Correlated electronic structure and optical response of rare-earth based semiconductors

Abstract

The coexistence of Mott localized $f$ states with wide conduction and valence bands in $f$-electron semiconductors results, quite generically, in a complex optical response with the nature of the absorption edge difficult to resolve both experimentally and theoretically. Here, we combine a dynamical mean-field theory approach to localized $4f$ shells with an improved description of band gaps by a semilocal exchange-correlation potential to calculate the optical properties of the light rare-earth fluorosulfides $Ln\mathrm{SF}$ ($Ln=\mathrm{Pr}$, Nd, Sm, Gd) from first principles. In agreement with experiment, we find the absorption edge in SmSF to stem from $\mathrm{S}\text{\ensuremath{-}}3p$ to $\mathrm{Sm}\text{\ensuremath{-}}4f$ transitions, while the Gd compound behaves as an ordinary $p\text{\ensuremath{-}}d$ gap semiconductor. In the unexplored PrSF and NdSF systems we predict a rather unique occurrence of strongly hybridized $4f\text{\ensuremath{-}}5d$ states at the bottom of the conduction band. The nature of the absorption edge results in a characteristic anisotropy of the optical conductivity in each system, which may be used as a fingerprint of the relative energetic positions of different states.