Direct observation of electron thermalization and electron-phonon coupling in photoexcited bismuth

TL;DR

This study uses time-resolved photoemission spectroscopy to directly observe electron thermalization and electron-phonon coupling in photoexcited bismuth, revealing fluence-dependent electron dynamics and differences between surface and bulk states.

Contribution

It provides the first direct visualization of electron-electron and electron-phonon interactions in bismuth, highlighting state-specific coupling and deviations from traditional Fermi liquid behavior.

Findings

Electron thermalization depends on fluence and can take hundreds of femtoseconds.

Surface states are less coupled to the A1g phonon mode than bulk states.

Data aligns qualitatively with Fermi liquid theory but shows notable deviations.

Abstract

We investigate the ultrafast response of the bismuth (111) surface by means of time resolved photoemission spectroscopy. The direct visualization of the electronic structure allows us to gain insights on electron-electron and electron-phonon interaction. Concerning electron-electron interaction, it is found that electron thermalization is fluence dependent and can take as much as several hundreds of femtoseconds at low fluences. This behavior is in qualitative agreement with Landau's theory of Fermi liquids but the data show deviations from the behavior of a common 3D degenerate electron gas. Concerning electron-phonon interaction, our data allows us to directly observe the coupling of individual Bloch state to the coherent $A_{1g}$ mode. It is found that surface states are much less coupled to this mode when compared to bulk states. This is confirmed by \textit{ab initio} calculations of surface and bulk bismuth.