Ab initio probing of the electronic band structure and Fermi surface of fluorine-doped WO3 as a novel low-TC superconductor

First-principles calculations were performed to investigate the electronic structure and the Fermi surface of the newly discovered low-temperature superconductor: fluorine-doped WO3. We find that F doping provides the transition of the insulating tungsten trioxide into a metallic-like phase WO3-xFx, where the near-Fermi states are formed mainly from W 5d with admixture of O 2p orbitals. The cooperative effect of fluorine additives in WO3 consists in change of electronic concentration as well as the lattice constant. At probing their influence on the near-Fermi states separately, the dominant role of the electronic factor for the transition of tungsten oxyfluoride into superconducting state was established. The volume of the Fermi surface gradually increases with the increase of the doping. In the sequence WO3 \rightarrow WO2.5F0.5 the effective atomic charges of W and O ions decrease, but much less, than it is predicted within the idealized ionic model - owing to presence of the covalent interactions W-O and W-F.Comment: 8 pages, 4 figure

Elastic properties of mono- and polycrystalline hexagonal AlB2-like diborides of s, p and d metals from first-principles calculations

We have performed accurate ab initio total energy calculations using the full-potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange-correlation potential to systematically investigate elastic properties of 18 stable, meta-stable and hypothetical hexagonal (AlB2-like) metal diborides MB2, where M = Na, Be, Mg, Ca, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ag and Au. For monocrystalline MB2 the optimized lattice parameters, independent elastic constants (Cij), bulk modules (B), shear modules (G) are obtained and analyzed in comparison with the available theoretical and experimental data. For the first time numerical estimates of a set of elastic parameters of the polycrystalline MB2 ceramics (in the framework of the Voigt-Reuss-Hill approximation), namely bulk and shear modules, compressibility, Young's modules, Poisson's ratio, Lame's coefficients are performed.Comment: 24 pages, 3 figure

Electronic structure of new quaternary superconductors LaONiBi and LaOCuBi from first principles

Based on first-principles FLAPW-GGA calculations, we have investigated the electronic structure of newly synthesized novel superconductors LaONiBi and LaOCuBi, the first bismuth-containing compounds from the family of quaternary oxypnictides which attract now a great deal of interest in search for novel 26-55K superconductors. The band structure, density of states and Fermi surfaces are discussed. Our results indicate that the bonding inside of the (La-O) and (Ni(Cu)-Bi) layers is covalent whereas the bonding between the (La-O) and (Ni(Cu)- Bi) blocks is mostly ionic. For both oxybismuthides, the DOSs at the Fermi level are formed mainly by the states of the (Ni(Cu)-Bi) layers, the corresponding Fermi surfaces have a twodimensional character and the conduction should be strongly anisotropic andhappen only on the (Ni(Cu)-Bi) layers. As a whole, the new oxybismuthides may be described as low-TC superconducting non-magnetic ionic metals.Comment: 13 pages, 5 figure

BN domains included into carbon nanotubes: role of interface

We present a density functional theory study on the shape and arrangement of small BN domains embedded into single-walled carbon nanotubes. We show a strong tendency for the BN hexagons formation at the simultaneous inclusion of B and N atoms within the walls of carbon nanotubes. The work emphasizes the importance of a correct description of the BN-C frontier. We suggest that BN-C interface will be formed preferentially with the participation of N-C bonds. Thus, we propose a new way of stabilizing the small BN inclusions through the formation of nitrogen terminated borders. The comparison between the obtained results and the available experimental data on formation of BN plackets within the single walled carbon nanotubes is presented. The mirror situation of inclusion of carbon plackets within single walled BN nanotubes is considered within the proposed formalism. Finally, we show that the inclusion of small BN plackets inside the CNTs strongly affects the electronic character of the initial systems, opening a band gap. The nitrogen excess in the BN plackets introduces donor states in the band gap and it might thus result in a promising way for n-doping single walled carbon nanotubes

Ginzburg-Landau theory of vortices in a multi-gap superconductor

The Ginzburg-Landau functional for a two-gap superconductor is derived within the weak-coupling BCS model. The two-gap Ginzburg-Landau theory is, then, applied to investigate various magnetic properties of MgB2 including an upturn temperature dependence of the transverse upper critical field and a core structure of an isolated vortex. Orientation of vortex lattice relative to crystallographic axes is studied for magnetic fields parallel to the c-axis. A peculiar 30-degree rotation of the vortex lattice with increasing strength of an applied field observed by neutron scattering is attributed to the multi-gap nature of superconductivity in MgB2.Comment: 11 page

Gap symmetry and structure of Fe-based superconductors

The recently discovered Fe-pnictide and chalcogenide superconductors display low-temperature properties suggesting superconducting gap structures which appear to vary substantially from family to family, and even within families as a function of doping or pressure. We propose that this apparent nonuniversality can actually be understood by considering the predictions of spin fluctuation theory and accounting for the peculiar electronic structure of these systems, coupled with the likely 'sign-changing s-wave' (s\pm) symmetry. We review theoretical aspects, materials properties and experimental evidence relevant to this suggestion, and discuss which further measurements would be useful to settle these issues.Comment: 86 pages, revie

Band structure of SrFeAsF and CaFeAsF as parent phases for a new group of oxygen-free FeAs superconductors

By means of first-principle FLAPW-GGA calculations, we have investigated the electronic properties of the newly discovered layered quaternary systems SrFeAsF and CaFeAsF as parent phases for a new group of oxygen-free FeAs superconductors. The electronic bands, density of states, Fermi surfaces, atomic charges, together with Sommerfeld coefficients and molar Pauli paramagnetic susceptibility have been evaluated and discussed in comparison with oxyarsenide LaFeAsO - a parent phase for a new class of high-temperature (Tc about 26-56K) oxygen-containing FeAs superconductors. Similarity of our data for SrFeAsF and CaFeAsF with the band structure of oxygen-containing FeAs superconducting materials may be considered as theoretical background specifying the possibility of superconductivity in these oxygen-free systems.Comment: 12 pages, 3 figure

Structural, Magnetic and Electronic Properties of the Iron-Chalcogenide Ax_xFe2−y_{2-y}Se2_2 (A=K, Cs, Rb, Tl and etc.) Superconductors

The latest discovery of a new iron-chalcogenide superconductor A$_x$Fe$_{2-y}$Se$_2$(A=K, Cs, Rb, Tl and etc.) has attracted much attention due to a number of its unique characteristics, such as the possible insulating state of the parent compound, the existence of Fe-vacancy and its ordering, a new form of magnetic structure and its interplay with superconductivity, and the peculiar electronic structures that are distinct from other Fe-based superconductors. In this paper, we present a brief review on the structural, magnetic and electronic properties of this new superconductor, with an emphasis on the electronic structure and superconducting gap. Issues and future perspectives are discussed at the end of the paper.Comment: 45 pages, 19 figure