T. Ishida

Critical temperature in nanoscopic superconductors

Recently, we have studied the properties of nano-scale superconductors different from that of bulk superconductors. One of the properties is the critical temperature of normal-superconducting transition. We evaluate the critical temperature with two method by solving Bogoliubov-de Gennes equation. The former method is the general approximation calculation, which is good approximation in bulk superconductors. The latter is the direct calculation of the T-dependence of the order parameter A by solving the Bogoliubov-de Gennes equation self-consistently. These methods lead to the much different critical temperature and the true critical temperature is higher than we expect in the bulk superconductors.

Phase Dynamics of a Closed 0-π Josephson Junction

The phase dynamics of a closed 0‐π junction is investigated. Such a system can be realized, for example, in a small d(x2−y2)‐wave superconductor embedded in a conventional s‐wave superconducting matrix. The Hamiltonian of the system is derived. We prove that the system can be mapped on a quantum two‐level system when the system size is small enough.

Physical Properties of composite structures of d‐ and s‐wave superconductors (d‐dot’s)

High‐Tc superconducting dot embedded in an s‐wave superconductor, which is called “d‐dot”, shows unusual properties. Especially, it shows spontaneous half‐quantized magnetic fluxes at the corners of the dot because of the anisotropy of d‐wave superconductivity. The d‐dot can be considered as an artificial spin, because most stable states are doubly degenerate. We study stability of these degenerate states, using two‐components Ginzburg‐Landau equation. We show the energy difference between these degenerate states and excited states is comparable to the condensation energy of s‐wave superconductor at T=0.

Matching effect between the density of screw dislocations and the break field in the irreversibility lines of YBa2Cu3O7−δ films

Abstract We have investigated the effect of screw dislocations on the irreversibility lines of YBaCuO films. The density nsd of screw dislocations is controlled between 2.0 × 1013 and 7.9 × 1013 m−2 by a subtle change in stoichiometry. We employ a nonlinearity creterion using the harmonic susceptibility to determine the irreversbility line, where the AC field amplitude μ0HAC (gm0 = 4π × 10−7 H/m) is fixed at 0.1 mT while the DC magnetic field μ0HDC varies up to 1 T. We have found a step-wise anomalous change in the irreversbility lines. A break field μ0Hbreak, where the step change starts as a function of decreasing field, is proportional to the pining capability of screw dislocations BΦ = μ0Φ0nsd (Φ0 is the flux quantum). The frequency dependence of the irreversibility temperature is also discussed.