Raman scattering in the transition-metal dichalcogenides of 1T'-MoTe2, Td-MoTe2, and Td-WTe2

We comparably performed polarized Raman scattering studies of MoTe2 and WTe2. By rotating crystals to tune the angle between the principal axis of the crystals and the polarization of the incident/scattered light, we obtained the angle dependence of the intensities for all the observed modes, which is perfectly consistent with careful symmetry analysis. Combining these results with first-principles calculations, we clearly identified the observed phonon modes in the different phases of both crystals. Fifteen Raman-active phonon modes (10 A(g) + 5 B-g) in the high-symmetry phase 1T' -MoTe2 (300 K) were well assigned, and all the symmetry-allowed Raman modes (11 A(1) + 6 A(2)) in the low-symmetry phase T-d-MoTe2 (10 K) and 12 Raman phonons (8 A(1) + 4 A(2)) in T-d-WTe2 were observed and identified. The present paper provides basic information about the lattice dynamics in transition-metal dichalcogenides and may shed some light on the understanding of the extremely large magnetoresistance in this class of materials.