T>0 ensemble state density functional theory revisited

A logical foundation of equilibrium state density functional theory in a Kohn-Sham type formulation is presented on the basis of Mermin's treatment of the grand canonical state. it is simpler and more satisfactory compared to the usual derivation of ground state theory, and free of remaining open points of the latter. It may in particular be relevant with respect to cases of spontaneous symmetry breaking like non-collinear magnetism and orbital order.Comment: 7 pages, no figure

Varying Cu-Ti hybridization near the Fermi energy in Cux_{x}TiSe2_{2}: Results from supercell calculations

The properties of Cu$_{x}$TiSe$_{2}$ are studied by band structure calculation based on the density functional theory for supercells. The density-of-states (DOS) for $x$=0 has a sharply raising shoulder in the neighborhood of the Fermi energy, $E_F$, which can be favorable for spacial charge modulations. The Cu impurity adds electrons and brings the DOS shoulder below $E_F$. Hybridization makes the Ti-d DOS at $E_F$, the electron-phonon coupling and the Stoner factor very large. Strong pressure dependent properties are predicted from the calculations, since the DOS shoulder is pushed to higher energy at a reduced lattice constant. Effects of disorder are also expected to be important because of the rapidly varying DOS near $E_F$.Comment: 5 pages, 4 figures 2 table

Calculated Cleavage Behavior and Surface States of LaOFeAs

The layered structure of the iron based superconductors gives rise to a more or less pronounced two-dimensionality of their electronic structure, most pronounced in LaOFeAs. A consequence are distinct surface states to be expected to influence any surface sensitive experimental probe. In this work a detailed density functional analysis of the cleavage behavior and the surface electronic structure of LaOFeAs is presented. The surface states are obtained to form two-dimensional bands with their own Fermi surfaces markedly different from the bulk electronic structure

Theoretical aspects of Andreev spectroscopy and tunneling spectroscopy in non-centrosymmetric superconductors: a topical review

Tunneling spectroscopy at surfaces of unconventional superconductors has proven an invaluable tool for obtaining information about the pairing symmetry. It is known that mid gap Andreev bound states manifest itself as a zero bias conductance peak in tunneling spectroscopy. The zero bias conductance peak is a signature for a non-trivial pair potential that exhibits different signs on different regions of the Fermi surface. Here, we review recent theoretical results on the spectrum of Andreev bound states near interfaces and surfaces in non-centrosymmetric superconductors. We introduce a theoretical scheme to calculate the energy spectrum of a non-centrosymmetric superconductor. Then, we discuss the interplay between the spin orbit vector field on the Fermi surface and the order parameter symmetry. The Andreev states carry a spin supercurrent and represent a helical edge mode along the interface. We study the topological nature of the resulting edge currents. If the triplet component of the order parameter dominates, then the helical edge mode exists. If, on the other hand, the singlet component dominates, the helical edge mode is absent. A quantum phase transition occurs for equal spin singlet and triplet order parameter components. We discuss the tunneling conductance and the Andreev point contact conductance between a normal metal and a non-centrosymmetric superconductor.Comment: 42 pages, 11 figure

Violation of non-interacting V\cal V-representability of the exact solutions of the Schr\"odinger equation for a two-electron quantum dot in a homogeneous magnetic field

We have shown by using the exact solutions for the two-electron system in a parabolic confinement and a homogeneous magnetic field [ M.Taut, J Phys.A{\bf 27}, 1045 (1994) ] that both exact densities (charge- and the paramagnetic current density) can be non-interacting $\cal V$-representable (NIVR) only in a few special cases, or equivalently, that an exact Kohn-Sham (KS) system does not always exist. All those states at non-zero $B$ can be NIVR, which are continuously connected to the singlet or triplet ground states at B=0. In more detail, for singlets (total orbital angular momentum $M_L$ is even) both densities can be NIVR if the vorticity of the exact solution vanishes. For $M_L=0$ this is trivially guaranteed because the paramagnetic current density vanishes. The vorticity based on the exact solutions for the higher $|M_L|$ does not vanish, in particular for small r. In the limit $r \to 0$ this can even be shown analytically. For triplets ($M_L$ is odd) and if we assume circular symmetry for the KS system (the same symmetry as the real system) then only the exact states with $|M_L|= 1$ can be NIVR with KS states having angular momenta $m_1=0$ and $|m_2|=1$. Without specification of the symmetry of the KS system the condition for NIVR is that the small-r-exponents of the KS states are 0 and 1.Comment: 18 pages, 4 figure

Giant thermoelectric effects in a proximity-coupled superconductor-ferromagnet device

The usually negligibly small thermoelectric effects in superconducting heterostructures can be boosted dramatically due to the simultaneous effect of spin splitting and spin filtering. Building on an idea of our earlier work [Phys. Rev. Lett. $\textbf{110}$, 047002 (2013)], we propose realistic mesoscopic setups to observe thermoelectric effects in superconductor heterostructures with ferromagnetic interfaces or terminals. We focus on the Seebeck effect being a direct measure of the local thermoelectric response and find that a thermopower of the order of $\sim200$ $\mu V/K$ can be achieved in a transistor-like structure, in which a third terminal allows to drain the thermal current. A measurement of the thermopower can furthermore be used to determine quantitatively the spin-dependent interface parameters that induce the spin splitting. For applications in nanoscale cooling we discuss the figure of merit for which we find enormous values exceeding 1 for temperature $\lesssim 1$K

Abrikosov flux-lines in two-band superconductors with mixed dimensionality

We study vortex structure in a two-band superconductor, in which one band is ballistic and quasi-two-dimensional (2D), and the other is diffusive and three-dimensional (3D). A circular cell approximation of the vortex lattice within the quasiclassical theory of superconductivity is applied to a recently developed model appropriate for such a two-band system [Tanaka et al 2006 Phys. Rev. B 73, 220501(R); Tanaka et al 2007 Phys. Rev. B 75, 214512]. We assume that superconductivity in the 3D diffusive band is "weak", i.e., mostly induced, as is the case in MgB$_2$. Hybridization with the "weak" 3D diffusive band has significant and intriguing influence on the electronic structure of the "strong" 2D ballistic band. In particular, the Coulomb repulsion and the diffusivity in the "weak" band enhance suppression of the order parameter and enlargement of the vortex core by magnetic field in the "strong" band, resulting in reduced critical temperature and field. Moreover, increased diffusivity in the "weak" band can result in an upward curvature of the upper critical field near the transition temperature. A particularly interesting feature found in our model is the appearance of additional bound states at the gap edge in the "strong" ballistic band, which are absent in the single-band case. Furthermore, coupling with the "weak" diffusive band leads to reduced band gaps and van Hove singularities of energy bands of the vortex lattice in the "strong" ballistic band. We find these intriguing features for parameter values appropriate for MgB$_2$.Comment: 11 pages, 14 figure