delaying the decoherence process. A special class of reservoirs of field modes is designed in order to obtain a decoherence process resulting, over estimated long time scales, in inverse power laws with powers decreasing continuously to unity according to the choice of the particular reservoir. The designed reservoirs exhibit a photonic band gap coinciding with the qubit transition frequency and are piecewise similar to those usually adopted in Quantum Optics, i.e., sub-ohmic at low frequencies and inverse power laws at high frequencies. Initially, the reservoir is assumed to be in the vacuum state and is unentangled from the qubit versing in a generic state. The exact dynamics results to be described by series of Fox $H$-functions. The simple form of the designed reservoir can be accessible experimentally.
The transfer of energy and information in quantum networks plays an important role in both quantum communication and quantum computation. The quantum system inevitably interacts with the surrounding environment, and such interaction leads to dissipation and decoherence, which are processes typically associated with a destruction of quantum coherence in the system. However, recently evidence of surprisingly long-lived quantum coherence has been identified in conjugate polymers and in photosynthetic light harvesting complexes. Here, the dynamics of a dimer under the influence of decoherence are studied. An exact analytical expression for the transition probability in the dimer system is obtained for different situations, i.e., for the dimer coupled to two independent environments, and coupled to two correlated environments, both in a spin star configuration. In all cases considered, it is shown that there exist well-defined ranges of parameters for which decoherent interaction with the environment assists energy transfer in the dimer system. In particular, it is found that correlated environments can assist energy transfer more efficiently than separate baths. This simple and analytically solvable model for energy transfer in a dimer system is easily extendible to more complex quantum networks, and more complex environmental models.
Quantum Random Walks have been introduced almost 20 years by Y. Aharonov et al. [Phys. Rev. A, 48(2):1687–1690, 1993] and have found wide applications in quantum computing. As is often the case in quantum theory, Quantum Walks differ strongly from classical random walks. In joint work of S. Attal, C. Sabot, F. Petruccione and I. Sinayskiy the concept of Open Quantum Random Walks was introduced, by taking into account dissipation and decoherence that occur in open quantum systems. The connection to classical and unitary random is discussed as well as the potential of Open Quantum Random Walks for quantum computing and efficient quantum transport.
We find non-local but non-signaling probabilities satisfying the 'nonlocality without inequality’ arguments for multiple two-level systems. Maximum probability of success of these arguments are obtained in the framework of a generalized nonlocal theory. Interestingly, for two two-level systems, the probability of success of these arguments converge to a common maximum in this framework. This is in sharp contrast with the quantum case, where for such systems, Cabello’s argument succeeds more than that of Hardy’s. We also find that the maximum probability of success of Hardy’s argument is the same for both the two and three two-level systems in the framework of this more generalized theory.
Ground-state properties of three-nucleon and four-nucleon systems are studied with the angular-momentum-projected and parity-projected antisymmetrized molecular dynamics. The Hamiltonian of the systems is constructed with semi-realistic nucleon-nucleon interactions. The results obtained for the ground-state energies, root-mean-square radii and magnetic dipole moments are compared with the findings of other theoretical methods.
The angular-momentum-projected and parity-projected antisymmetrized molecular dynamics is used to analyse the charge and magnetic form factors of the three-nucleon systems. Non-relativistic nuclear charge and current operators with relativistic corrections are employed. The Hamiltonian of the nuclear systems is described with a semi-realistic nucleon-nucleon potential. The results obtained are compared with results obtained using other theoretical methods as well as some experimental data.
Astrophysics and Space Science
A number of solutions to the dark energy problem have been proposed in literature, the simplest is the cosmological constant Λ. But the cosmological constant lacks theoretical explanation for its extremely small value, thus dark energy is more generally modeled as quintessence scalar field rolling down a flat potential. For the quintessence scalar field to be evolving on cosmological scales to day its mass must be of order H0 , which is the present value of the Hubble constant. A scalar field φ whose mass varies with the background energy density was proposed by Khoury and Weltman(2003). This scalar field can evolve cosmologically while having coupling (β) to different matter fields of order unity. Such a scalar field also couples to photons in the presence of an external magnetic field via the φF2 interaction, where F stands for the electromagnetic field strength tensor. The chameleon(φ)-photon coupling of this nature causes a conversion of photons to light Chameleon(φ) particles and vice versa. In this work we investigate this effect on pulsars, and we constrain the parameter space of this theory.
Applied and Industrial Physics
The amidases catalyse the conversion of amides to the corresponding carboxylic acid and ammonia. We have visualized the structure of the amidase (MW= 30 kDa) from a bacterium, Nesterenkonia sp. at atomic resolution by X-ray crystallography. The primary components of the active site are two negatively charged carboxylates (E1 and E2) arising from two conserved glutamate side chains, a positively charged amino group (K) arising from a conserved lysine, a sulphydryl arising from a conserved cysteine and a water molecule. Mutation of the active site residues has enabled us to visualize the unreacted substrate and a number of artificial intermediate states which have led to our insights. We propose a reaction mechanism which passes through a thioester adduct to the cysteine. Although this in itself is not controversial, our studies demonstrate for the first time how exquisitely accurate geometrical placement of the various groups leads to accurate stereoelectronic alignment which allows the transition states to form. The configuration of E1, E2 and K ensures that K remains positively charged throughout the reaction thus enabling it to act as an acid catalyst and provide an oxyanion hole to stabilize the tetrahedral transition states. The configuration also accurately positions the substrate for nucleophilic attack by the sulphydryl by hydrogen bonding to the amide, positions E1 as a base catalyst and positions the water with the correct stereoelectronic alignment for a nucleophilic attack on the thioester. The transition state energies, electron densities and hydrogen placements calculated using Gaussian09 demonstrate the plausibility of our proposal.
The transition from intermediate valence (IV) behaviour in CeNi2Si2 to magnetically ordered Kondo lattice behaviour in CeAu2Si2 is investigated through measurements of X-ray diffraction (XRD), electrical resistivity &rho(T), magnetization &sigma(µ0H) and magnetic susceptibility &chi(T) on the polycrystalline Ce(Au1-xNix)2Si2 (0 ≤ x ≤ 1) alloy series. Lattice parameters as derived from XRD measurements deviate from Vegard’s rule around x = 0.6 – 0.8. &rho(T) data indicate Kondo lattice behavior in the presence of a crystal field for x ≤ 0.6, the occurrence of magnetic ordering for x = 0 and fluctuating valency for x ≥ 0.8. &chi(T) data at high temperatures, follow the Curie-Weiss relation for alloys in the concentration 0 ≤ x ≤ 0.6 (Kondo lattice region) and give effective magnetic moment values µeff close to that expected for the free Ce3+-ion. The low temperature &chi(T) data indicate the onset of antiferromagnetic ordering for x ≤ 0.78. For alloys in the concentration range 0.8 ≤ x ≤ 1 (IV region), &chi(T) data are described within the framework of the paramagnon model. &sigma(µ0H) measurements indicate metamagnetic behavior for alloys in the concentration range 0 ≤ x ≤ 0.1.
The use of tetrahedral amorphous carbon thin films in bolometers depends strongly on the ability to tune its optical properties. This can be achieved by controlling the density of dangling bond defects and the disorder in the material. The pi-orbital axis vector theory is used in this study to investigate the phenomenon of rehybridization of the carbon bonding in tetrahedral amorphous carbon. It is shown that the correlated pi - pi* orbitals is locally entangled due to the competition between the on-site Coulomb interaction and the hybridization. A microscopic model is proposed for the reduction in strain in the material due to hydrogenation, based on the the sp3 – sp2 conversion reaction. It is demonstrated that the activation energy for this conversion is dependent on the Urbach energy, and therefore on the disorder. The implication of the results on the manifestation of strain in similar, carbon-based, materials is discussed.
Microcrystalline germanium-carbide formation was studied in germanium substrate using perturbed gamma-gamma angular correlation (PAC) method. Information about the lattice location of carbon atoms, in the host matrix, can be obtained via the interaction between carbon and unstable probe nucleus (111In). At high dose carbon implantation, in crystal germanium substrate, two defect complexes have been identified by the corresponding unique nuclear quadrupole interaction frequencies. The measured frequencies, Q = 207(1) MHz (eta = 0.2) and Q = 500(1) MHz ( eta= 0), are associated with the formation of carbon related microcrystalline system in germanium. The frequencies are attributed to two different types of carbon-indium pairs in the substrate lattice. The orientations of the measured electric field gradients and thermal stability of the defect complexes are studied. The results are encouraging towards attaining germanium carbide crystal which has interesting potential for applications.
We use density functional theory to investigate the structural, elastic properties and energetic stabilities of Zr, Nb and its alloys in the alpha and beta phases, employing pseudopotential plane wave methods within the Pardew-Burke-Ernzerhof Generalized Gradient Approximation (PBE-GGA). The structures were fully optimized in a non-magnetic state, allowing atomic positions, cell volume and shape to change. Lattice parameters for the pure phases gave excellent agreement with the available experimental data. We also found that the phonon dispersions curves display soft-modes for the metastable beta-Zr phase which is lacking on the alpha-Zr and beta-Nb, condition of mechanical stability. The solid solution calculations show that an increase in the Nb contents destabilizes the structures in both the alpha and beta phases. Interestingly, the calculated elastic moduli for Zr-2.3%Nb are in good agreement to the experimental results of Zr-2.5%Nb.
The formation energy of self-interstitial defects in silicon was studied. The 〈110〉split site was found to be the lowest energy position for the silicon self-interstitial at neutral charge state, as well as singly and doubly negative charged state. The tetrahedral site is found to be the lowest energy site for the singly and doubly charged positive states. The energy barriers for diffusion from the 〈110〉split site to the hexagonal and tetrahedral sites were found to be 0.370 and 0.361 eV respectively. Diffusion between the tetrahedral and hexagonal sites has an energy barrier of 0.074 eV. These energy barriers are found to be too great to account for athermal diffusion of the silicon self-interstitial, and diffusion paths involving cyclic changes in charge state are most likely to be responsible for the athermal diffusion observed at low temperature.
Fuel cells have been intensively researched for the past few decades for macro applications. However, micro fuel cells have not received so much attention. A micro fuel cell delivers power in the mW-W range, and would be suitable for mobile purposes, e.g. as a charger for cell-phone or other small devices. We consider polymers such as polydimethylsiloxane (PDMS) as a prospective structural material candidate for a Micro Fuel Cell. The introduction of such a polymer as structural material will reduce the mass and the size of the micro fuel cell and therefore optimize the current micro fuel cell models. In this perspective, our work focuses on the tailorability and optimization of thermal, mechanical, and electrical properties of PDMS with different micro/nano fillers such as silver particles, carbon black,. The mechanical property is characterized by Dynamic Mechanical Analysis(DMA). The thermal properties are measured using Thermal Gravimetric Analysis (TGA) and Cut-Bar thermal conductivity measurement. Four point probe was used for electrical conductivity measurement. Scanning Electron Microscopy (SEM) was used for surface morphology characterization.
Lasers, Optics and Spectroscopy
Time-domain terahertz (TD-THz) spectroscopy has emerged as a valuable tool for studying material properties in the THz region of the electromagnetic spectrum. Many classes of materials exhibit characteristic absorption signatures in this previously unstudied portion of the electromagnetic spectrum. This opens up the possibility for non-destructive and non-invasive testing and analysis of materials which previously have proven difficult. Time domain T Hz spectroscopy entails measuring the electric field of the generated THz pulse in the time domain, and extracting the spectral information from the pulse through a Fourier Transform. Two main mechanisms for generating short, broadband THz pulses have emerged namely optical rectification in a nonlinear crystal of a femtosecond laser pulse and THz generation from a photo-conductive antenna (PCA). Recently, a photo-conductive antenna TD-THz setup has been established at the Laser Research Institute, University of Stellenbosch. This presentation will focus on the basic principles of operation and on the layout of the setup as well as the possible applications envisaged using the setup.
We observe ultrafast structural dynamics in crystals. For that, we use the 'Pump Probe Technique': The 'Pump' triggers a fast process, the 'Probe' takes a snapshot of it at a specific time. We are interested in observing processes, which happen on a picosecond timescale. That means, the duration of our 'Probe' must be in the order of ~ 100 fs. The only source of a 100 fs short signal, produced and controlled by humans, is a short pulsed Laser. We