Electronic structure of rare-earth pnictides

The results of first-principles calculations of the electronic band structures, equilibrium lattice constants, cohesive energies, bulk moduli, and magnetic moments are presented for the rare-earth pnictides with the rocksalt structure and chemical formula R-V, where R = Gd, Er, and the group-V elements N, P, and As. The linear-muffin-tin-orbital method was used in the atomic sphere approximation. The 4f states were treated as localized corelike states with fixed spin occupancies. Justifications for this procedure are presented. The systems were studied with the 4f spins on all rare-earth ions aligned (ferromagentic phase) and with the spins randomly oriented (paramagentic phase). Within the local spin-density approximation, all systems studied were found to be semimetallic with a hole section of the Fermi surface near Gamma and electron section near X. The nitrides, however, have a nearly zero band-gap overlap. We estimated quasiparticle self-energy corrections using an approach previously used for semiconductors. With these corrections, GdN is found to be a semiconductor in the paramagnetic phase and a semimetal in the ferromagnetic phase. ErN, on the other hand, is found to be a semiconductor in both phases. All systems correspond to a trivalent state of the rare-earth element and are characterized by ionic bonding. The results for the lattice constants and the qualitative conclusion about the semimetallic nature are in agreement with experimental data and with the previous calculations for Gd-pnictides. For ErAs, the calculated magnetic exchange splittings, electron and hole concentrations, Fermi-surface cross-sectional areas, and cyclotron masses are in satisfactory agreement with the available Shubnikov-de Haas data on ErxSc1-xAs when account is taken of the differences due to the presence of Sc and of the self-energy corrections to the local-density approximation.