Electronic structure of UGe$_2$ at ambient pressure: comparison with X-ray photoemission spectra
Based on experimental crystallographic data, electronic structure of UGe$_2$ have been calculated and compared with our results of X-ray photoelectron spectroscopy (XPS) measurements. We employed two different advanced full potential (FP) methods: FP-local-orbital (FPLO) and FP-linear augmented plane waves (Wien2k) codes for non-magnetic and ferromagnetic states. Starting from the local spin-density approximation (LSDA) or generalised gradient approximation (GGA), we verified either the orbital polarisation (OP) correction or the GGA+U approach for the U 5f-electrons, changing Coulomb-repulsion energies U in the range 0-4 eV. Satisfying agreement was achieved between experimental and our calculated magnetic moments using ab-initio LSDA+OP and non-ab-initio GGA+U approaches, the latter for realistic U values of 2-3 eV. We proved by the LSDA+OP approach an existence of the Fermi surface nesting vector along the a axis, possibly responsible for the triplet superconducting pairing. The calculated data reveal predominantly an itinerant U 5f-electron character of bands near the Fermi level, EF, with only small contributions from the U 6d and Ge 4p states. The experimental XPS spectrum of valence bands (VB) also contains the sharp main 5f-electron peak at EF, a wide hump (around -2 eV), and broad small peaks at higher energies. In the calculated XPS spectrum, the width of the main 5f-electron peak varies between 0.8 and 1.4 eV, depending on a method used in computations, but the hump remains unresolved. A newly observed asymmetric 1-eV satellite in the experimental 4f-core XPS spectrum together with known 3-eV and 7-eV satellites suggest dual behaviour of U-5f-electrons in UGe$_2$, the feature is inferred also from the VB studies.