Density Expansion for the Mobility in a Quantum Lorentz Model

We consider the mobility of electrons in an environment of static hard-sphere scatterers, which provides a realistic description of electrons in Helium gas. A systematic expansion in the scatterer density is carried to second order relative to the Boltzmann result, and the analytic contribution at this order is derived, together with the known logarithmic term in the density expansion. It is shown that existing experimental data are consistent with the existence of the logarithmic term in the density expansion, but more precise experiments are needed in order to unambiguously detect it. We show that our calculations provide the necessary theoretical information for such an experiment, and give a detailed discussion of a suitable parameter range.Comment: 17pp., REVTeX, 7 figure attached as 8 postscript files, db/94/

Gap Nodes and Time Reversal Symmetry Breaking in Strontium Ruthenate

We study the superconducting state of Sr$_2$RuO$_4$ on the bases of a phenomenological but orbital specific description of the electron-electron attraction and a realistic quantitative account of the electronic structure in the normal state. We found that a simple model which features both `in plane' and `out of plane' coupling with strengths $U_{\parallel}=40$meV and $U_{\perp}=48$meV respectively reproduced the experimentally observed power law behaviour of the low temperature specific heat $C_v(T)$, superfluid density $n_s(T)$ and thermal conductivity in quantitative detail. Moreover, it predicts that the quasi-particle spectrum on the $\gamma$ -sheet is fully gaped and the corresponding order parameter breaks the time reversal symmetry. We have also investigated the stability of this model to inclusion of further interaction constants in particular %those which describe `proximity coupling' between orbitals contributing to the $\gamma$ sheet of the Fermi surface and the $\alpha$ and $\beta$ sheets. We found that the predictions of the model are robust under such changes. Finally, we have incorporated a description of weak disorder into the model and explored some of its consequences. For example we demonstrated that the disorder has a more significant effect on the $f$-wave component of the order parameter than on the p-wave one.Comment: EPJ B submitte

Eliashberg-type equations for correlated superconductors

The derivation of the Eliashberg -- type equations for a superconductor with strong correlations and electron--phonon interaction has been presented. The proper account of short range Coulomb interactions results in a strongly anisotropic equations. Possible symmetries of the order parameter include s, p and d wave. We found the carrier concentration dependence of the coupling constants corresponding to these symmetries. At low hole doping the d-wave component is the largest one.Comment: RevTeX, 18 pages, 5 ps figures added at the end of source file, to be published in Phys.Rev. B, contact: [email protected]

Is Sr2RuO4 a Chiral P-Wave Superconductor?

Much excitement surrounds the possibility that strontium ruthenate exhibits chiral p-wave superconducting order. Such order would be a solid state analogue of the A phase of He-3, with the potential for exotic physics relevant to quantum computing. We take a critical look at the evidence for such time-reversal symmetry breaking order. The possible superconducting order parameter symmetries and the evidence for and against chiral p-wave order are reviewed, with an emphasis on the most recent theoretical predictions and experimental observations. In particular, attempts to reconcile experimental observations and theoretical predictions for the spontaneous supercurrents expected at sample edges and domain walls of a chiral p-wave superconductor and for the polar Kerr effect, a key signature of broken time-reversal symmetry, are discussed.Comment: To appear in the proceedings of LT25 (Amsterdam, August 2008