Electronic structure and Fermi surface ofUNZ(Z=Seand Te) byab initiocalculations

Abstract

The electronic structures of ferromagnetic (FM) UNTe and its nonmagnetically ordered (NMO) isostructural (tetragonal $P4/nmm$) and isoelectronic counterpart, UNSe, have been calculated from first principles in the framework of the fully relativistic and full-potential local-orbital band-structure code within local-spin density approximation (LSDA) including also an orbital polarization correction by Eriksson, Brooks, and Johansson (OPB). The results predict that both ternaries have a covalently metallic character and solely uranium atoms, located in (001) planes, form a metallic bond due to the $\text{U}\text{ }5f\text{\ensuremath{-}}6d$ electrons. The $\text{U}\text{ }5f$ electrons contribute also to a covalent bond with the ligand N and Te or Se atoms and they reveal a dual character, i.e., partly localized and itinerant. Contrary to UNSe, UNTe is a collinear FM with the magnetic moment alignment along the $c$ axis, as observed experimentally in the past and now is well reproduced by the $\text{LSDA}+\text{OPB}$ calculations. In NMO states of both systems, band pseudogaps are opening merely $\ensuremath{\sim}0.25\text{ }\text{eV}$ below the Fermi level, which cause an instability of the metallic state under small perturbations leading to a semiconducting behavior. The two-band Fermi surfaces (FSs) of both compounds (in NMO state) have similar quasi-two-dimensional (Q2D) properties with nesting vectors along the [100] direction. In turn, UNTe in the FM state possesses three-band FS with also Q2D properties and nesting features along the [100] and [110] directions, being important, e.g., in arising such collective phenomena as superconductivity.