Effects of Y substitution on the electronic structure and charge dynamics of SmS

Abstract

Evolution of the electronic state by Y doping was explored by optical conductivity covering a wide energy and compositional region deduced from systematic reflectivity measurements of ${\mathrm{Sm}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}\mathrm{S}$ single crystals. Although electrons doped onto the $5d$ band by Y dopants immediately realize a metallic state, the charge dynamics do not obey a simple Drude response and the spectral intensity is condensed in the mid-infrared region. Contrary to the pressure-induced case, a metallic phase appears in a certain compositional region before reaching the valence transition. The Fermi level is shifted upwards by electron doping. Therefore, the $5d$ band must overlap with the weakly dispersive $4f$ band more deeply than in the pressure-induced case to reach the valence transition, which can explain the stability of the divalent phase up to higher temperatures in the doped system than in the pressure-induced case. The present optical study provides us with a strategy that sulfur deficiency might increase operating temperature of negative thermal expansion in this system.