E. Schachinger

Gap anisotropy in superconductors with paramagnetic impurities

The effect of magnetic impurities with localized moments on the anisotropy in the gap and, hence, of the critical temperature is studied by numerical solution of the Eliashberg equations with a simple model for the anisotropy in the interaction. Quantitative results are given for lead for the transition temperature as a function of normal and spin-flip lifetimes. A significantly higher critical concentration of spin-flip scatterers is needed to suppress superconductivity in the anisotropic system than in the isotropic one with the same mean electron-phonon interaction; for concentrations between that critical for the anisotropic and that critical for the isotropic system superconductivity persists because the anisotropy is found to have increased above that in the pure material. This anisotropy increase is studied in detail.

Magnetic resonance at 41 meV and charge dynamics in YBa2Cu3O6.95

We report an Eliashberg analysis of the electron dynamics in YBa2Cu3O6.95. The magnetic resonance at 41 meV couples to charge carriers and defines the characteristic shape in energy of the scattering rate τ−1(T,ω), which allows us to construct the charge-spin spectral density I2χ(ω,T) at temperature T. The T-dependence of the weight under the resonance peak in I2χ(ω) agrees with experiment as does that of the London penetration depth and of the microwave conductivity. Also, the T = 0 condensation energy, the fractional oscillator strength in the condensate, and the ratio of gap-to-critical temperature agree well with the data.

On the BCS theory of paramagnetic impurities in anisotropic superconductors

We develop a theory based on BCS theory which treats the effect of paramagnetic impurities in an anisotropic superconductor. It is found that in a separable model for the order parameter its anisotropy is expected to increase with increasing concentration of paramagnetic impurities. The critical temperature, the order parameter, and the quasiparticle density of states are computed numerically from our theoretical results. We find that there exists a regime which is superconducting by virtue of the fact that the paramagnetic impurities make the material highly anisotropic. In this regime the critical temperature can be reduced rapidly to zero by adding dilute concentrations of normal scattering impurities.

Magnetic-field effects on the density of states of orthorhombic superconductors

The quasiparticle density of states in a two-dimensional d-wave superconductor depends on the orientation of the in-plane external magnetic field H. This is because. in the region of the gap nodes, the Doppler shift due to the circulating supercurrents around a vortex depend on the direction of H. For a tetragonal system the induced pattern is four-fold symmetric and, at zero energy, the density of states exhibits minima along the node directions. But YBa_2C_3O_{6.95} is orthorhombic because of the chains and the pattern becomes two-fold symmetric with the position of the minima occuring when H is oriented along the Fermi velocity at a node on the Fermi surface. The effect of impurity scattering in the Born and unitary limit is discussed.

Relation of spin susceptibility and a.c. conductivity in nearly antiferromagnetic Fermi liquid

Coupling of the charge carriers through the magnetic spin susceptibility which is strongly peaked at (π, π) in the two dimensional CuO2Brillouin zone leads to strong momentum anisotropies and energy dependencies in the inelastic optical scattering rates. The a.c. conductivity is isotropic and can be used to extract an effective, isotropic spin-charge carrier spectral density which reproduces almost exactly the optical scattering rates. The technique is then applied to data in superconducting, optimally doped YBCO and used to measure the charge carrier coupling to the 41 meV peak observed in spin polarized neutron scattering.

Critical temperature of a proximity junction using Eliashberg theory

We present calculations for the critical temperatureTc of a proximity junction using the McMillan tunneling model but treating the metals on either sides within Eliashberg theory formulated on the imaginary axis. In addition we derive formulas for, and evaluate numerically, the functional derivative ofTc with respect to the electron-phonon spectral density of either side of the junction.