Transport Properties of "Extended-s" State Superconductors

Superconducting states with "extended s-wave" symmetry have been suggested in connection with recent ARPES experiments on BSCCO. In the presence of impurities, thermodynamic properties of such states reflect a residual density of states $N(0)$ for a range of concentrations. While properties reflecting $N(\omega)$ alone will be similar to those of d-wave states, transport measurements may be shown to qualitatively distinguish between the two. In contrast to the d-wave case with unitarity limit scattering, limiting low-temperature residual conductivities in the s-wave state are large and scale inversely with impurity concentration.Comment: 4 pages, 5 figures, uuencoded compressed postscript fil

Multichannel Kondo impurities in superconductors

We discuss the effect of multichannel Kondo impurities on superconductivity. In the strong coupling regime such impurities are pairbreakers, in contrast to the ordinary Kondo effect. Measurements of $T_c$-suppression may help in identifying impurities displaying this more exotic exchange coupling to the conduction band.Comment: 3 pages, 4 figures, uuencoded compressed postscript file, submitted to SCES'94 (Amsterdam

Thermal Conductivity Anisotropy in Superconducting UPt3UPt_3

Recent thermal conductivity measurements on $UPt_3$ single crystals by Lussier et al. indicate the existence of a strong b--c anisotropy in the superconducting state. We calculate the thermal conductivity in various unconventional candidate states appropriate for the $UPt_3$ ``B phase" and compare with experiment, specifically the $E_{2u}$ and $E_{1g}$ $(1,i)$ states predicted in some Ginzburg-Landau analyses of the phase diagram. For the simplest realizations of these states over spherical or ellipsoidal Fermi surfaces, the normalized $E_{2u}$ conductivity is found, surprisingly, to be completely isotropic. We discuss the effects of inelastic scattering and realistic Fermi surface anisotropy, and deduce constraints on the symmetry class of the $UPt_3$ ground state.Comment: 4 postscript pages, UFL102

Effect of Order-Parameter Suppression on Scattering by Isolated Impurities in Asymmetric Bands

The single-impurity problem in d-wave superconductors with asymmetric bands is discussed. The effect of local order parameter suppression near the impurity is to shift the quasiparticle resonance. Contrary to previous work [A. Shnirman et al., Phys. Rev. B 60, 7517 (1999)] we find that the direction of the shift is not universally towards the strong scattering limit.Comment: 2 pages, 1 figure, proceedings for M2S-HTSC-V

Residual absorption at zero temperature in d-wave superconductors

In a d-wave superconductor with elastic impurity scattering, not all the available optical spectral weight goes into the condensate at zero temperature, and this leads to residual absorption. We find that for a range of impurity parameters in the intermediate coupling regime between Born (weak) and unitary (strong) limit, significant oscillator strength remains which exhibits a cusp like behavior of the real part of the optical conductivity with upward curvature as a function of frequency, as well as a quasilinear temperature dependence of the superfluid density. The calculations offer an explanation of recent data on ortho-II YBa$_2$Cu$_3$O$_{6.5}$ which has been considered anomalous.Comment: Accepted for publication by Phys. Rev. B 7 Pages and 4 Figure

Renormalization of thermal conductivity of disordered d-wave superconductors by impurity-induced local moments

The low-temperature thermal conductivity \kappa_0/T of d-wave superconductors is generally thought to attain a "universal" value independent of disorder at sufficiently low temperatures, providing an important measure of the magnitude of the gap slope near its nodes. We discuss situations in which this inference can break down because of competing order, and quasiparticle localization. Specifically, we study an inhomogeneous BCS mean field model with electronic correlations included via a Hartree approximation for the Hubbard interaction, and show that the suppression of \kappa_0/T by localization effects can be strongly enhanced by magnetic moment formation around potential scatterers.Comment: 2 pages, 1 figure, submitted to M2S-HTSC VIII, Dresden 200

A possible cooling effect in high temperature superconductors

We show that an adiabatic increase of the supercurrent along a superconductor with lines of nodes of the order parameter on the Fermi surface can result in a cooling effect. The maximum cooling occurs if the supercurrent increases up to its critical value. The effect can also be observed in a mixed state of a bulk sample. An estimate of the energy dissipation shows that substantial cooling can be performed during a reasonable time even in the microkelvin regime.Comment: 5 pages, to appear in Phys. Rev.

Surface effects in multiband superconductors. Application to MgB2_2

Metals with many bands at the Fermi level can have different band dependent gaps in the superconducting state. The absence of translational symmetry at an interface can induce interband scattering and modify the superconducting properties. We dicuss the relevance of these effects to recent experiments in MgB$_2$