Measurements of Nonequilibrium Interatomic Forces in Photoexcited Bismuth

TL;DR

This study experimentally measures how interatomic forces in bismuth change upon photoexcitation, revealing force weakening along bonds that influences lattice dynamics and Peierls distortion reversal.

Contribution

It provides the first detailed experimental determination of excited-state interatomic forces in bismuth using femtosecond x-ray scattering and a Born-von Karman model.

Findings

Photoexcitation causes softening of optical and acoustic phonon modes.

Weakening of three nearest neighbor forces along bonding directions.

Results clarify the mechanisms behind Peierls distortion reversal.

Abstract

We determine experimentally the excited-state interatomic forces in photoexcited bismuth. The forces are obtained by a constrained least-squares fit of the excited-state dispersion obtained by femtosecond time-resolved x-ray diffuse scattering to a fifteen-nearest neighbor Born-von Karman model. We find that the observed softening of the zone-center $A_{1g}$ optical mode and transverse acoustic modes with photoexcitation are primarily due to a weakening of three nearest neighbor forces along the bonding direction. This provides a more complete picture of what drives the partial reversal of the Peierls distortion previously observed in photoexcited bismuth.