Electron cooling and Debye-Waller effect in photoexcited bismuth

TL;DR

This study combines first principles calculations and thermodynamical modeling to analyze electron cooling and Debye-Waller effects in photoexcited bismuth, successfully reproducing experimental diffraction data.

Contribution

It introduces a detailed model linking electron-phonon coupling, electron temperature, and Bragg peak dynamics in bismuth, highlighting the importance of optical mode softening.

Findings

Electron-phonon coupling G_0 increases with electron temperature.

The model accurately reproduces femtosecond diffraction experiments.

Debye-Waller effects are crucial for understanding Bragg peak evolution.

Abstract

By means of first principles calculations, we computed the effective electron-phonon coupling constant $G_0$ governing the electron cooling in photoexcited bismuth. $G_0$ strongly increases as a function of electron temperature, which can be traced back to the semi-metallic nature of bismuth. We also used a thermodynamical model to compute the time evolution of both electron and lattice temperatures following laser excitation. Thereby, we simulated the time evolution of (1 -1 0), (-2 1 1) and (2 -2 0) Bragg peak intensities measured by Sciaini et al [Nature 458, 56 (2009)] in femtosecond electron diffraction experiments. The effect of the electron temperature on the Debye-Waller factors through the softening of all optical modes across the whole Brillouin zone turns out to be crucial to reproduce the time evolution of these Bragg peak intensities.