Fully symmetric and doubly degenerate coherent phonons in semimetals at low temperature and high excitation: similarities and differences

We report the ultrafast dynamics of simultaneously excited coherent A(1g) and E-g phonons in semimetals Bi and Sb. At a low level of excitation, the doubly degenerate E-g phonons in pump-probe data occur at low temperature only, and they nearly vanish at room temperature. Their initial phase is shifted by pi/2 with respect to that of the fully symmetric A(1g) phonons indicating that the E-g and A(1g) phonons are excited predominantly impulsively and displacively, respectively. At a high level of excitation, both phonons display an asymmetric (varying in time) line shape with more spectral weight at low frequencies testifying to quantum interference between the one-phonon state and a continuum of states. Furthermore, above a threshold fluence, these phonons of different symmetry exhibit collapse and revival, albeit with different characteristic times.