Suppression of shear ionic motions in bismuth by coupling with large-amplitude internal displacement

Abstract

Bismuth, with its rhombohedral crystalline structure and two Raman active phonon modes corresponding to the internal displacement ($A_{1g}$) and shear ($E_g$) ionic motions, offers an ideal target for the investigation of the nonequilibrium phonon-phonon and electron-phonon couplings. We investigate the $E_g$ phonon dynamics under intense photoexcitation by performing anisotropic transient reflectivity (TR) measurements on a 1-mm-thick bismuth single crystal at 11 K. The amplitude of the coherent $E_g$ phonon is found to increase with incident pump fluence up to $10\mathrm{mJ}/{\mathrm{cm}}^2$ and then turns to an apparent decrease. This behavior is in stark contrast to the amplitude of the $A_{1g}$ phonon in standard TR measurements, which increases monotonically up to $20\mathrm{mJ}/{\mathrm{cm}}^2$ and then saturates. The contrasting behaviors of the two phonon modes can be interpreted in terms of the strong coupling of the $E_g$ oscillation with large-amplitude $A_{1g}$ displacement in a highly excited electronic state, where dynamic fluctuation of the vibrational potential will lead to a quick loss in the $E_g$ vibrational coherence. Unlike previous studies on thin Bi films on substrates we observe no sign of a transition to a high-symmetry phase and instead observe signs of partial damage of the crystalline surface at $28\mathrm{mJ}/{\mathrm{cm}}^2$, possibly due to less efficient cooling at the surface of the bulk crystal.