9-13 July 2012
Africa/Johannesburg timezone
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Observing photo-induced dynamics in the charge density wave compound 4HB-TaSe2 by means of femtosecond electron diffraction

Presented by Ms. Andrea VON FLOTOW on 11 Jul 2012 from 08:20 to 08:40
Type: Oral Presentation
Session: Photonics
Track: Track C - Photonics

Abstract

Layered transition metal dichalcogenides MX<sub>2</sub> (M=Ta, Ti, Nb ..., X=S, Se...) are quasi-two-dimensional crystals consisting of strongly bound 3 atom thick planar layers of X-M-X which are weakly bonded to each other by van der Waals forces along the c-axis. Due to strong electron correlation in these compounds, the electron density is not uniform throughout the compound but rather modulated with a periodicity different to that of the crystal lattice. This modulation is accompanied by an induced lattice distortion with the same periodicity. The modulation in the electron density together with the periodic lattice distortion (PLD) is referred to as a charge density wave (CDW) and only develops below a corresponding phase transition temperature. In reciprocal space the CDW manifests into a superstructure in the diffraction pattern with CDW peaks between the lattice Bragg peaks. The CDW and its associated PLD can be disturbed or destroyed by introducing energy into the system by means of photons. Depending on the applied fluence the CDW is either only weakened for a short period of time or the transition to a higher temperature phase is driven, from which it might require longer time to return into the ordered state. Here we investigate the photo induced dynamics of the host lattice and the CDW/PLD in 4HB-TaSe<sub>2</sub> by means of femtosecond electron diffraction (FED), which has proven to be a powerful technique to observe structural dynamics in crystalline material with atomic temporal and spatial resolution: a femtosecond laser pulse deposits energy into the sample, initiates the dynamic process and defines time zero; a succeeding subpicosecond electron pulse is diffracted off the sample at an optically defined and variable time t and monitors the instantaneous structure of the sample. Successive recordings of diffraction patterns for varying t produces an atomic-level 'movie' of the crystal dynamics. We will present the wealth of dynamical information retrieved from the femtosecond electron diffraction experiment, and discuss its interpretation in the context of CDW/PLD in TaSe<sub>2</sub>.

Award

Yes

Level

PhD

Supervisor

Heinrich Schwoerer, heso@sun.ac.za

Paper

No

Place

Location: IT 4-5


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