Non-equilibrium supercurrent through mesoscopic ferromagnetic weak links

We consider a mesoscopic normal metal, where the spin degeneracy is lifted by a ferromagnetic exchange field or Zeeman splitting, coupled to two superconducting reservoirs. As a function of the exchange field or the distance between the reservoirs, the supercurrent through this device oscillates with an exponentially decreasing envelope. This phenomenon is similar to the tuning of a supercurrent by a non-equilibrium quasiparticle distribution between two voltage-biased reservoirs. We propose a device combining the exchange field and non-equilibrium effects, which allows us to observe a range of novel phenomena. For instance, part of the field-suppressed supercurrent can be recovered by a voltage between the additional probes.Comment: 7 pages, 8 figures, Europhys. Lett., to be published, corrected two reference

The Flux-Line Lattice in Superconductors

Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.Comment: Review paper for Rep.Prog.Phys., 124 narrow pages. The 30 figures do not exist as postscript file

Pairing in planar organic superconductors

The nature of superconductivity in planar organics is still a controversial problem. We investigate two types of d-wave and extended s-wave pairing proposed theoretically in a model of kappa-(BEDT-TTF)(2)X organic superconductors. As probes of the pairing symmetry, we calculate the density of states, the tunnelling conductance, the specific heat, and the spin susceptibility. The sensibility of superconductivity to the sample preparation suggests that beside the symmetric d(x2-y2) gap, which appears to be consistent with some experiments (for the specific heat, see Nakazawa and Kanoda, Phys. Rev. B 55 (1995) R8670, for STM, Arai et al., Phys. Rev. B 63 (2001) 194518, and for NMR, Mayaffre et al., Phys. Rev. Lett. 75 (1995) 4122) the asymmetric d(xy)-like or extended s-wave gaps could occur. We show that in comparison with the d(x2-y2) case this should be manifested by a more rapid increase of the specific heat at low temperature, by the presence of inner peaks on the density of states and conductance curves at low temperature, and by a different curvature of the spin susceptibility temperature dependence

Superfluid density and pairing in planar organic superconductors

We calculate self-consistently in-plane superfluid density rho(parallel to)(s) in planar organic superconductors to]for three types of pairing symmetry: d(x2-y2), d(xy)-like and extended s-wave pairing. Assuming strong dimerization, we consider a single band model with elliptical Fermi surface to evaluate: the temperature dependence rho(parallel to)(s) (T) in K-(BEDT-TTF)(2)X compounds for those types of pairing. The obtained results, compared with measurements of the superfluid density in k-(BEDT-TTF)(2)Cu[N(CN)(2)]Br, strongly suggest that in these compounds the gap symmetry, influenced by cooling history, in the "ground state" with the least disorder is of the extended s-wave type. Possible physical reasons for agreement of d(x2-y2), and/or isotropic s-wave pairing with experirncntal results in more disordered "interniediate state" are discussed

Theory of the pi junctions formed in atomic-scale superconductor ferromagnet superlattices

no abstrac