Extreme Phonon Softening in Laser-excited Bismuth – Towards an Inverse Peierls-transition

AbstractLarge amplitude coherent optical phonons have been investigated in laser-excited Bismuth by means of femtosecond time-resolved X-ray diffraction. For absorbed laser fluences above 2 mJ/cm2, the experimental data reveal an extreme softening of the excited A1g-mode down to frequencies of about 1 THz, only 1/3 of the unperturbed A1g-frequency. At even stronger excitation the measured diffraction signals no longer exhibit an oscillatory behavior presenting strong indication that upon intense laser-excitation the Peierls-distortion, which defines the equilibrium structure of Bismuth, can be transiently reversed.

Download Full-text

Evolution of laser-induced strain in a Ge crystal for the [111] and [100] directions probed by time-resolved X-ray diffraction

Journal of Applied Crystallography ◽

10.1107/s1600576721010281 ◽

2021 ◽

Vol 54 (6) ◽

Author(s):

Ranjana Rathore ◽

Himanshu Singhal ◽

Ajmal Ansari ◽

Juzer Ali Chakera

Keyword(s):

Propagation Velocity ◽

Laser Excitation ◽

Laser Pulses ◽

X Ray Diffraction ◽

Time Resolved ◽

Carrier Diffusion ◽

X Ray ◽

Strained Crystal ◽

Different Depths ◽

Silicon Based

Ultra-short laser-pulse-induced strain propagation in a Ge crystal is studied in the [111] and [100] directions using time-resolved X-ray diffraction (TXRD). The strain propagation velocity is derived by analysis of the TXRD signal from the strained crystal planes. Numerical integration of the Takagi–Taupin equations is performed using open source code, which provides a very simple approach to estimate the strain propagation velocity. The present method will be particularly useful for relatively broad spectral bandwidths and weak X-ray sources, where temporal oscillations in the diffracted X-ray intensity at the relevant phonon frequencies would not be visible. The two Bragg reflections of the Ge sample, viz. 111 and 400, give information on the propagation of strain for two different depths, as the X-ray extinction depths are different for these two reflections. The strain induced by femtosecond laser excitation has a propagation velocity comparable to the longitudinal acoustic velocity. The strain propagation velocity increases with increasing laser excitation fluence. This fluence dependence of the strain propagation velocity can be attributed to crystal heating by ambipolar carrier diffusion. Ge is a promising candidate for silicon-based optoelectronics, and this study will enhance the understanding of heat transport by carrier diffusion in Ge induced by ultra-fast laser pulses, which will assist in the design of optoelectronic devices.

Download Full-text

Out-of-equilibrium dynamics of photoexcited spin-state concentration waves

Faraday Discussions ◽

10.1039/c4fd00164h ◽

2015 ◽

Vol 177 ◽

pp. 363-379 ◽

Cited By ~ 14

Author(s):

A. Marino ◽

M. Buron-Le Cointe ◽

M. Lorenc ◽

L. Toupet ◽

R. Henning ◽

...

Keyword(s):

Spin State ◽

Spin Crossover ◽

Laser Excitation ◽

Intermediate Phase ◽

Concentration Wave ◽

X Ray Diffraction ◽

Time Resolved ◽

X Ray ◽

Equilibrium Dynamics ◽

State Concentration

The spin crossover compound [FeIIH2L2-Me][PF6]2 presents a two-step phase transition. In the intermediate phase, a spin state concentration wave (SSCW) appears resulting from a symmetry breaking (cell doubling) associated with a long-range order of alternating high and low spin molecular states. By combining time-resolved optical and X-ray diffraction measurements on a single crystal, we study how such a system responds to femtosecond laser excitation and we follow in real time the erasing and rewriting of the SSCW.

Download Full-text