Importance of van der Waals interactions forab initiostudies of topological insulators.

We investigate the lattice and electronic structures of the bulk and surface of the prototypical layered topological insulators Bi2Se3and Bi2Te3usingab initiodensity functional methods, and systematically compare the results of different methods of including van der Waals (vdW) interactions. We show that the methods utilizing semi-empirical energy corrections yield accurate descriptions of these materials, with the most precise results obtained by properly accounting for the long-range tail of the vdW interactions. The bulk lattice constants, distances between quintuple layers and the Dirac velocity of the topological surface states (TSS) are all in excellent agreement with experiment. In Bi2Te3, hexagonal warping of the energy dispersion leads to complex spin textures of the TSS at moderate energies, while in Bi2Se3these states remain almost perfectly helical away from the Dirac point, showing appreciable signs of hexagonal warping at much higher energies, above the minimum of the bulk conduction band. Our results establish a framework for unified and systematic self-consistent first principles calculations of topological insulators in bulk, slab and interface geometries, and provides the necessary first step towardab initiomodeling of topological heterostructures.

Orbital order and Hund's rule frustration in Kondo lattices.

We analyze a microscopic origin of the Kondo effect-assisted orbital order in heavy-fermion materials. By studying the periodic two-orbital Anderson model with two local electrons, we show that frustration of Hund's rule coupling due to the Kondo effect leads to an incommensurate spiral orbital and magnetic order, which exists only inside the Kondo screened (heavy-electron) phase. This spiral state can be observed in neutron and resonant x-ray scattering measurements in U- and Pr-based heavy-fermion compounds, and realized in cold atomic gases, e.g., fermionic 173Yb.

Inhomogeneous superconducting States of mesoscopic thin-walled cylinders in external magnetic fields.

We theoretically investigate the appearance of spatially modulated superconducting states in mesoscopic superconducting thin-wall cylinders in a magnetic field at low temperatures. Quantization of the electron motion around the circumference of the cylinder leads to a discontinuous evolution of the spatial modulation of the superconducting order parameter along the transition line T(c)(H). We show that this discontinuity leads to the nonmonotonic behavior of the specific heat jump at the onset of superconductivity as a function of temperature and field. We argue that this geometry provides an excellent opportunity to directly and unambiguously detect distinctive signatures of the Fulde-Ferrell-Larkin-Ovchinnikov modulation of the superconducting order.

Heavy antiferromagnetic phases in Kondo lattices.

We propose a microscopic physical mechanism that stabilizes the coexistence of the Kondo effect and antiferromagnetism in heavy-fermion systems. We consider a two-dimensional quantum Kondo-Heisenberg lattice model and show that long-range electron hopping leads to a robust antiferromagnetic Kondo state. By using a modified slave-boson mean-field approach we analyze the stability of the heavy antiferromagnetic phase across a range of parameters, and discuss transitions between different phases. Our results may be used to guide future experiments on heavy fermion compounds.

Antiferromagnetic order in Pauli-limited unconventional superconductors.

We develop a theory of the coexistence of superconductivity (SC) and antiferromagnetism (AFM) in CeCoIn(5). We show that in Pauli-limited nodal superconductors the nesting of the quasiparticle pockets induced by Zeeman pair breaking leads to incommensurate AFM with the magnetic moment normal to the field. We compute the phase diagram and find a first order transition to the normal state at low temperatures, the absence of normal state AFM, and the coexistence of SC and AFM at high fields, in agreement with experiments. We also predict the existence of a new double-Q magnetic phase.

Nodes versus minima in the energy gap of iron pnictide superconductors from field-induced anisotropy.

We develop the formalism for computing the oscillations of the specific heat and thermal transport under rotated magnetic field in multiband superconductors with anisotropic gap and apply it to iron-based materials. We show that these oscillations change sign at low temperatures and fields, which strongly influences the experimental conclusions about the gap structure. We find that recent measurements of the specific heat oscillations indicate that the iron-based superconductors possess an anisotropic gap with deep minima or nodes close to the line connecting electron and hole pockets. We predict the behavior of the thermal conductivity that will help distinguish between these cases.

Surface bound states and spin currents in noncentrosymmetric superconductors.

We investigate the ground state properties of a noncentrosymmetric superconductor near a surface. We determine the spectrum of Andreev bound states due to surface-induced mixing of bands with opposite spin helicities for a Rashba-type spin-orbit coupling. We find that the order parameter suppression qualitatively changes the bound state spectrum. The spin structure of Andreev states leads to a spin supercurrent along the interface, which is strongly enhanced compared to the normal state spin current. Particle and hole coherence amplitudes show Faraday-like rotations of the spin along quasiparticle trajectories.

Thermal conductivity evidence for a dx2-y2 pairing symmetry in the heavy-fermion CeIrIn5 superconductor.

The phase diagram of the quasi-2D Ce(Ir,Rh)In5 system contains two distinct superconducting domes. By the thermal transport measurements in rotating magnetic fields H, we pinned down the superconducting gap structure of CeIrIn5 in the second dome, located distant from the first dome in proximity to an antiferromagnetic quantum critical point. Clear fourfold oscillation was observed when H is rotated within the ab plane, while no oscillation was observed within the bc plane. In sharp contrast to previous reports, our results are most consistent with dx2-y2 symmetry, implying that the superconductivity in the second phase is also mediated by antiferromagnetic spin fluctuations.

Nodal structure of quasi-two-dimensional superconductors probed by a magnetic field.

We consider a quasi-two-dimensional superconductor with line nodes in the presence of an in-plane magnetic field, and compute the dependence of the specific heat C and the in-plane heat conductivity kappa on the angle between the field and the nodal direction in the vortex state. We use a variation of the microscopic Brandt-Pesch-Tewordt method that accounts for the scattering of quasiparticles off vortices, and analyze the signature of the nodes in C and kappa. At low to moderate fields the specific heat anisotropy changes sign with increasing temperature. Comparison with measurements of C and kappa in CeCoIn(5) resolves the contradiction between the two in favor of the d((x(2)-y(2)) gap.

4f-electron localization in CexLa 1-xM In5 with M=Co, Rh, or Ir.

de Haas-van Alphen measurements on Ce(x)La(1-x)MIn(5) yield contrasting types of behavior that depend on whether M=Co and Ir or M=Rh. A stronger x-dependent scattering in the case of M=Co and Ir is suggestive of a stronger relative coupling, J/W, of the conduction electrons to the 4f electrons, which would then account for the development of a heavy composite Fermi-liquid state as x-->1. The failure of a composite Fermi-liquid state to form for any x in the case of M= Rh is shown to be inconsistent with theoretical models that propose antiferromagnetism to result from spin-density-wave formation.

Evidence for the coexistence of an anisotropic superconducting gap and nonlocal effects in the nonmagnetic superconductor LuNi2B2C.

A study of the dependence of the heat capacity C(p)(alpha) on the field angle in LuNi2B2C reveals an anomalous disorder effect. For pure samples, C(p)(alpha) exhibits a fourfold variation as the field H (alpha=0). A slightly disordered sample, however, develops anomalous secondary minima along <110> for mu(0)H>1 T, leading to an eightfold pattern at 2 K and 1.5 T. The anomalous pattern is discussed in terms of coexisting superconducting gap anisotropy and nonlocal effects.

Proposal to determine the spectrum of pairing glue in high-temperature superconductors.

We propose a method for an analysis of the angle-resolved photoemission data in two-dimensional anisotropic superconductors which directly yields the spectral function of the bosons mediating Cooper pairing. The method includes a self-consistency check for the validity of the approximations made in the analysis. We explicitly describe the experimental data needed for implementing the proposed procedure.

Tail states in clean superconductors with magnetic impurities.

We analyze the behavior of the density of states in a singlet s-wave superconductor with weak magnetic impurities in the clean limit. By using the method of optimal fluctuation and treating the order parameter self-consistently we show that the density of states is finite everywhere in the superconducting gap, and that it varies as ln(N(E) proportional to -/E-Delta(0)/((7-d)/4) near the mean field gap edge Delta(0) in a d-dimensional superconductor. In contrast to most studied cases the optimal fluctuation is strongly anisotropic.

Avoided antiferromagnetic order and quantum critical point in CeCoIn5.

We measured the specific heat and resistivity of heavy fermion CeCoIn5 between the superconducting critical field H(c2)=5 T and 9 T, with the field in the [001] direction, and at temperatures down to 50 mK. At 5 T the data show a non-Fermi liquid (NFL) behavior down to the lowest temperatures. At the field above 8 T the data exhibit a crossover from the Fermi liquid to a non-Fermi liquid behavior. We analyzed the scaling properties of the specific heat and compared both the resistivity and the specific heat with the predictions of a spin-fluctuation theory. Our analysis leads us to suggest that the NFL behavior is due to incipient antiferromagnetism (AFM) in CeCoIn5 with the quantum critical point in the vicinity of H(c2). Below H(c2) the AFM phase which competes with the paramagnetic ground state is superseded by the superconducting transition.

Interlayer quasiparticle transport in the vortex state of josephson coupled superconductors

We calculate the dependence of the interlayer quasiparticle conductivity, sigma(q), in a Josephson coupled d-wave superconductor on the magnetic field B parallelc and the temperature T. We consider a clean superconductor with resonant impurity scattering and a dominant coherent interlayer tunneling. When pancake vortices in adjacent layers are weakly correlated, at low T the conductivity increases sharply with B over a field scale determined by the impurity scattering, before reaching an extended region of slow linear growth. At high T the conductivity initially decreases and then reaches the same linear regime. For correlated pancakes, sigma(q) increases much more strongly with the applied field.