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

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 $10\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$ 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 $20\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$ 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 $28\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$, possibly due to less efficient cooling at the surface of the bulk crystal.