Inelastic light scattering in the europium chalcogenides

Abstract

Anomalous, symmetry-forbidden first-order Raman scattering has been observed in the cubic magnetic semiconductors EuO, EuS, EuSe, and EuTe. The scattering observed for $T>{T}_{C}$ in the europium chalcogenides is characterized by (1) the occurrence of a peak in the scattering intensity at ${\ensuremath{\omega}}_{0}$ where ${\ensuremath{\omega}}_{\mathrm{TO}}<{\ensuremath{\omega}}_{0}<{\ensuremath{\omega}}_{\mathrm{LO}}$, (2) a broad linewidth corresponding to the frequency range of the LO phonons throughout the Brillouin zone, (3) a lack of polarization sensitivity of the spectrum, (4) a lack of sensitivity to whether the sample is electrically conducting or insulating, (5) a quenching of the scattered intensity when the temperature is lowered below ${T}_{C}$, (6) a quenching of the scattered intensity by application of an external magnetic field when the temperature is close to but greater than ${T}_{C}$, and (7) the observation of harmonic structures at multiples of ${\ensuremath{\omega}}_{0}$ in EuS and EuSe. The properties of this scattering cannot be explained by conventional models for symmetry-forbidden Raman scattering. A new light-scattering mechanism is proposed, in which the spin disorder lifts the translational and inversion symmetries of the electronic wave functions in these materials. With this spin-disorder mechanism it is possible to explain qualitatively many of the observed features of this broad-line Raman scattering. It is also possible to account for the relative intensities of the higher harmonics of the broad-line scattering observed in EuS and EuSe for $T>{T}_{C}$ by considering a model proposed by Williams and Smit.