Hiromichi Ebisawa

Conductance Formula for Mesoscopic Systems with a Superconducting Segment

Simple conductance formula for small systems with a single superconducting segment is derived through the linear response theory. It is an extension of the Landauer formula. The expression consists of the usual transmission and reflection coefficients as well as the off-diagonal elements which are given by the processes where an electron is reflected to a hole due to the Andreev reflection.

Conductance and Its Fluctuations of Mesoscopic Wires in Contact with a Superconductor

The conductance and its fluctuations in mesoscopic NS (N is a normal metal and S is a superconductor) wires are studied numerically. It is shown that the conductance fluctuations in the NS wire is enhanced as compared with an ordinary normal metal wire (N wire) by about a factor two. For the conductance itself, an enhancement which has been expected is found to vanish when the length of the normal segment is longer than the mean free path.

Hall resistivity of type-II superconductors in the renormalized regime

Abstract Hall resistivity in high temperature superconductors is investigated theoretically by taking account of the renormalized order parameter fluctuation. The results reproduce the characteristics of the Hall resistivity data including the sign change when the sign of the energy derivative of the density of states is positive for the hole-type normal state Hall resistivity. This implies that the existence of the renormalized regime below the mean field critical temperature is essential to explain the characteristics of the Hall resistivity like other thermodynamic quantities in the fluctuation region.

Conductance of Normal-Superconductor Contacts Due to the Andreev Reflection

Low-bias conductance of impure normal-superconductor contacts is studied theoretically in the limit of the small Andreev reflection probability. Its dependence on both magnetic field and temperature is obtained. In the calculation, we focus our attention on the interference effect between an electron and a hole that is suggested to be significant by van Wees et al. It is shown that the conductance is sensitively suppressed by a magnetic field or temperature.

Quasi-particle band structure and gapless superconductivity in the vortex-lattice state

The quasi-particle spectrum for s -wave superconductors in the vortex-lattice state remains gapless in magnetic fields much below the upper critical field H c2 where the cyclotron frequency is smal...

Superconducting fluctuations in a mesoscopic ring structure: Little–Parks oscillation in fluctuation regime

Abstract Superconducting fluctuation in a mesoscopic superconducting ring is studied and its effect on the Little–Parks oscillation is discussed. We introduce Ginzburg–Landau type effective action to describe the fluctuation of the order parameter and treat the nonlinear term by Hartree–Fock approximation. It is shown that the strength of the superconducting fluctuation varies as a function of the magnetic flux enclosed in the ring. This phenomenon is expected to be responsible for the experimentally observed reduction of the amplitude of Little–Parks oscillation.

Numerical study on the conductance of normal-superconductor contacts

Conductance of normal-superconductor contacts with disorder in the normal segment is studied numerically using a simple lattice model. When the probability of the Andreev reflection is small, the ensemble-averaged conductance is shown to be suppressed by a magnetic field. It is also found that the conductance is enhanced with an increase of randomness in the normal segment in the weak disordered regime. These anomalous features disappear in the case of complete Andreev reflection.

Cluster variation method for two-dimensional Hubbard and t-J models

Abstract A new cluster variation method including the wave nature of electron is formulated for the two-dimensional Hubbard and t-J models and the magnetic properties are discussed.

Effect of Quantum Fluctuation on the Andreev Reflection.

We investigate the dynamical quantum fluctuation effect on an Andreev current through an ultrasmall superconductor in the Coulomb blockade regime. We calculate the current using a phenomenological gauge invariant model and the real-time path-integral method. We choose a special gauge which absorbs the scalar potential to convert the phase of a superconductor as a dynamical variable. A zero-energy phase fluctuation of the superconductor suppresses the Andreev current at low bias voltage. This suppression is expected to be observed in samples with large level spacing and small charging energy which allow the coexistence of a large quantum fluctuation and Andreev reflection process.

Numerical Study on Josephson-Current Fluctuations in a Dirty SNS Junction: The Effect of Incomplete Andreev Reflection

Mesoscopic fluctuations of the DC Josephson current in a dirty SNS junction (S: superconductor, N: normal conductor) are studied numerically in the presence of barrier potentials at the NS interfaces, which reduce the Andreev reflection probability. The results show that the critical-current fluctuations in the long-junction limit are suppressed as the Andreev reflection probability decreases while those in the short-junction limit are relatively insensitive to reductions in the Andreev reflection probability. Therefore, the effect of incomplete Andreev reflection is significant in the long-junction limit. We show that the quantitative difference between our previous theory and the experimental results of Takayanagi et al. is well explained by taking the effect of incomplete Andreev reflection into account.