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Phonon-driven ultrafast symmetry lowering in a Bi2Se3 crystal
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 E1u 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 E2g symmetry, experiences dynamical splitting during the first two picoseconds after excitation. The corresponding angular scattering pattern is modified indicating the 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 E2g 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 E1u mode. We discuss possible mechanisms of the dynamical splitting: nonlinear lattice deformation caused by the intense E1u 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 E2g phonons in Bi2Se3 excited by terahertz pulses.