The band-gap of Tl-doped gallium nitride alloys

Structural and electronic properties of hypothetical zinc blende Tl(x)Ga(1-x)N alloys have been investigated from first principles. The structural relaxation, preformed within the LDA approach, leads to a linear dependence of the lattice parameter a on the Tl content x. In turn, band structures obtained by MBJLDA calculations are significantly different from the corresponding LDA results. The decrease of the band-gap in Tl-doped GaN materials (for x<0.25) is predicted to be a linear function of x, i.e. 0.08 eV per atomic % of thallium. The semimetallic character is expected for materials with x>0.5. The obtained spin-orbit coupling driven splitting between the heavy-hole and split-off band at the Gamma point of the Brillouin zone in Tl(x)Ga(1-x)N systems is significantly weaker when compared to that of Tl-doped InN materials.

Pressure effects on the superconductivity of HfPd2Al Heusler compound: Experimental and theoretical study

Polycrystalline HfPd2Al has been synthesized using the arc-melting method and studied under ambient pressure conditions by x-ray diffraction from room temperature up to 450^oC. High pressure x-ray diffraction up to 23 GPa was also performed using Diacell-type membrane diamond anvil cells. The estimated linear thermal expansion coefficient was found to be {alpha} = 1.40(3)x10^{-5} K^{-1}, and the bulk modulus derived from the fit to the 3rd order Birch-Murnaghan EOS (BMEOS) is B0 = 97(2) GPa. Resistivity studies under applied pressure (p < 7.49 GPa) showed a linear decrease of superconducting critical temperature with increasing pressure and the slope dTc/dp = -0.13(1) K GPa^{-1}. The same behavior is observed for the electron-phonon coupling constant {lambda_{ep}}(p) that changes from 0.67 to 0.6, estimated for p = 0.05 GPa and 7.49 GPa, respectively. First principles electronic structure and phonon calculation results are presented and used to estimate the magnitude of electron-phonon interaction {lambda_{ep}} and its evolution with pressure. Theoretical results explain the experimentally observed decrease in Tc due to considerable lattice stiffening.

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.

Ab initio study of InxGa(1-x)N - performance of the alchemical mixing approximation

The alchemical mixing approximation which is the ab initio pseudopotential specific implementation of the virtual crystal approximation (VCA), offered in the ABINIT package, has been employed to study the wurtzite (WZ) and zinc blende (ZB) InxGa(1-x)N alloy from first principles. The investigations were focused on structural properties (the equilibrium geometries), elastic properties (elastic constants and their pressure derivatives), and on the band-gap. Owing to the ABINIT functionality of calculating the Hellmann-Feynmann stresses, the elastic constants have been evaluated directly from the strain-stress relation. Values of all the quantities calculated for parent InN and GaN have been compared with the literature data and then evaluated as functions of composition x on a dense, 0.05 step, grid. Some results have been obtained which, to authors knowledge, have not yet been reported in the literature, like composition dependent elastic constants in ZB structures or composition dependent pressure derivatives of elastic constants. The band-gap has been calculated within the MBJLDA approximation. Additionally, the band-gaps for pure $InN$ and GaN have been calculated with the Wien2k code, for comparison purposes. The evaluated quantities have been compared with the available literature reporting supercell-based ab initio calculations and on that basis conclusions concerning the performance of the alchemical mixing approach have been drawn. ...

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.

Anomalous band-gap bowing of AlN(1-x)Px alloy

Electronic structure of zinc blende AlN(1-x)$Px alloy has been calculated from first principles. Structural optimisation has been performed within the framework of LDA and the band-gaps calculated with the modified Becke-Jonson (MBJLDA) method. Two approaches have been examined: the virtual crystal approximation (VCA) and the supercell-based calculations (SC). The composition dependence of the lattice parameter obtained from the SC obeys Vegards law whereas the volume optimisation in the VCA leads to an anomalous bowing of the lattice constant. A strong correlation between the band-gaps and the structural parameter in the VCA method has been observed. On the other hand, in the SC method the supercell size and atoms arrangement (clustered vs. uniform) appear to have a great influence on the computed band-gaps. In particular, an anomalously big band-gap bowing has been found in the case of a clustered configuration with relaxed geometry. Based on the performed tests and obtained results some general features of MBJLDA are discussed and its performance for similar systems predicted.

First principles prediction of structural and electronic properties of TlxIn(1-x)N alloy

Structural and electronic properties of zinc blende TlxIn(1-x)N alloy have been evaluated from first principles. The band structures have been obtained within the density functional theory (DFT), the modified Becke-Johnson (MBJLDA) approach for the exchange-correlation potential, and fully relativistic pseudopotentials. The calculated band-gap dependence on Tl content in this hypothetical alloy exhibits a linear behaviour up to the 25 % of thalium content where its values become close to zero. In turn, the split-off energy at the Gamma point of the Brillouin zone, related to the spin-orbit coupling, is predicted to be comparable in value with the band-gap for relatively low thalium contents of about 5 %. These findings suggest TlxIn(1-x)N alloy as a promising material for optoelectronic applications. Furthermore, the band structure of TlN reveals some specific properties exhibited by topological insulators.