Picosecond strain dynamics in Ge$_{2}$Sb$_{2}$Te$_{5}$ monitored by time-resolved x-ray diffraction

TL;DR

This study investigates ultrafast lattice dynamics in Ge$_{2}$Sb$_{2}$Te$_{5}$ using time-resolved x-ray diffraction to understand coherent phonon effects for potential phase-change applications.

Contribution

It demonstrates the use of 100 ps x-ray pulses to measure coherent phonon-induced lattice distortions in GST, revealing non-thermal effects beyond thermal expansion.

Findings

Measured lattice shifts exceed thermal predictions, indicating non-thermal electron heating.

Linear-chain model describes diffraction changes well, but magnitudes suggest additional effects.

Transient non-equilibrium electron heating influences lattice dynamics.

Abstract

Coherent phonons (CP) generated by laser pulses on the femtosecond scale have been proposed as a means to achieve ultrafast, non-thermal switching in phase-change materials such as Ge$_{2}$Sb$_{2}$Te$_{5}$(GST). Here we use ultrafast optical pump pulses to induce coherent acoustic phonons and stroboscopically measure the corresponding lattice distortions in GST using 100 ps x-ray pulses from the ESRF storage ring. A linear-chain model provides a good description of the observed changes in the diffraction signal, however, the magnitudes of the measured shifts are too large to be explained by thermal effects alone implying the presence of transient non-equilibrium electron heating in addition to temperature driven expansion. The information on the movement of atoms during the excitation process can lead to greater insight into the possibilities of using CP-induced phase-transitions in GST.