Showing papers by "Saburo Takahashi published in 2000"

Conductance quantization and magnetoresistance in magnetic point contacts.

Abstract: We theoretically study the electron transport through a magnetic point contact (PC) with special attention given to the effect of an atomic scale domain wall (DW). The spin precession of a conduction electron is forbidden in such an atomic scale DW and the sequence of quantized conductances depends on the relative orientation of magnetizations between left and right electrodes. The magnetoresistance is strongly enhanced for the narrow PC and oscillates with the conductance.

Coulomb staircase in STM current through granular films

Abstract: The electron transport through an array of tunnel junctions consisting of an STM tip and a granular film is studied both theoretically and experimentally. When the tunnel resistance between the tip and a granule on the surface is much larger than those between granules, a bottleneck of the tunneling current is created in the array. It is shown that the period of the Coulomb staircase(CS) is given by the capacitance at the bottleneck. Our STM experiments on Co-Al-O granular films show the CS with a single period at room temperature. This provides a new possibility for single-electron-spin-electronic devices at room temperature.

Spin injection and magnetoresistance in ferromagnet– superconductor–ferromagnet tunnel junctions

Abstract: We theoretically study the spin-dependent transport in a ferromagnet–superconductor–ferromagnet double barrier tunnel junction. The spin-polarized tunneling currents give rise to spin imbalance in the superconductor. The resulting nonequilibrium spin density suppresses the superconductivity with increase of the tunneling currents. We focus on the effect of asymmetry in the double tunnel junction, where the barrier height of the tunnel junction and the spin polarization of the ferromagnets are different, on spin injection, and discuss how the superconductivity is suppressed in the asymmetric junction. Our results explain recent experimental results on the critical current suppression in high-Tc superconductors by spin injection.

Spin injection in ferromagnet/superconductor/ferromagnet tunnel junctions

Abstract: We theoretically study the spin-dependent transport in a ferromagnet/superconductor/ferromagnet double barrier tunnel junction. The spin-polarized tunneling currents create spin and charge imbalance in a superconductor. Nonequilibrium effects are dominated by spin imbalance, causing an unusual spin transport and suppression of superconductivity depending on whether the magnetizations of the electrodes are parallel or antiparallel. Recent experimental results on the critical current suppression by spin injection are discussed.

TEMPERATURE DEPENDENCE OF JOSEPHSON PLASMA MODES IN Bi2Sr2CaCu2O8+δ NEAR Tc

Abstract: A strong temperature dependent phenomenon of the Josephson plasma resonance mode has been found in high-Tc superconductor Bi2Sr2CaCu2O8+δ in a microwave frequency region between 9 and 50 GHz. The longitudinal plasma frequency sharply decreases and disappears just below Tc. The extrapolated plasma energy is estimated to be ℏωp(0)=2.59× 10-4eV. Since the plasma frequency, ωp, is determined by the Anderson-Higgs–Kibble mechanism and it is expected to be temperature independent, this phenomenon can not be accounted for by the conventional underst and ing of the plasma mode in superconductors. Experimental results are discussed in terms of the two fluid model, in which the intrinsic Josephson nature of the coupling restricting the tunneling probability of quasiparticles between layers is considered to be as a dumping mechanism of the quasiparticles in this system.