Ultrafast scattering dynamics of coherent phonons in Bi$_{1-x}$Sb$_{x}$ in the Weyl semimetal phase

TL;DR

This study explores ultrafast coherent phonon dynamics in Bi$_{1-x}$Sb$_{x}$ alloys, revealing composition-dependent phonon frequency shifts and significantly reduced relaxation times in certain compositions, influenced mainly by anharmonic phonon-phonon interactions.

Contribution

It provides new insights into phonon relaxation mechanisms in Bi$_{1-x}$Sb$_{x}$ alloys, especially in the Weyl semimetal phase, highlighting the role of anharmonic phonon-phonon scattering.

Findings

Phonon frequencies vary with Sb composition.

Phonon relaxation times decrease dramatically for x between 0.5 and 0.8.

Anharmonic phonon-phonon scattering dominates phonon relaxation.

Abstract

We investigate ultrafast phonon dynamics in the Bi$_{1-x}$Sb$_{x}$ alloy system for various compositions $x$ using a reflective femtosecond pump-probe technique. The coherent optical phonons corresponding to the A$_{1g}$ local vibrational modes of Bi-Bi, Bi-Sb, and Sb-Sb are generated and observed in the time domain with a few picoseconds dephasing time. The frequencies of the coherent optical phonons were found to change as the Sb composition $x$ was varied, and more importantly, the relaxation time of those phonon modes was dramatically reduced for $x$ values in the range 0.5--0.8. We argue that the phonon relaxation dynamics are not simply governed by alloy scattering, but are significantly modified by anharmonic phonon-phonon scattering with implied minor contributions from electron-phonon scattering in a Weyl-semimetal phase.