Nobukatsu Yoshida

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 Spin Polarized Tunneling in Superconducting Sr2RuO4.

A theory of tunneling conductance in ferromagnetic-metal / insulator / triplet-superconductor junctions is presented for unitary and non-unitary spin triplet pairing states which are promising candidates for the superconducting pairing symmetry of Sr 2 RuO 4 . As the magnitude of the exchange interaction in the ferromagnetic metal is increased, the conductance for the unitary pairing state below the energy gap is reduced in contrast to that for the non-unitary pairing state. This is due to the fact that the retro-reflectivity in the Andreev reflection for unitary pairing state is broken due to the influence from the exchange interaction.

Observation of the second harmonic in superconducting current-phase relation of Nb/Au/(001)YBa2Cu3Ox heterojunctions

The superconducting current-phase relation (CPR) of Nb/Au/(001)YBa2Cu3Ox heterojunctions prepared on epitaxial c-axis–oriented YBa2Cu3Ox thin films has been measured in a single-junction interferometer. For the first time, the second harmonic of the CPR of such junctions has been observed. The appearance of the second harmonic and the relative sign of the first and second harmonics of the CPR can be explained assuming that the macroscopic pairing symmetry of our YBa2Cu3Ox thin films is of the d + s type.

Parity-Sensitive Measurements Based on Ferromagnet/Superconductor Tunneling Junctions

A method for identifying the parity of unconventional superconductors based on tunneling spectroscopy is proposed. For a model of calculation, we adopt a ferromagnet/superconductor (F/S) junction, ...

Spin-polarized tunneling of La0.67Sr0.33MnO3/YBa2Cu3O7−δ junctions

Abstract The transport properties in contacts between ferromagnets and high- T c superconductors were investigated in La 0.67 Sr 0.33 MnO 3 /YBa 2 Cu 3 O 7− δ cross-strip junctions. The conductance spectra show zero-bias conductance peaks (ZBCPs), reflecting charge transport along the ab plane. When an external magnetic field is applied to the junctions, the conductance spectra show two notable features, i.e., an increase in background conductance and an asymmetric field response of ZBCP whose width responds nonlinearly to the applied field. These features are shown to be qualitatively consistent with a theoretical prediction of tunneling spectroscopy, in which presence of a ferromagnetic insulator layer between a spin-polarized ferromagnet and a d-wave superconductor is assumed. Possible origins for the ferromagnetic insulator are discussed.

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 4e

Tunneling Conductance and Spatial Dependences of Pair Potentials in Normal Metal-Triplet Superconductor Junctions

Tunneling conductance spectra in normal metal / insulator / triplet superconductor junctions are theoretically investigated. For the triplet paring states, we select several kinds of the symmetries...

Theory of tunneling conductance in quantum-wire/unconventional superconductor junctions

Tunneling conductance in quantum-wire/insulator/unconventional superconductor (q/I/S) junctions is calculated in the case where the unconventional superconductor has triplet symmetry. Differently from the case in d-wave superconductor junctions, the magnitude of the tunneling conductance near zero voltage is enhanced due to the Andreev bound state by the quasiparticle with perpendicular injection. From the tunneling spectroscopy of q/I/S junctions, we can distinguish p-wave (triplet) superconductor from d-wave (singlet) one.

A relation between the zero-bias conductance and spin polarization in ferromagnetic metal/d-wave superconductor junctions

Abstract The relation between the zero-bias conductance and spin polarization in ferromagnetic metal/d-wave superconductor junctions is studied theoretically. It is found that, in the presence of the zero-energy states (ZES), the spin polarization of the ferromagnetic metal at low temperatures hardly depends on the magnitude of the barrier potential in contrast to that in the absence of ZES. The obtained results suggest that junctions comprised of Mn oxides compounds and high-Tc superconductor make it possible to measure the spin polarization of Mn oxides.

Influence of the Impurity-scattering on Zero-bias Conductance Peak in Ferromagnet/Insulator/d-wave Superconductor Junctions

Effects of impurity-scattering on a zero-bias conductance peak in ferromagnet/insulator/ d -wave superconductor junctions are theoretically studied. The impurities are introduced through the random potential in ferromagnets near the junction interface. As in the case of normal-metal/insulator/ d -wave superconductor junctions, the magnitude of zero-bias conductance peak decreases with increasing the degree of disorder. However, when the magnitude of the exchange potential in ferromagnet is sufficiently large, the random potential can enhance the zero-bias conductance peak in ferromagnetic junctions.