9-13 July 2012
Africa/Johannesburg timezone
<a href="http://events.saip.org.za/internalPage.py?pageId=11&confId=14"><font color=#ff0000>SAIP2012 PROCEEDINGS AVAILABLE</font></a>

Observing photo-induced dynamics in the charge density wave compound 4HB-TaSe<sub>2</sub> by means of femtosecond electron diffraction

11 Jul 2012, 08:20
20m
Oral Presentation Track C - Photonics Photonics

Speaker

Ms Andrea von Flotow (Stellenbosch University)

Would you like to <br> submit a short paper <br> for the Conference <br> Proceedings (Yes / No)?

No

Level for award<br>&nbsp;(Hons, MSc, <br> &nbsp; PhD)?

PhD

Apply to be<br> consider for a student <br> &nbsp; award (Yes / No)?

Yes

Main supervisor (name and email)<br>and his / her institution

Heinrich Schwoerer, heso@sun.ac.za

Abstract content <br> &nbsp; (Max 300 words)

Layered transition metal dichalcogenides MX2 (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-TaSe2 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 TaSe2.

Primary author

Ms Andrea von Flotow (Stellenbosch University, South Africa)

Co-authors

Dr Gunther Kassier (Stellenbosch University, South Africa) Prof. Heinrich Schwoerer (Stellenbosch University, South Africa) Prof. Jure Demsar (Konstanz University, Germany) Ms Kerstin Haupt (Stellenbosch University, South Africa) Mr Maximilian Eichberger (Konstanz University, Germany) Mr Nicolas Erasmus (Stellenbosch University, South Africa)

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