Electronic structure of rare-earth mononitrides: quasiatomic excitations and semiconducting bands

The electronic structure of the rare-earth mononitrides Ln N (where Ln = rare-earth), which are promising materials for future spintronics applications, is difficult to resolve experimentally due to a strong influence of defects on their transport and optical properties. At the same time, Ln N are challenging for theory, since wide semiconducting 2 p and 5 d bands need to be described simultaneously with strongly correlated 4 f states. Here, we calculate the many-body spectral functions and optical gaps of a series of Ln N (with Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er) by a density-functional + dynamical mean-field theory (DFT + DMFT) approach treating the correlated Ln 4 f shells within the quasi-atomic Hubbard-I approximation. The on-site Coulomb interaction in the 4 f shell is evaluated by a constrained DFT + Hubbard-I approach. Furthermore, to improve the treatment of semiconducting bands in DFT + DMFT, we employ the modified Becke–Johnson semilocal exchange potential. Focusing on the paramagnetic high-temperature phase, we find that all investigated Ln N are pd semiconductors with gap values ranging from 1.02 to 2.14 eV along the series. The pd band gap is direct for light Ln = La…Sm and becomes indirect for heavy rare-earths. Despite a pronounced evolution of the Ln 4 f states along the series, empty 4 f states are invariably found above the bottom of the 5 d conduction band. The calculated spectra agree well with those available from x-ray photoemission, x-ray emission and x-ray absorption measurements.