Electronic structure of black SmS. I.4d−4fresonance and angle-integrated valence-band photoemission spectroscopy

Abstract

We study the electronic structure of single-crystal ``black'' semiconducting SmS using x-ray absorption and resonance photoemission spectroscopy across the $4d\ensuremath{-}4f$ threshold and high-resolution temperature-dependent valence-band photoemission spectroscopy. The $4d\ensuremath{-}4f$ on-resonance spectra show mixed valency of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ states in semiconducting SmS at low temperature $(T=30 \mathrm{K}).$ The high-resolution spectra show a pseudogap within 20 meV of the Fermi level at low temperatures. This pseudogap is gradually filled up accompanied by a redistribution of spectral weight over a larger energy scale of $\ensuremath{\sim}200 \mathrm{meV}$ on increasing temperature, resulting in an incipient metallic phase at room temperature. The two energy scales can be associated with the coherence temperature ${T}_{c}$ and the Kondo temperature ${T}_{K},$ respectively. The changes in the single-particle density of states at and about the Fermi level are compatible with recent theoretical results on ``exhaustion physics'' in the periodic Anderson model. The present study indicates a Kondo lattice picture for SmS and suggests a relation between the mixed valency and the temperature dependence of the pseudogap for ``black'' semiconducting SmS.