Polarized Raman Spectroscopy of 2H-NbSe₂ and 1T-VSe₂ Single Crystals

We present the polarized Raman spectra of 2H-NbSe (Formula presented.) and 1T-VSe (Formula presented.) single crystals. The phonon symmetries of the first-order Raman-active modes in both materials are identified using linearly and circularly polarized Raman spectroscopy. In particular, the Raman peak of 1T-VSe (Formula presented.) at 250 cm (Formula presented.), previously assigned to the E (Formula presented.) mode, is reinterpreted as two-phonon combinational modes at the M point in the Brillouin zone. The helicity-dependent Raman spectroscopy enables the separation of the overlapping A (Formula presented.) and E (Formula presented.) modes in 2H-NbSe (Formula presented.). The second-order phonon modes exhibit either helicity-conserving or helicity-changing behavior. The first-principles calculations of phonon dispersion and the two-phonon density of states support the assignment of the two-phonon modes. The calculated shear and layer breathing modes of 2H-NbSe (Formula presented.) appear at 37 and 53 cm (Formula presented.), respectively, while the calculated E (Formula presented.) mode of 1T-VSe (Formula presented.) appears at 139 cm (Formula presented.).