[en] We present a theoretical study on the structural, electronic and magnetic properties of Mn-doping in amorphous group IV-semiconductors. In order to understand the effect of local disorder on the properties of the system, we consider the ST12 structure, a 12-atom unit cell which is known to locally reproduce the coordination of amorphous semiconductors. Based on ab-initio calculations we predict enhanced magnetic properties and higher solubility of Mn ions in model amorphous systems suggesting that amorphous semiconductors can represent an alternative route for spintronic applications.

[en] We present the result of a complete ab-initio characterization of elemental phosphorus in its structural and superconducting properties. At low pressure we discuss the possibility of superconductivity in Black-P upon doping. In the high pressure regime we investigate thermodynamic stability by performing structural search with the minima hopping method. This way identifying structures that are likely to form upon compression. On the low enthalpy structures we perform full electronic and phononic characterization within DFT-PBE. Superconducting properties are predicted by means of Density Functional Theory for Superconductors. Apart from several interesting predictions, calculations provide a nice interpretation of the existing experimental data, in particular of the evidence that different experimental procedures lead to significantly different values of the critical temperature.

[en] Successful fabrication of one monolayer FeSe on SrTiO_3 represented a real breakthrough in searching for high-T_c Fe-based superconductors ([1]). Motivated by this important discovery, we studied the effects of tensile strain on one monolayer and bulk iron-chalcogenide superconductors (FeSe and FeTe), showing that it produces important magnetic and electronic changes in the systems. We found that the magnetic ground state of bulk and monolayer FeSe is the block-checkerboard phase, which turns into the collinear stripe phase under in-plane tensile strain. FeTe, in both bulk and monolayer phases, shows two magnetic transitions upon increasing the tensile strain: from bicollinear in the ground state to block-checkerboard ending up to the collinear antiferromagnetic phase which could bring it in the superconducting state. Finally, the study of the mechanical properties of both FeSe and FeTe monolayers reveals their enormous tensile strain limits and opens the possibility to grow them on different substrates. (paper)

[en] In this work we explore, by first-principles density functional theory (DFT) calculations, the possibility of inducing electron-phonon mediated superconductivity in a graphene sheet by doping its surface with alkaline metal adatoms. We demonstrate that, contrary to what could be naively believed, simple exfoliation to one layer of superconducting graphite intercalated compounds (GICs) does not necessarily lead to superconducting graphene, as it is the case in CaC₆. On the contrary, it is meaningful to look for superconductivity in monolayers obtained by exfoliating non-superconducting GICs. In particular, we demonstrate that Li coating and double-coating of graphene leads to superconductivity in graphene with T_c that could be as large as 18 K. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] We present first-principles calculations of the superconducting properties of molecular metallic hydrogen under pressure, obtained within the density functional theory of superconductivity. Our study is able to single out the features which drive the system towards superconductivity: mainly, a rich and complex Fermi surface and strongly coupled phonon modes driving the intra- or intermolecular charge transfer. We predict three-gap superconductivity and a critical temperature that rises with increasing pressure up to 242 K at 450 GPa. Our study clearly demonstrates that a very high superconducting critical temperature can be reached purely from electron-phonon and Coulombic electron-electron interactions, thus confirming Ashcroft's early speculations.

[en] We present first-principles calculations of the superconducting properties of CaC₆, obtained within the density functional theory of superconductivity (SCDFT). We find an anisotropic gap which is larger on the Fermi surface sheet with interlayer character. In contrast to MgB₂ the intraband anisotropy is large and the gaps on the three Fermi surface sheets overlap. The resulting critical temperature of 9.5 K is in good agreement with the experimental value of 11.5 K. We show that anisotropy improves the agreement between calculated and experimental specific heat and is consistent with tunnelling experiments. A direct evidence of the gap anisotropy in CaC₆ has been recently observed in directional point contact measurements. We also investigate the system under pressure in order to analyse the increase of the superconducting critical temperature reported experimentally but not reproduced in the McMillan approach. Within our SCDFT implementation we intend to improve the theoretical description with respect to previous studies introducing an ab-initio and pressure-dependent Coulomb interaction

[en] By means of ab initio calculations within the local density approximation to density functional theory, we investigate the electronic structure of the 60 K superconductor YSr₂Cu₃O_6+x (YSCO). We focus on the effects of the Sr/Ba substitution and on the main structural modifications induced by this substitution experimentally found in the Sr compound, namely the tetragonal symmetry and the oxygen disorder in the basal plane. In the calculations, this disorder is simulated by using a supercell approach. Due to band structure effects, we find a larger stabilisation free energy of the orthorhombic structure in YBa₂Cu₃O_6+x (YBCO). In YSCO, the tetragonal disordered phase is found to be stabilized by oxygen overdoping (x  >  1) and by sufficiently large mass-enhancement factors, . The analysis of the atomic site projected density of states suggests that oxygen disorder in the CuO basal planes of YSCO induces hole localisation, which accounts for the large 30 K reduction of with respect to YBCO. (paper)

[en] We present a comparative and detailed study of transition metal doping in CaFe₂As₂. Comparing with several experimental results and carefully analyzing how the states at the Fermi level are affected by doping we show that: i) simulation of real doping and considering induces structural relaxations are crucial to correctly address the physical mechanisms induced by transition metal substitutions; ii) different dopant concentration induces changes on the band structure that can not be described within a simple rigid-band picture; iii) careful comparison with the available ARPES results shows that the main effects on band filling and symmetry can be caught within DFT.

[en] We present an application of Eliashberg theory of superconductivity to study a set of novel superconducting systems with a wide range of structural and chemical properties. The set includes three intercalated group-IV honeycomb layered structures, SH₃ at 200 GPa (the superconductor with the highest measured critical temperature), the similar system SeH₃ at 150 GPa, and a lithium doped mono-layer of black phosphorus. The theoretical approach we adopt is a recently developed, fully ab initio Eliashberg approach that takes into account the Coulomb interaction in a full energy-resolved fashion avoiding any free parameters like μ*. This method provides reasonable estimations of superconducting properties, including T_C and the excitation spectra of superconductors. (author)

[en] We present ab initio electronic structure calculations within density functional theory of the structural, electronic and magnetic properties of transition metal impurities into a Ge matrix. We examine the case of dopant clusters, made of up to three transition metal impurity atoms in substitutional and/or interstitial sites, embedded in the Ge matrix. We find that while isolated interstitial TM impurities are energetically unfavored over the substitutional sites, they can be stabilized thanks to the interaction with substitutional sites nearby. Mn impurities show a tendency to cluster and exhibit magnetic alignment strongly dependent on the occupied sites. The electronic properties of the most stable configurations are analyzed and discussed