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

The effect of spin fluctuations on the electronic structure in iron based superconductors

Magnetic inelastic neutron scattering (INS) studies of iron-based superconductors reveal a strongly temperature-dependent spin-fluctuation spectrum in the normal conducting state, which develops a prominent low-energy resonance feature when entering the superconducting state. Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS) allow to study the fingerprints of fluctuation modes via their interactions with electronic quasiparticles. We calculate such fingerprints in 122 iron pnictides using an experimentally motivated spin-fluctuation spectrum and make a number of predictions that can be tested in ARPES and STS experiments. This includes discussions of the quasiparticle scattering rate and the superconducting order parameter. In quantitative agreement with experiment we reproduce the quasiparticle dispersions obtained from momentum distribution curves as well as energy distribution curves. We discuss the relevance of the coupling between spin fluctuations and electronic excitations for the superconducting mechanism.Comment: 22 pages, 22 figure

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

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

Theory of surface spectroscopy for noncentrosymmetric superconductors

We study noncentrosymmetric superconductors with the tetrahedral $T_d$, tetragonal $C_{4v}$, and cubic point group $O$. The order parameter is computed self-consistently in the bulk and near a surface for several different singlet to triplet order parameter ratios. It is shown that a second phase transition below $T_c$ is possible for certain parameter values. In order to determine the surface orientation's effect on the order parameter suppression, the latter is calculated for a range of different surface orientations. For selected self-consistent order parameter profiles the surface density of states is calculated showing intricate structure of the Andreev bound states (ABS) as well as spin polarization. The topology's effect on the surface states and the tunnel conductance is thoroughly investigated, and a topological phase diagram is constructed for open and closed Fermi surfaces showing a sharp transition between the two for the cubic point group $O$.Comment: 19 pages, 15 figures, accepted for publication in Phys. Rev.

Crystal Symmetry, Electron-Phonon Coupling, and Superconducting Tendencies in Li2_2Pd3_3B and Li2_2Pt3_3B

After theoretical determination of the internal structural coordinates in Li$_2$Pd$_3$B, we calculate and analyze its electronic structure and obtain the frequencies of the two $A_g$ phonons (40.6 meV for nearly pure Li mode, 13.0 meV for the strongly mixed Pd-Li mode). Pd can be ascribed a $4d^{10}$ configuration, but strong 4d character remains up to the Fermi level. Small regions of flat bands occur at -70 meV at both the $\Gamma$ and X points. Comparison of the Fermi level density of states to the linear specific heat coefficient gives a dynamic mass enhancement of $\lambda$ = 0.75. Rough Fermi surface averages of the deformation potentials $D$ of individual Pd and Li displacements are obtained. While is small, ~ 1.15 eV/\AA is sizable, and a plausible case exists for its superconductivity at 8 K being driven primarily by coupling to Pd vibrations. The larger d bandwidth in Li$_2$Pt$_3$B leads to important differences in the bands near the Fermi surface. The effect on the band structure of spin-orbit coupling plus lack of inversion is striking, and is much larger in the Pt compound.Comment: 8 pages and 8embedded figures, to be appeared in PR

Calculation of the static and dynamical correlation energy of pseudo-one-dimensional beryllium systems via a many-body expansion

Low-dimensional beryllium systems constitute interesting case studies for the test of correlation methods because of the importance of both static and dynamical correlation in the formation of the bond. Aiming to describe the whole dissociation curve of extended Be systems we chose to apply the method of increments (MoI) in its multireference (MR) formalism. However, in order to do so an insight into the wave function was necessary. Therefore we started by focusing on the description of small Be chains via standard quantum chemical methods and gave a brief analysis of the main characteristics of their wave functions. We then applied the MoI to larger beryllium systems, starting from the Be6 ring. First, the complete active space formalism (CAS-MoI) was employed and the results were used as reference for local MR calculations of the whole dissociation curve. Despite this approach is well established for the calculation of systems with limited multireference character, its application to the description of whole dissociation curves still requires further testing. After discussing the role of the basis set, the method was finally applied to larger rings and extrapolated to an infinite chain