We study the electronic structure of single-crystal "black" semiconducting SmS using x-ray absorption and resonance photoemission spectroscopy across the 4d-4f threshold and high-resolution temperature-dependent valence-band photoemission spectroscopy. The 4d-4f on-resonance spectra show mixed valency of Sm2+ and Sm3+ states in semiconducting SmS at low temperature (T = 30 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 ∼200 meV on increasing temperature, resulting in an incipient metallic phase at room temperature. The two energy scales can be associated with the coherence temperature Tc and the Kondo temperature TK, 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.
|Number of pages
|Physical Review B - Condensed Matter and Materials Physics
|Published - 2002 Apr 15