Theory of thermal conductivity in extended-ss state superconductors: application to ferropnictides

Within a two-band model for the recently discovered ferropnictide materials, we calculate the thermal conductivity assuming general superconducting states of $A_{1g}$ ("s-wave") symmetry, considering both currently popular isotropic "sign-changing" $s$ states and states with strong anisotropy, including those which manifest nodes or deep minima of the order parameter. We consider both intra- and interband disorder scattering effects, and show that in situations where a low-temperature linear-$T$ exists in the thermal conductivity, it is not always "universal" as in d-wave superconductors. We discuss the conditions under which such a term can disappear, as well as how it can be induced by a magnetic field. We compare our results to several recent experiments.Comment: 13 page

Disorder induced transition between s_+- and s_++ states in two-band superconductors

We have reexamined the problem of disorder in two-band superconductors, and shown within the framework of the T-matrix approximation, that the suppression of T_c can be described by a single parameter depending on the intraband and interband impurity scattering rates. T_c is shown to be more robust against nonmagnetic impurities than would be predicted in the trivial extension of Abrikosov-Gor'kov theory. We find a disorder-induced transition from the s_{\pm} state to a gapless and then to a fully gapped s_{++} state, controlled by a single parameter -- the sign of the average coupling constant . We argue that this transition has strong implications for experiments.Comment: 5 pages, 4 figures; suppl. material: 3 pages, 2 figures; published versio

Nesting symmetries and diffusion in disordered d-wave superconductors

The low-energy density of states (DOS) of disordered 2D d-wave superconductors is extremely sensitive to details of both the disorder model and the electronic band structure. Using diagrammatic methods and numerical solutions of the Bogoliubov-de Gennes equations, we show that the physical origin of this sensitivity is the existence of a novel diffusive mode with momentum close to $(\pi,\pi)$ which is gapless only in systems with a global nesting symmetry. We find that in generic situations, the DOS vanishes at the Fermi level. However, proximity to the highly symmetric case may nevertheless lead to observable non-monotonic behavior of the DOS in the cuprates

Superconducting Junctions with Ferromagnetic, Antiferromagnetic or Charge-Density-Wave Interlayers

Spectra and spin structures of Andreev interface states and the Josephson current are investigated theoretically in junctions between clean superconductors (SC) with ordered interlayers. The Josephson current through the ferromagnet-insulator-ferromagnet interlayer can exhibit a nonmonotonic dependence on the misorientation angle. The characteristic behavior takes place if the pi state is the equilibrium state of the junction in the particular case of parallel magnetizations. We find a novel channel of quasiparticle reflection (Q reflection) from the simplest two-sublattice antiferromagnet (AF) on a bipartite lattice. As a combined effect of Andreev and Q reflections, Andreev states arise at the AF/SC interface. When the Q reflection dominates the specular one, Andreev bound states have almost zero energy on AF/ s-wave SC interfaces, whereas they lie near the edge of the continuous spectrum for AF/d-wave SC boundaries. For an s-wave SC/AF/s-wave SC junction, the bound states are found to split and carry the supercurrent. Our analytical results are based on a novel quasiclassical approach, which applies to interfaces involving itinerant antiferromagnets. Similar effects can take place on interfaces of superconductors with charge density wave materials (CDW), including the possible d-density wave state (DDW) of the cuprates.Comment: LT24 conference proceeding, 2 pages, 1 figur

Evolution of superconductivity in Fe-based systems with doping

We study the symmetry and the structure of the gap in Fe-based superconductors by decomposing the pairing interaction obtained in the RPA into s- and d-wave components and into contributions from scattering between different Fermi surfaces. We show that each interaction is well approximated by the lowest angular harmonics and use this simplification to analyze the origin of the attraction in the two channels, the competition between s- and d-wave solutions, and the origin of superconductivity in heavily doped systems, when only electron or only hole pockets are present.Comment: 4pp, 2 figures, 2 table

Fe-based superconductors: seven years later

Iron-based superconductors were discovered seven years ago, in 2008. This short review summarizes what we learned about these materials over the last seven years, what are open questions, and what new physics we expect to extract from studies of this new class of high-temperature superconductors.Comment: 19 pp, 7 figs, article for Physics Today. Apologies to those not cited --a very limited number of citations is allowe

Impurity-Induced Quasiparticle Transport and Universal Limit Wiedemann-Franz Violation in d-Wave Superconductors

Due to the node structure of the gap in a d-wave superconductor, the presence of impurities generates a finite density of quasiparticle excitations at zero temperature. Since these impurity-induced quasiparticles are both generated and scattered by impurities, prior calculations indicate a universal limit (\Omega -> 0, T -> 0) where the transport coefficients obtain scattering-independent values, depending only on the velocity anisotropy v_f/v_2. We improve upon prior results, including the contributions of vertex corrections and Fermi liquid corrections in our calculations of universal limit electrical, thermal, and spin conductivity. We find that while vertex corrections modify electrical conductivity and Fermi liquid corrections renormalize both electrical and spin conductivity, only thermal conductivity maintains its universal value, independent of impurity scattering or Fermi liquid interactions. Hence, low temperature thermal conductivity measurements provide the most direct means of obtaining the velocity anisotropy for high T_c cuprate superconductors.Comment: 22 pages, 6 figures; revised version to be published in Phys Rev