Defect-activated versus intrinsic Raman spectra of GdN and LuN

We report Raman studies of GdN and LuN, two of the rare-earth mononitrides that form a series of mostly intrinsic ferromagnetic semiconductors. Their rock-salt structure supports no first-order Raman scattering, but, nonetheless, we find a density-of-modes pattern expected for nearly point-defect scattering, an intrinsic two-phonon scattering spectrum, and an LO feature that appears independent of defect concentration. The results are compared with the phonon dispersion calculated within a density functional theory treatment indicating that computed phonon frequencies are smaller than measured, with the disagreement rising to $\ensuremath{\sim}30%$ for the TA branch. Contrary to the previous assumption of defect-induced scattering at the $L$ point, we propose that the observed LO feature arises from $\mathrm{LO}(\mathrm{\ensuremath{\Gamma}}$) scattering induced by the Fr\"ohlich interaction, similar to what has been observed in YbS.