Selective excitation of coherent high-amplitude vibrations of atoms in a solid can induce exotic nonequilibrium states, in which the character of interactions between electronic, magnetic and lattice degrees of freedom is considerably altered and the underlying symmetries are broken. Here we use intense single-cycle terahertz pulses to drive coherently the dipole-active Eu1 phonon mode of a Bi2Se3 crystal. As a result, several Raman-active modes are simultaneously excited in a nonlinear process, while one of them, having the Eg2 symmetry, experiences dynamical splitting during the first two picoseconds after excitation. The corresponding angular scattering pattern is modified indicating coexistence of two phonon modes characteristic of a nonequilibrium state with a lower crystal symmetry. We observe also a short-lived frequency splitting of the original Eg2 mode that immediately after excitation amounts to ∼25% of the unperturbed value. This transient state relaxes with a characteristic time of ∼ 1 ps, that is close to the decay time of the squared amplitude of the resonantly excited infrared-active Eu1 mode. We discuss possible mechanisms of the dynamical splitting: nonlinear lattice deformation caused by the intense Eu1 vibrations and excitation of anisotropic electronic distribution due to nonlinear electron-phonon interaction. Our data also contain an evidence in favor of the sum-frequency Raman mechanism of generation of the coherent Eg2 phonons in Bi2Se3 excited by terahertz pulses.
@article{arxiv.2006.11923,
title = {Phonon-driven ultrafast symmetry lowering in a Bi$_2$Se$_3$ crystal},
author = {A. A. Melnikov and Yu. G. Selivanov and S. V. Chekalin},
journal= {arXiv preprint arXiv:2006.11923},
year = {2020}
}