Large-Amplitude Anharmonic Decay of Coherent A1g phonon motion in Bismuth

Large amplitude coherent motion of the A1g phonon in bismuth can be generated by ultrafast optical excitation. At low amplitude, the decay rate agrees with that observed in Raman scattering. At high levels of photoexcitation, the observed phonon damping is substantially increased, compared to low-amplitude motion. We present a classical simulation of the anharmonic decay of the phonon, including third-order anharmonic terms in the energy, calculated using density functional perturbation theory, coupling the A1g motion to modes throughout the Brillouin Zone. At low amplitude, the classical decay can be shown in perturbation theory to be almost identical in classical and quantum dynamics at room temperature, demonstrating the validity of a classical simulation of the dynamics. For very large A1g amplitude, the amplitude of motion in the final state modes is substantially increased over their thermal average values, leading to an increase in the decay rate of the A1g mode.