Spectroscopy of the rare-earth hexaborides: From correlation gaps to colossal magnetoresistance

We present a temperature- and field-dependent Raman scattering study of the "metal-semiconductor" transitions in SmB₆ and EuB₆. Below a characteristic temperature T* ∼ 50 K, the Raman scattering spectrum of SmB₆ is characterized by (i) an abrupt redistribution of electronic scattering intensity across a temperature-independent energy, Δ_c, reflecting the opening of a pseudo-gap, and (ii) the appearance of several in-gap resonances which appear to be crystal-electric-field transitions of the Sm^{3 +} ion which are split by local lattice distortions. By contrast, the metal-semiconductor (MS) transition in EuB₆ manifests itself as a change from a diffusive electronic scattering response in the high-temperature paramagnetic phase, to a flat continuum scattering response in the low-temperature ferromagnetic metal phase which is characteristic of a strongly-correlated metal. Most interesting is evidence that the MS transition in EuB₆ is precipitated by the formation of bound magnetic polarons.