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

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.