No abstract available

[en] The coupling constants appearing in the two-band model are derived from recent microscopic calculations. The resulting order parameters are close to the BCS value. The model allows the effects of interband impurity scattering to be discussed. (author)

[en] Evidence for d-wave superconductivity from measurements of the local conductivity is rather indirect and not very compelling. Very recently, experimental data on the surface resistance taken at five different frequencies on very high quality single crystals of YBa₂Cu₃O₇ have become available. Having the frequency as additional variable provides a much more stringent test for the predictive powers of the d-wave model. Because of the high quality of the samples the authors expect that at low temperatures the nonlocal limit applies to the surface resistance. A nonlocal relation between current and field would allow a direct investigation of the order parameter anisotropy by varying the wave vector q of the radiation relative to the crystal axes

[en] Li et al. found that the critical current density J_c^J across atomically clean c-axis twist junctions of Bi₂Sr₂CaCu₂O_8+δ is the same as that of the constituent single crystal, J^J_c, independent of the twist angle φ_o, even at and below Tc.The authors investigated theoretically if a d_x²-y²-wave order parameter might twist by mixing in d_xy-wave components, but found that such mixing cannot possibly explain the data near to Tc. Combined with group theoretical arguments, they then conclude that the order parameter contains at least a substantial s-wave component, but does not contain any purported d_x²-y²-wave component, except possibly below a second, unobserved phase transition. By studying tunneling models, they further conclude that the intrinsic c-axis Josephson tunneling in Bi₂Sr₂CaCu₂O_8+δ is likely to be mostly incoherent

[en] It is shown that the attenuation of longitudinal ultrasound in superconducting niobium cannot be explained by the two-band model. Attempts to fit both the data near Tsub(c) and much below Tsub(c) lead to contradictory assumptions for the model parameters. Experiments, on the other hand, do not at present exclude the existence of multiple gaps if these are close to the BSC-gap. (author)

[en] A theory of the upper critical field is developed which is sufficiently general to incorporate strong coupling effects, anisotropy of both Fermi surface and pairing interaction, and momentum-dependent scattering, including exchange and spin-orbit scattering. It is not necessary to assume that spin-orbit scattering is a small fraction of the total scattering. An important improvement in this theory is the correct treatment of the spatial dependence of the anomalous Green function and the order parameter in the mixed state. In the presence of anisotropy this is, in general, not described with sufficient accuracy by the Abrikosov vortex lattice. This paper concentrates on the effects arising from the momentum dependence of the elastic scattering. If spin-orbit scattering forms a sizeable fraction of the total scattering, it is necessary to take its explicit momentum dependence into account. For a complete description of the upper critical field, other effects mentioned above need to be taken into account, but for superconductors of intermediate purity the momentum dependence of the scattering cannot be ignored. 85 refs

[en] It is shown that the difference between the Eliashberg function of pure Nb and that of Nbsub(.75)Zrsub(.25), which is observed at high energies, can be explained in terms of force constant disorder, provided one includes in addition to force constant disorder an overall softening of the lattice and a change in the density of states at the Fermi level. At low energies the Eliashberg function of Nbsub(.75)Zrsub(.25) shows a broad shoulder, which cannot be explained by the model employed to describe force constant disorder, and which is probably due to certain phonons becoming soft. (author)

[en] In the presence of interband scattering of quasiparticles by impurities only a single gap in the excitation spectrum is found. The dependence of this gap and the two order parameters on interband coupling and impurity content is discussed. Graphical representations of the density of states for several sets of parameters are given. For certain ranges of parameters the calculated density of states is qualitatively similar to the experimental results. (author)

[en] Li et al. found that the critical current density J_c^J across atomically clean c-axis twist junctions of Bi₂Sr₂CaCu₂O_8+δ is the same as that of the constituent single crystal, J_c^S, independent of the twist angle φ₀, even at and below Tc. They investigated theoretically if a d_x²-y²-wave order parameter might twist by mixing in d_xy--wave components, but found that such mixing cannot possibly explain the data near to Tc. Combined with group theoretical arguments, they then conclude that the order parameter contains at least a substantial s-wave component, but does not contain any purported d_x²_-y²-wave component, except possibly below a second, unobserved phase transition. By studying tunneling models, they further conclude that the intrinsic c-axis Josephson tunneling in Bi₂Sr₂CaCu₂O_8+δ is likely to be mostly incoherent. They also propose a c-axis junction version of a tricrystal experiment, which does not rely upon expensive substrates

[en] Use of the general many-body formalism allows us to derive a system of equations describing the propagation of electromagnetic shear waves coupled to ultrasonic shear waves in normal and superconducting metals with arbitrary electron mean free paths. From this system of equations we derive general expressions in terms of correlation functions for the attenuation coefficient as well as for the generation and detection (radiation) efficiency of ultrasonic shear waves, assuming specular reflection. It is shown that, for arbitrary mean free path, generation and radiation efficiency are equal. Furthermore, they are related in a simple way to the residual surface resistance caused by ultrasound generation. In order to incorporate the frictional force between electrons and lattice, a reactive part of which remains finite even at T=0, one has to introduce the current-stress tensor correlation function. The calculation of this correlation function for an impure BCS superconductor is more complicated than the calculation of the conductivity from the current-current correlation function. If suitable normalized, electromagnetic contribution to the attenuation coefficient depends on mean free path only through the conductivity. For sufficiently short mean free path the generation and radiation efficiency in a superconductor can become equal to and even larger than the efficiency in the normal metal because the frictional force, which opposes the electromagnetic field driving the ions, is less effective in the superconducting than in the normal state. It is shown that the interpretation of experiments on the radiation efficiency in superconducting tin has to be modified when the finite electron mean free path is taken into account. Agreement between theory and experiment can still be achieved by a suitable choice of the Fermi velocity and the mean free path. The London penetration depth calculated from this Fermi velocity turns out to be smaller than generally accepted values