Raman scattering in the transition-metal dichalcogenides of 1T′−MoTe2,Td−MoTe2, and Td−WTe2

We performed comparable polarized Raman scattering studies of MoTe2 and WTe2.\nBy rotating crystals to tune the angle between the principal axis of the\ncrystals and the polarization of the incident/scattered light, we obtained the\nangle dependence of the intensities for all the observed modes, which is\nperfectly consistent with careful symmetry analysis. Combining these results\nwith first-principles calculations, we clearly identified the observed phonon\nmodes in the different phases of both crystals. Fifteen Raman-active phonon\nmodes (10Ag+5Bg) in the high-symmetry phase 1T'-MoTe2 (300 K) were well\nassigned, and all the symmetry-allowed Raman modes (11A1+6A2) in the\nlow-symmetry phase Td-MoTe2 (10 K) and 12 Raman phonons (8A1+4A2) in Td-WTe2\nwere observed and identified. The present work provides basic information about\nthe lattice dynamics in transition-metal dichalcogenides and may shed some\nlight on the understanding of the extremely large magnetoresistance (MR) in\nthis class of materials.\n