Activation of New Raman Modes by Inversion Symmetry Breaking in Type II Weyl Semimetal Candidate T′-MoTe2

We synthesized distorted octahedral (T') molybdenum ditelluride (MoTe2) and investigated its-vibrational properties with Raman spectroscopy, density functional, theory, and symmetry analysis. Compared to results from the high temperature centrosymmetric monoclinic (T-mo(')) phase, four new Raman bands emerge in the low-temperature orthorhombic (T'(or)) phase, which was recently predicted to be a type II Weyl semimetal. Crystal-angle-dependent, light-polarization-resolved measurements indicate that all the observed Raman peaks belong to,two categories: those vibrating along the zigzag Mo atomic chain (z-modes) and those vibrating in the mirror plane (m-modes) perpendicular to the zigzag chain. Interestingly, the low energy shear z-mode and shear m-mode, absent from the T'(mo), spectra, become activated when sample cooling induces a phase transition to the T'(or) crystal structure. We interpret this observation as a consequence of inversion-symmetry breaking, which is crucial for the existence of Weyl fermions in the layered crystal. Our temperature-dependent Raman measurements further show that both the high-energy m-mode at similar to 130 cm(-1) and the low-energy shear m-mode at similar to 12 cm(-1) provide useful gauges for monitoring the broken inversion symmetry in the crystal.