Spin-disorder-induced Raman scattering from phonons in europium chalcogenides. I. Experiment

The symmetry-forbidden first-order Raman scattering in the paramagnetic phase of europium chalcogenides has been analyzed in terms of the Raman tensor components, their resonance enhancement, and the phonons involved. The most dominant scattering contribution is due to the antisymmetric ${\Gamma }_{15}^{+}$ Raman tensor component. On the basis of the phenomenological theory of spin-dependent Raman scattering from phonons, the ${\Gamma }_{15}^{+}$ component is a direct proof of simultaneous one-phonon one-spin excitations. Less intense but more resonantly enhanced contributions from the symmetric ${\Gamma }_{12}^{+}$ and ${\Gamma }_{25}^{+}$ components are due to one-phonon two-spin excitations. The anion-mass-dependent peak position of the first-order Raman scattering in the paramagnetic phase has been attributed to a locally full symmetric ($4f$) hole-phonon coupling, involving zone-boundary optic phonons. The associated breakdown in $\overrightarrow{\mathrm{k}}$-selection rule originates from the disorder of the spin system in the paramagnetic phase. A proposed model for the scattering process under optical $4f^7\to 4f^{6}5d^1$ excitation predicts a transition from one-phonon zero-frequency spin excitations in the paramagnetic phase to one-phonon one-magnon excitations in the magnetically ordered phase, consistent with our experimental observations.