[en] We investigate the adsorption of a Ni monolayer on the β-SiC (001) surface by means of highly precise first-principles all-electron full-potential linearized augmented plane wave calculations. Total-energy calculations for the Si- and C-terminated surfaces reveal high Ni-SiC adsorption energies, with respect to other metals, confirmining the strong reactivity and the stability of the transition metal/SiC interface. These high binding energies, about 7.3--7.4 eV, are shown to be related to strong p-d hybridization, common to both surface terminations and different adsorption sites and despite the large mismatch, can stabilize overlayer growth. A detailed analysis of the bonding mechanism, hybridization of the surface states, charge transfer, and surface core level shifts reveals the strong covalent character of the bonding. After a proper accounting of the Madelung term, the core-level shift is shown to follow the charge-transfer trend

[en] The chemical, electronic and magnetic properties of Co super-soft silicides are investigated by means of all-electron density functional calculations. From the study of the electronic properties in terms of chemical bond, band structure and density of states we are able to give relevant information regarding the stability of the non-magnetic and magnetic phases of this compound and the possible structural modifications that can be stabilized. In addition, we studied the super-soft behaviour for both the paramagnetic and ferromagnetic phases. A parallel study on the free surface properties of CoSi shows enhanced magnetic moments due to the reduced dimensionality

[en] The use of lattice cells in real space that are arbitrarily larger than the primitive one, is nowadays more and more often required by ab initio calculations to study disorder, vacancy or doping effects in real materials. This leads, however, to complex band structures which are hard to interpret. Therefore an unfolding procedure is sought for in order to obtain useful data, directly comparable with experimental results, such as angle-resolved photoemission spectroscopy measurements. Here, we present an extension of the unfolding procedure recently implemented in the VASP code, which includes a projection scheme that leads to a full reconstruction of the primitive space. As a test case, we apply this newly implemented scheme to the Ru-doped BaFe_2As_2 superconducting compound. The results provide a clear description of the effective electronic band structure in the conventional Brillouin zone, highlighting the crucial role played by doping in this compound. (paper)

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No abstract available

[en] Superconductivity in Pb, H under extreme pressure and CaBeSi, in the framework of the density functional theory for superconductors, is discussed. A detailed analysis on how the electron-phonon and electron-electron interactions combine together to determine the superconducting gap and critical temperature of these systems is presented. Pb, H under pressure and CaBeSi are multigap superconductors. We will address the question under which conditions does a system exhibits this phenomenon. The presented results contribute to the understanding of multiband and anisotropic superconductivity, which has received a lot of attention since the discovery of MgB₂, and show how it is possible to describe the superconducting properties of real materials on a fully ab initio basis.

[en] We present first-principles calculations on the normal and superconducting state of CaBe_xSi_2-x (x=1), in the framework of the density functional theory for superconductors (SCDFT). CaBeSi is isostructural and isoelectronic to MgB₂ and this makes possible a direct comparison of the electronic and vibrational properties and the electron-phonon interaction of the two materials. Although our calculations show that CaBeSi has a low critical temperature (T_c∼0.6 K), it exhibits a complex gap structure, with three gaps at the Fermi level. Besides the two σ and π gaps, present also in MgB₂, the appearance of a third gap is related to the anisotropy of the Coulomb repulsion, acting in different way on the bonding and antibonding electronic π states

[en] Superconductivity in intercalated graphite CaC₆ and H under extreme pressure is discussed in the framework of superconducting density functional theory. A detailed analysis of how the electron-phonon and electron-electron interactions combine together to determine the superconducting gap and critical temperature (T_c) of these systems is presented. In particular, we discuss the effect on the calculated T_c of the anisotropy of the electron-phonon interaction and of the different approximations for screening the Coulomb repulsion. These results contribute to the understanding of multigap and anisotropic superconductivity, which has received a lot of attention since the discovery of MgB₂, and show how it is possible to describe the superconducting properties of real materials on a fully ab initio basis.

[en] We report a detailed first-principles local-density-functional investigation of the structural, electronic, dynamical and superconducting properties of MgB₂ focusing on different aspects related to this material. In particular, we examine Al doping, as well as reduced dimensionality and pressure effects on the electronic and superconducting properties of this compound. Our ab initio calculations for the case of 50% Al doping are able to correctly reproduce the measured frequencies of the E_2g phonon and explain the disappearance of superconductivity in terms of filling effects on both carrier concentration and electron-phonon coupling. The surface study shows that an enhanced density of states at the Fermi level is found in the B-terminated case. However, we find the Mg-terminated surface to be the most stable structure in the whole range admitted by the chemical potentials, in agreement with very recent experimental results. Finally, the study of the E_2g phonon frequency under pressure is able to explain the critical temperature lowering under applied pressure

[en] First principles calculations demonstrate the metallization of phosphorene by means of Li doping filling the unoccupied antibonding p _z states. The electron–phonon coupling in the metallic phase is strong enough to eventually lead to a superconducting phase at T _c = 17 K for LiP₈ stoichiometry. Using angle-resolved photoemission spectroscopy we confirm that the surface of black phosphorus can be chemically functionalized using Li atoms which donate their 2s electron to the conduction band. The combined theoretical and experimental study demonstrates the semiconductor-metal transition indicating a feasible way to induce a superconducting phase in phosphorene and few-layer black phosphorus. (paper)