Direct observation of non-fully-symmetric coherent optical phonons by femtosecond x-ray diffraction

TL;DR

This study uses femtosecond x-ray diffraction to directly observe non-fully-symmetric coherent optical phonons in bismuth, revealing their small amplitude and ruling out certain coupling scenarios.

Contribution

First direct measurement of E$_g$ symmetry coherent phonons in bismuth using femtosecond x-ray diffraction, providing new insights into phonon dynamics.

Findings

E$_g$ mode displacement is 0.2 pm peak-to-peak

E$_g$ motion is much smaller than A$_{1g}$ mode

Results rule out strong coupling scenario for phonon dynamics

Abstract

We directly measure by femtosecond time-resolved x-ray diffraction the E$_g$ symmetry coherent phonon excited in bismuth by strong optical excitation. The magnitude of the E$_g$ mode observed is 0.2 pm peak-to-peak, compared against the 2.7 pm initial displacement of the fully-symmetric A$_{1g}$ mode. The much smaller motion of the E$_g$ mode is a consequence of the short lifetime of the electronic states that drive the atomic motion. The experimentally measured magnitude of the E$_g$ motion allows us to rule out a previously suggested scenario for explaining the dynamics in bismuth that relies on strong coupling between these modes.