Combining semilocal exchange with dynamical mean-field theory: Electronic structure and optical response of rare-earth sesquioxides

In rare-earth semiconductors, wide ligand p and rare-earth 5d bands coexist with localized, partially filled 4f shells. A simultaneous description for both extended and localized states represents a significant challenge for first-principles theories. Here, we combine an ab initio dynamical mean-field theory approach to strong local correlations with a perturbative application of the semilocal modified Becke-Johnson exchange potential to correct the semiconducting gap. We apply this method to calculate the electronic structure and optical response of the light rare-earth sesquioxides R2O3 (R= La, Ce, Pr, and Nd). Our calculations correctly capture a nontrivial evolution of the optical gap in R2O3 due to a progressive lowering of the 4f states along the series and their multiplet structure. 2p - 4f hybridization is found to induce a substantial upward shift for the occupied 4f states occurring within the p - d gap, thus reducing the magnitude of the optical gap. We show that a characteristic plateau observed in the optical conductivity in the Pr and Nd sesquioxides right above their absorption edge is a fingerprint of 4f states located within the p - d gap.