Revealing the Microscopic Mechanism of Displacive Excitation of Coherent Phonons in a Bulk Rashba Semiconductor

Changing the macroscopic properties of quantum materials by optically activating collective lattice excitations has recently become a major trend in solid state physics. One of the most commonly employed light-matter interaction routes is the displacive mechanism. However, the fundamental contribution to this process remains elusive, as the effects of free-carrier density modification and raised effective electronic temperature have not been disentangled yet. Here we use time-resolved pump-probe spectroscopy to address this issue in the Rashba semiconductor BiTeI. Exploring the conventional regime of electronic interband transitions for different excitation wavelengths as well as the barely accessed regime of electronic intraband transitions, we answer a long-standing open question regarding the displacive mechanism: the lattice modes are predominantly driven by the rise of the effective electronic temperature. In the intraband regime, which allows to increase the effective carrier temperature while leaving their density unaffected, the phonon coherence time does not display significant fluence-dependent variations. Our results thus reveal a pathway to displacive excitation of coherent phonons, free from additional scattering and dissipation mechanisms typically associated with an increase of the free-carrier density.