Satoshi Kashiwaya

Tunnelling effects on surface bound states in unconventional superconductors

Recent studies on high-Tc superconductors have aroused new interest in tunnelling effects in unconventional superconductors. Unlike in conventional s-wave superconductors, the d-wave pairing state in these materials has an internal phase of the pair potential. The internal phase as a function of the wavevector of the Cooper pairs has a large influence on the electric properties of tunnelling junctions. Important effects of the internal phase on the Josephson current were first predicted theoretically. The idea has been established through several experiments using high-Tc Josephson junctions, which detect ?-phase shift between the a- and b-axis directions and fractional flux quanta. These results give convincing evidence for d-wave symmetry in high-Tc superconductors. In addition, the existence of new interference effects in the quasiparticle states near surfaces and boundaries has been suggested through theoretical predictions. Experimentally, a large number of tunnelling spectroscopy data showed zero-bias conductance peaks (ZBCPs), the origin of which cannot be explained in terms of the classical concept that a tunnelling conductance spectrum is a phase-insensitive probe of the electronic states. It is clarified theoretically that the observed ZBCPs reflect the formation of zero-energy states on the surface due to the ?-phase shift of internal phase in the d-wave pairing symmetry. The formulation developed for tunnelling spectroscopy suggests that tunnelling spectroscopy is essentially phase sensitive. In addition, the formation of the bound states has been shown to have a serious influence on the electrical properties of Josephson junctions. Several anomalous properties including strong enhancement of the Josephson current in the low-temperature region have been predicted theoretically. In this report, recent developments in tunnelling effects on surface bound states in unconventional superconductors are reviewed.

SPIN CURRENT IN FERROMAGNET-INSULATOR-SUPERCONDUCTOR JUNCTIONS

A theory of spin polarized tunneling spectroscopy based on a scattering theory is given for tunneling junctions between ferromagnets and d-wave superconductors. The spin filtering effect of an exchange field in the insulator is also treated. We clarify that the properties of the Andreev reflection are largely modified due to a presence of an exchange field in the ferromagnets, and consequently the Andreev reflected quasiparticle shows an evanescent-wave behavior depending on the injection angle of the quasiparticle. Conductance formulas for the spin current as well as the charge current are given as a function of the applied voltage and the spin-polarization in the ferromagnet for arbitrary barrier heights. It is shown that the surface bound states do not contribute to the spin current and that the zero-bias conductance peak expected for a d-wave superconductor splits into two peaks under the influence of the exchange interaction in the insulator.

THEORY OF TUNNELING SPECTROSCOPY IN SUPERCONDUCTING SR2RUO4

A theory for tunneling spectroscopy in normal metal /insulator/triplet superconductor junction is presented. We assume two kinds of non-unitary triplet superconducting states which are the most promising states for Sr

Anomalous charge transport in triplet superconductor junctions

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Edge states of Sr2RuO4 detected by in-plane tunneling spectroscopy.

RuO

Crystal growth of ZnO

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Theory of Tunneling Conductance for Normal Metal/Insulator/ Triplet Superconductor Junction

. The calculated conductance spectra showzero-bias peaks as well as gap structures. The existences of residual components in the spectra reflect the non-unitary properties of superconducting states.

Theory of Local Density of States of d x2- y2-Wave Superconducting State Near the Surfaces of the t- J Model

Charge transport properties of a diffusive normal metal /triplet superconductor (DN/TS) junction are studied based on the Keldysh-Nambu quasiclassical Greens function formalism. % Contrary to the unconventional singlet superconductor junction case, the mid gap Andreev resonant state (MARS) at the interface of the TS is shown to enhance the proximity effect in the DN. % The total resistance of the DN/TS junction is drastically reduced and is completely independent of the resistance of the DN in the extreme case. % Such anomalous transport accompanies a giant zero-bias peak in the conductance spectra and a zero-energy peak of the local density of states in the DN region. % These striking features manifest the presence of novel proximity effect peculiar to triplet superconductor junctions.

Josephson current in s-wave-superconductor/Sr2RuO4 junctions

Tunneling spectroscopy has been performed on Sr(2)RuO(4) searching for the edge states peculiar to topological superconductivity. Conductance spectra exhibit broad humps with three types of peak shape: domelike peak, split peak, and two-step peak. By comparing the experiments with predictions for unconventional superconductivity, these varieties are shown to originate from multiband chiral p-wave symmetry with weak anisotropy of pair amplitude. The broad hump in the conductance spectrum is a direct manifestation of the edge state due to chiral p-wave superconductivity.

Phenomenological theory of zero-energy Andreev resonant states

Abstract Centimeter-sized crystals of zinc oxide have been grown by the top-seeded solution growth method and traveling-solvent floating-zone method using a mixed solvent of V 2 O 5 +B 2 O 3 and MoO 3 +V 2 O 5 . The crystals were brown, pale yellow and white semi-transparent in color depending on the solvent used. The characterization of the crystals by scanning electron microscopy X-ray energy-dispersion spectroscopy is reported.