Ultrafast dynamics of the low frequency shear phonon in 1T’−MoTe₂

We report on the dynamics of coherent phonons in semimetal 1T’−MoTe₂ using femtosecond pump−probe spectroscopy. On an ultrafast sub−picosecond time scale at room temperature, a low frequency and long−lifetime shear phonon mode was observed at 0.39 THz, which was previously reported in the form of a characteristic phonon only in the low temperature T_d − MoTe2 phase. Unlike the other optical phonon modes, the shear phonon mode was found to strongly couple with photoexcited carriers. Moreover, the amplitude of the shear mode surprisingly decreased with increasing excitation density, a phenomenon which can be attributed to be a consequence of the lattice temperature rising after excitation. These results provide useful physical information on ultrafast lattice symmetry switching between the normal semimetal 1T’ and the lattice inversion symmetry breaking Type−II Weyl semimetal T_d phases.