Abnormal Raman Scattering in Hexagonal FeTe by Symmetry Breaking

We performed experimental and theoretical investigations into the Raman optical phonon modes of hexagonal FeTe (h-FeTe) nanosheets, which is a two-dimensional yet nonlayered ferromagnetic material. Prior studies have reported two Raman peaks at around 120 and 138 cm–1. Our helicity-dependent Raman experiments reveal two novel features: (i) the peak at 138 cm–1 is, in fact, a composite of two closely spaced modes located at 137 and 142 cm–1, and (ii) an additional Raman mode appears at 158 cm–1. In our theoretical analysis, however, the three upper modes at 137, 142, and 158 cm–1 are inherently Raman-inactive within the symmetry of the h-FeTe bulk structure. We discuss how these Raman-inactive modes become active through the breaking of hexagonal and inversion symmetry.