Magnetic field induced rotation of the d-vector in Sr_2RuO_4

The superconductor Sr_2RuO_4 is widely believed to be a spin triplet system with a chiral order parameter analogous to the A phase of superfluid helium-3. The best evidence for this pairing state is that the Knight shift or spin susceptibility measured in neutron scattering is constant below T_c, unlike in a spin-singlet superconductor. The original Knight shift and neutron scattering measurements were performed for magnetic fields aligned in the ruthenate a-b plane. These would be consistent with a triplet d-vector d{k} aligned along the c-axis. However recently the Knight shift for fields along c was also found to be constant below T_c, which is not expected for this symmetry state. In this paper we show that while spin-orbit interaction stabilises the c-axis oriented d-vector, it is possible that only a very small external B field may be sufficient to rotate the d-vector into the a-b plane. In this case the triplet pairing model remains valid. We discuss characteristics of the transition and the prospects to detect it in thermodynamic quantities.Comment: 2 pages, 1 figure, Proceedings of the M2S-HTSC conference in Dresden. Requires elsart.sty (included

Van Hove Singularity and D-Wave Pairing in Disordered Superconductors

We apply the coherent potential approximation (CPA) to a simple model for disordered superconductors with d-wave pairing. We demonstrate that whilst the effectiveness of an electronic Van Hove singularity to enhance the transition temperature T$_c$ is reduced by disorder it is not eliminated. In fact we give a qualitative account of changes in the T$_c$ vs. doping curve with increasing disorder and compare our results with experiments on the Y_{0.8}Ca_{0.2}Ba_2(Cu_{1-c}Zn_c)_{3}O_{7-\delta} alloys.Comment: 4 pages of text and 7 postscript file

Charge on the quantum dot in the presence of tunneling current

The calculation of the charge present in central region of the double barrier structure at non-equilibrium conditions is discussed. We propose here a simple method to calculate non equilibrium Green's functions which allows consistent calculations of retarded and distribution functions. To illustrate the approach we calculate the charge on the quantum dot coupled {\it via} tunnel barriers to two external leads having different chemical potentials $\mu_L$ and $\mu_R$. The obtained results have been compared with other approaches existing in the literature. They all agree in the equilibrium situation and the departures grow with increasing the difference $\mu_L-\mu_R$.Comment: 9 pages, 2 (.eps) figures, to be published in Solid State Commu

Effect of disorder on superconductivity in the boson-fermion model

We study how a randomness of either boson or fermion site energies affects the superconducting phase of the boson fermion model. We find that, contrary to what is expected for s-wave superconductors, the non-magnetic disorder is detrimental to the s-wave superconductivity. However, depending in which subsystem the disorder is located, we can observe different channels being affected. Weak disorder of the fermion subsystem is responsible mainly for renormalization of the single particle density of states while disorder in the boson subsystem directly leads to fluctuation of the strength of the effective pairing between fermions.Comment: 7 pages, 6 figures. Physical Review B (accepted for publication