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

We comparably performed polarized Raman scattering studies of $\mathrm{MoT}{\mathrm{e}}_{2}$ and $\mathrm{WT}{\mathrm{e}}_{2}$. 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 $(10\phantom{\rule{0.16em}{0ex}}{A}_{g}+5\phantom{\rule{0.16em}{0ex}}{B}_{g})$ in the high-symmetry phase $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{MoT}{\mathrm{e}}_{2}$ (300 K) were well assigned, and all the symmetry-allowed Raman modes $(11\phantom{\rule{0.16em}{0ex}}{A}_{1}+6\phantom{\rule{0.16em}{0ex}}{A}_{2})$ in the low-symmetry phase ${T}_{d}\text{\ensuremath{-}}\mathrm{MoT}{\mathrm{e}}_{2}$ (10 K) and 12 Raman phonons $(8\phantom{\rule{0.16em}{0ex}}{A}_{1}+4\phantom{\rule{0.16em}{0ex}}{A}_{2})$ in ${T}_{d}\text{\ensuremath{-}}\mathrm{WT}{\mathrm{e}}_{2}$ 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.