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 phonon-phonon and electron-phonon couplings under photoexcitation. We perform transient reflectivity measurements of bismuth single crystal at 11 K over wide range of absorbed laser fluence up to $F_\text{abs}=9$ mJ/cm$^2$, at which a sign of an irreversible surface damage is observed. At the minimum fluence examined (0.1 mJ/cm$^2$) the coherent $A_{1g}$ and $E_g$ oscillations are a cosine and a sine functions of time, as are consistent with their generations in the displacive and impulsive limits, respectively. With increasing fluence the initial phases of the both modes deviate from their low-fluence values, indicating a finite time required for the transition from the ground-state potential energy surface (PES) to the excited-state one. Surprisingly, the $E_g$ amplitude increases with increasing fluence up to 3 mJ/cm$^2$ and then turns to an apparent decrease, in contrast to the monotonic increase of the $A_{1g}$ amplitude up to 6 mJ/cm$^2$. The contrasted behaviors can be understood by considering a two-dimensional PES, where the strongly driven $A_{1g}$ oscillation leads to a temporal fluctuation of the PES along the $E_g$ coordinate and thereby to a loss in the $E_g$ oscillation coherence at high fluences.