A possible cooling effect in high temperature superconductors

We show that an adiabatic increase of the supercurrent along a superconductor with lines of nodes of the order parameter on the Fermi surface can result in a cooling effect. The maximum cooling occurs if the supercurrent increases up to its critical value. The effect can also be observed in a mixed state of a bulk sample. An estimate of the energy dissipation shows that substantial cooling can be performed during a reasonable time even in the microkelvin regime.Comment: 5 pages, to appear in Phys. Rev.

On the low temperature properties and specific anisotropy of pure anisotropically paired superconductors

Dependences of low temperature behavior and anisotropy of various physical quantities for pure unconventional superconductors upon a particular form of momentum direction dependence for the superconducting order parameter (within the framework of the same symmetry type of superconducting pairing) are considered. A special attention is drawn to the possibility of different multiplicities of the nodes of the order parameter under their fixed positions on the Fermi surface, which are governed by symmetry. The problem of an unambiguous identification of a type of superconducting pairing on the basis of corresponding experimental results is discussed. Quasiparticle density of states at low energy for both homogeneous and mixed states, the low temperature dependences of the specific heat, penetration depth and thermal conductivity, the I-V curves of SS and NS tunnel junctions at low voltages are examined. A specific anisotropy of the boundary conditions for unconventional superconducting order parameter near $T_c$ for the case of specular reflection from the boundary is also investigated.Comment: 20 page

Effect of point-contact transparency on coherent mixing of Josephson and transport supercurrents

The influence of electron reflection on dc Josephson effect in a ballistic point contact with transport current in the banks is considered theoretically. The effect of finite transparency on the vortex-like currents near the contact and at the phase difference $\phi =\pi ,$ which has been predicted recently \cite{KOSh}, is investigated. We show that at low temperatures even a small reflection on the contact destroys the mentioned vortex-like current states, which can be restored by increasing of the temperature.Comment: 6 pages, 8 Figures, Latex Fil

Quasiclassical description of transport through superconducting contacts

We present a theoretical study of transport properties through superconducting contacts based on a new formulation of boundary conditions that mimics interfaces for the quasiclassical theory of superconductivity. These boundary conditions are based on a description of an interface in terms of a simple Hamiltonian. We show how this Hamiltonian description is incorporated into quasiclassical theory via a T-matrix equation by integrating out irrelevant energy scales right at the onset. The resulting boundary conditions reproduce results obtained by conventional quasiclassical boundary conditions, or by boundary conditions based on the scattering approach. This formalism is well suited for the analysis of magnetically active interfaces as well as for calculating time-dependent properties such as the current-voltage characteristics or as current fluctuations in junctions with arbitrary transmission and bias voltage. This approach is illustrated with the calculation of Josephson currents through a variety of superconducting junctions ranging from conventional to d-wave superconductors, and to the analysis of supercurrent through a ferromagnetic nanoparticle. The calculation of the current-voltage characteristics and of noise is applied to the case of a contact between two d-wave superconductors. In particular, we discuss the use of shot noise for the measurement of charge transferred in a multiple Andreev reflection in d-wave superconductors

Josephson current in unconventional superconductors through an Anderson impurity

Josephson current for a system consisting of an Anderson impurity weakly coupled to two unconventional superconductors is studied and shown to be driven by a surface zero energy (mid-gap) bound-state. The repulsive Coulomb interaction in the dot can turn a $\pi$ junction into a 0-junction. This effect is more pronounced in p-wave superconductors while in high-temperature superconductors with $d_{x^2-y^2}$ symmetry it can exit for rather large artificial centers at which tunneling occurs within a finite region.Comment: 4 pages 3.eps figure

Interface effects on the shot noise in normal metal- d-wave superconductor Junctions

The current fluctuation in normal metal / d-wave superconductor junctions are studied for various orientation of the crystal by taking account of the spatial variation of the pair potentials. Not only the zero-energy Andreev bound states (ZES) but also the non-zero energy Andreev bound states influence on the properties of differential shot noise. At the tunneling limit, the noise power to current ratio at zero voltage becomes 0, once the ZES are formed at the interface. Under the presence of a subdominant s-wave component at the interface which breaks time-reversal symmetry, the ratio becomes 4eComment: 13 pages, 3 figure

Josephson Current in S-FIF-S Junctions: Nonmonotonic Dependence on Misorientation Angle

Spectra and spin structures of Andreev interface states in S-FIF-S junctions are investigated with emphasis on finite transparency and misorientation angle between in-plane magnetizations of ferromagnetic layers in a three-layer interface. It is demonstrated that the Josephson current in S-FIF-S quantum point contacts can exhibit a nonmonotonic dependence on the misorientation angle. The characteristic behavior takes place, if the pi-state is the equilibrium state of the junction in the particular case of parallel magnetizations.Comment: 5 pages, 4 figure

Surface-impedance approach solves problems with the thermal Casimir force between real metals

The surface impedance approach to the description of the thermal Casimir effect in the case of real metals is elaborated starting from the free energy of oscillators. The Lifshitz formula expressed in terms of the dielectric permittivity depending only on frequency is shown to be inapplicable in the frequency region where a real current may arise leading to Joule heating of the metal. The standard concept of a fluctuating electromagnetic field on such frequencies meets difficulties when used as a model for the zero-point oscillations or thermal photons in the thermal equilibrium inside metals. Instead, the surface impedance permits not to consider the electromagnetic oscillations inside the metal but taking the realistic material properties into account by means of the effective boundary condition. An independent derivation of the Lifshitz-type formulas for the Casimir free energy and force between two metal plates is presented within the impedance approach. It is shown that they are free of the contradictions with thermodynamics which are specific to the usual Lifshitz formula for dielectrics in combination with the Drude model. We demonstrate that in the impedance approach the zero-frequency contribution is uniquely fixed by the form of impedance function and does not need any of the ad hoc prescriptions intensively discussed in the recent literature. As an example, the computations of the Casimir free energy between two gold plates are performed at different separations and temperatures. It is argued that the surface impedance approach lays a reliable framework for the future measurements of the thermal Casimir force.Comment: 21 pages, 3 figures, to appear in Phys. Rev.

Tilt Grain-Boundary Effects in S- and D-Wave Superconductors

We calculate the s- and d-wave superconductor order parameter in the vicinity of a tilt grain boundary. We do this self-consistently within the Bogoliubov de Gennes equations, using a realistic microscopic model of the grain boundary. We present the first self-consistent calculations of supercurrent flows in such boundaries, obtaining the current-phase characteristics of grain boundaries in both s-wave and d-wave superconductors

Influence of impurity scattering on tunneling conductance in normal metal- d -wave superconductor junctions

Tunneling conductance spectra between a normal metal / d-wave superconductor junction under the presence of bulk impurities in the superconductor are studied. The quasiclassical theory has been applied to calculate the spatial variation of the pair potential and the effect of impurity scattering has been introduced by t-matrix approximation. The magnitude of a subdominant s-wave component at the interface is shown to robust against the impurity scattering while that for a subdominant $d_{xy}$-wave component is largely suppressed with the increase of the impurity scattering rate. The zero-bias conductance peak due to the zero-energy Andreev bound states is significantly broadened for the case of Born limit impurity compared with that of unitary limit impurity.Comment: 14 pages, 5 figure