Ultrafast scattering dynamics of coherent phonons in Bi1−x Sb x in the Weyl semimetal phase

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 _1 _g 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.