Raman scattering and far-infrared studies of the vanadium dihalides with layered structure

The vanadium dihalides with layered structure, VCl₂, VBr₂, and VI₂, have been investigated by Raman and infrared spectroscopy. A compilation of the measured optical frequencies is given. The new lines appearing in the Raman spectrum of VI₂ below its N\'eel temperature TN are attributed to zone-boundary-phonon Raman scattering induced by Bragg scattering from the spin superstructure. We discuss the symmetry properties and selection rules for the proposed spin-dependent electron-phonon coupling mechanism due to a phonon modulation of the exchange interaction. The measured two-phonon absorption spectra of the three materials show a shift to lower frequencies with respect to the calculated two-phonon density of states. Some possible reasons for this unusually large shift are discussed. The Raman spectra of VCl₂ and VBr₂ show additional features at low temperature, different from the sharp lines observed in VI₂ below TN. This additional scattering is also shifted to lower frequencies compared to the calculated one-phonon density of states. We explain the appearance of the additional Raman scattering by a spin-phonon coupling via the exchange modulation mechanism, whereas the shift in the Raman spectra should be due to a self-energy renormalization by the coupling mechanism.