Y. Sawa

Andreev bound states and tunneling characteristics of a noncentrosymmetric superconductor

The tunneling characteristics of planar junctions between a normal metal and a non-centrosymmetric superconductor like CePt3Si are examined. It is shown that the superconducting phase with mixed parity can give rise to characteristic zero-bias anomalies in certain junction directions. The origin of these zero-bias anomalies are Andreev bound states at the interface. The tunneling characteristics for different directions allow to test the structure of the parity-mixed pairing state.

Odd-frequency pairs and Josephson current through a strong ferromagnet

We study Josephson current in superconductor/diffusive ferromagnet/superconductor junctions by using the recursive Green function method. When the exchange potential in a ferromagnet is sufficiently large compared to the pair potential in a superconductor, an ensemble average of Josephson current is much smaller than its mesoscopic fluctuations. The Josephson current vanishes when the exchange potential is extremely large so that a ferromagnet is half-metallic. Spin-flip scattering at junction interfaces drastically changes the characteristic behavior of Josephson current. In addition to spin-singlet Cooper pairs, equal-spin triplet pairs penetrate into a half metal. Such equal-spin pairs have an unusual symmetry property called odd-frequency symmetry and carry the Josephson current through a half metal. The penetration of odd-frequency pairs into a half metal enhances the low energy quasiparticle density of states, which could be detected experimentally by scanning tunneling spectroscopy. We will also show that odd-frequency pairs in a half metal cause a nonmonotonic temperature dependence of the critical Josephson current.

Quasiclassical Green's function theory of the Josephson effect in chiral p-wave superconductor/diffusive normal metal/chiral p-wave superconductor junctions

We study the Josephson effect in chiral p-wave superconductor/diffusive normal metal (DN)/chiral p-wave superconductor (CP/DN/CP) junctions using quasiclassical Greens function formalism with proper boundary conditions. The px+ipy-wave symmetry of superconducting order parameter is chosen which is believed to be the pairing state in Sr2RuO4. We show that the superconducting pair amplitude induced in DN has an odd-frequency spin-triplet s-wave symmetry and is an odd function of Matsubara frequency. Despite the peculiar symmetry properties of the Cooper pairs, the behavior of the Josephson current is rather conventional. We have found that the current-phase relation is almost sinusoidal and the Josephson current is proportional toexp(−L/xi), where xi is the coherence length in DN and L is the thickness of the DN layer. We also calculate the Josephson current in CP/diffusive ferromagnet metal/CP junctions and show that the 0-pi transition can be realized by varying temperature or thickness L similar to the case of conventional s-wave junctions. The obtained results may help to explore the properties of superconducting state in Sr2RuO4.

Angular dependence of Josephson currents in unconventional superconducting junctions

Josephson effect in junctions between unconventional superconductors is studied theoretically within the model describing the effects of interface roughness. The particularly important issue of applicability of the frequently used Sigrist-Rice (SR) formula for Josephson current in d-wave superconductor/insulator/d-wave superconductor junctions is addressed. We show that although the SR formula is not applicable in the ballistic case, it works well for rough interfaces when the diffusive normal metal regions exist between the d-wave superconductor and the insulator. It is shown that the SR approach only takes into account the component of the d-wave pair potential symmetric with respect to an inversion around the plane perpendicular to the interface. Similar formula can be derived for general unconventional superconductors with arbitrary angular momentum l.

Quantitative model for the ICR product in d-wave Josephson junctions

We study theoretically the Josephson effect in d-wave superconductor/diffusive normal metal/insulator/diffusive normal metal/d-wave superconductor (D/DN/I/DN/D) junctions. This model aims to describe practical junctions in high-TC cuprate superconductors, in which the product of the critical Josephson current (IC) and the normal state resistance (R) (the so-called ICR product) is very small compared to the prediction of the standard theory for clean d-wave superconductor/insulator/d-wave superconductor (DID) junctions. We show that the ICR product in D/DN/I/DN/D junctions can be much smaller than that in DID junctions. The proposed theory describes the behavior of ICR products quantitatively in high-TC cuprate junctions.

Spin-polarized carrier injection effect in ferromagnetic semiconductor/diffusive semiconductor/superconductor junctions

We study the transport properties of a p-InMnAs/n-InAs/Nb junction where a p-InMnAs can be regarded as a spin injector. Differential conductance of the n-InAs channel is measured as a function of injection current from p-InMnAs or from Nb at 20 mK. A conductance minimum appears at zero-bias voltage with no current injection. As the injection current from p-InMnAs increases, the minimum gradually disappears. This conductance behaviour is very different from that of the injection case from Nb. We also calculate the conductance in the n-InAs channel by taking account of the exchange field in the InAs channel that is induced by InMnAs ferromagnet. The difference between the conductance behaviours on injection current direction can be explained by the inverse proximity effect that the exchange field is also induced in the superconducting electrode.

JOSEPHSON π-STATE DUE TO SPIN-ACTIVE JUNCTION INTERFACE

Motivated by a recent experiment [R. S. Keizer, et. al., Nature 439, 825 (2006)], we study the Josephson effect in superconductor/diffusive half metal/superconductor junctions. The Josephson π-state is more stable than the 0-junction when the spin-flip scattering at the junction interface opens the Josephson channel of the odd-frequency Cooper pairs.