Electronic structure of ruthenium-doped iron chalcogenides

The structural and electronic properties of hypothetical Ru$_x$Fe$_{1-x}$Se and Ru$_x$Fe$_{1-x}$Te systems have been investigated from first principles within the density functional theory (DFT). Reasonable values of lattice parameters and chalcogen atomic positions in the tetragonal unit cell of iron chalcogenides have been obtained with the use of norm-conserving pseudopotentials. The well known discrepancies between experimental data and DFT-calculated results for structural parameters of iron chalcogenides are related to the semicore atomic states which were frozen in the used here approach. Such an approach yields valid results of the electronic structures of the investigated compounds. The Ru-based chalcogenides exhibit the same topology of the Fermi surface (FS) as that of FeSe, differing only in subtle FS nesting features. Our calculations predict that the ground states of RuSe and RuTe are nonmagnetic, whereas those of the solid solutions Ru$_x$Fe$_{1-x}$Se and Ru$_x$Fe$_{1-x}$Te become the single- and double-stripe antiferromagnetic, respectively. However, the calculated stabilization energy values are comparable for each system. The phase transitions between these magnetic arrangements may be induced by slight changes of the chalcogen atom positions and the lattice parameters $a$ in the unit cell of iron selenides and tellurides. Since the superconductivity in iron chalcogenides is believed to be mediated by the spin fluctuations in single-stripe magnetic phase, the Ru$_x$Fe$_{1-x}$Se and Ru$_x$Fe$_{1-x}$Te systems are good candidates for new superconducting iron-based materials.

Electronic structure of the 344-type superconductors La3(Ni;Pd)4(Si;Ge)4 by ab initio calculations

Electronic structures of superconducting ternaries: La3Ni4Si4, La3Ni4Ge4, La3Pd4Si4, La3Pd4Ge4, and their non-superconducting counterpart, La3Rh4Ge4, have been calculated employing the full-potential local-orbital method within the density functional theory. Our investigations were focused particularly on densities of states (DOSs) at the Fermi level with respect to previous experimental heat capacity data, and Fermi surfaces (FSs) being very similar for all considered here compounds. In each of these systems, the FS originating from several bands contains both holelike and electronlike sheets possessing different dimensionality, in particular quasi-two-dimensional cylinders with nesting properties. A comparative analysis of the DOSs and FSs in these 344-type systems as well as in nickel (oxy)pnictide and borocarbide superconductors indicates rather similar phonon mechanism of their superconductivity.

Electronic structure of non-centrosymmetric superconductor LaPdSi3 and its reference compound LaPdGe3

Electronic structures of a superconductor without inversion symmetry, LaPdSi3, and its non-superconducting counterpart, LaPdGe3, have been calculated employing the full-potential local-orbital method within the density functional theory. The investigations were focused on analyses of densities of states at the Fermi level in comparison with previous experimental heat capacity data and an influence of the antisymmetric spin-orbit coupling on the band structures and Fermi surfaces (FSs) being very similar for both considered here compounds. Their FSs sheets originate from four bands and have a holelike character, but exhibiting pronounced nesting features only for superconducting LaPdSi3. It may explain a relatively strong electron-phonon coupling in the latter system and its lack in non-superconducting LaPdGe3.

The electronic structure of CeNiGe3 and YNiGe3 superconductors by ab initio calculations

Band structures of pressure-induced CeNiGe3 and exotic BCS-like YNiGe3 superconductors have been calculated employing the full-potential local-orbital code. Both the local density approximation (LDA) and LDA+U treatment of the exchange-correlation energy were used. The investigations were focused on differences between electronic properties of both compounds. Our results indicate that the Ce-based system exhibits higher density of states at the Fermi level, dominated by the Ce 4f states, in contrast to its non f-electron counterpart. The Fermi surface (FS) of each compound originates from three bands and consists of both holelike and electronlike sheets. The specific FS nesting properties of only CeNiGe3 enable an occurrence of spin fluctuations of a helicoidal antiferromagnetic character that may lead to unconventional pairing mechanism in this superconductor. In turn, the topology of the FS in YNiGe3 reveals a possibility of multi-band superconductivity, which can explain the observed anomalous jump at Tc in its specific heat.