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>TC in the europium chalcogenides is characterized by (1) the occurrence of a peak in the scattering intensity at 0 where TO<0<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 TC, (6) a quenching of the scattered intensity by application of an external magnetic field when the temperature is close to but greater than TC, and (7) the observation of harmonic structures at multiples of 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>TC by considering a model proposed by Williams and Smit. © 1974 The American Physical Society.