音 賢一

We investigate commensurate oscillations in ordered and disordered artificial lateral superlattice (ALS) systems, in which the anti-dots are arranged in a square or triangular lattice. With increasing disorder of the anti-dot location, the peaks of the commensurate oscillations fade out. The peak heights are more strongly affected by the disorder along perpendicular direction to the current than by that along the parallel direction. In the square ALS system, the commensurate oscillations seem to be determined principally by the order along the perpendicular direction to the current, while in the triangular ALS system, the commensurate oscillations would be determined by the nearest neighbor distance between anti-dots and the order along the perpendicular direction. In addition, the appearance of each peak is determined by the ratio of the anti-dot diameter to the ALS period. The weak localization effect in very low magnetic field and the strongly temperature dependent conductance in the absence of magnetic field are also observed in the ALS systems.

Normal and superconducting properties of PtSi films with thicknesses of d=2-20 nm have been investigated. The superconducting transition has been observed on the thin films down to d=4 nm. The systematic reduction of the transition temperature with decreasing d (increasing the sheet resistance) has been explained by the localization and Coulomb interaction effects on superconductivity. The temperature dependence of the upper critical field indicates that the PtSi films behave as homogeneous superconductors.

The magnetic field dependence of the capacitance of a GaAs/AlGaAs heterostructure through a two-dimensional electron system (2DES) is measured in the quantum Hall regime. It is found that the values of the capacitance minima at the Hall plateaus are determined not by the 2DES area under the gate but by the edge length of the 2DES. These results cannot be explained by an existing interpretation of the magnetocapacitance, where the capacitance is directly related to the density of states of the 2DES. We suggest an alternative model in which the bottom values are essentially determined by the current carrying area at the edge. The widths of the current channels are estimated and compared with the recent theoretical and experimental estimates. © 1994 The American Physical Society.

The nonlocal Shubnikov-de Haas (S-dH) resistance has been investigated in a GaAs/AlGaAs wire with Schottky gates across the separation path between current and voltage probes. With decreasing electron concentration below the gate, the up-spin S-dH oscillation peak vanishes at higher electron concentration than the down-spin peak. This can be understood by the fact that the bulk current at the down-spin peak actually consists of both down- and up-spin states and that the one at the up-spin peak consists mostly of the up-spin state. Further the influence of extra probes, which have ostensibly nothing to do with the four-terminal resistance, has been studied by disconnecting the extra probes electrically by the gate across the extra terminal. When the extra probe is disconnected, both normal and nonlocal S-dH oscillation peaks at high magnetic fields are altered.

The Shubnikov-de Haas (SdH) effects of multi-terminal GaAs/AlGaAs wires in the nonlocal geometry, where the nominal current path is spatially separated from and does not intersect with the voltage probes, have been measured in the wide range of the separation length (0.4-mu-m < DELTA-L < 1.5 mm) between the current and voltage probes, which will be related to the quantum effect in connection with the edge currents. Further, it is found that the signal of nonlocal SdH resistance becomes even negative, when the contact resistance is very large. This will be explained by the non-equal chemical potentials among the edge currents and the different transmission probabilities of the edge current to the probe due to the selective contact resistance for each edge state.

The Shubnikov-de Haas (SdH) oscillations in the nonlocal geometry (RNL) have been observed in the Si-MOSFET, where the voltage probes are well separated from the nominal current path. The mobility, excitation current and temperature dependencies of the RIN have been investigated. The separation length between the current path and voltage probes is much longer than the electron mean free path in the sample. The nonlocal SdH effect can be understood by considering both edge and extended (bulk) states. It is demonstrated that the edge state plays an important role also in the Si-MOSFET quantum transport. © 1992.

Nonlocal Shubnikov-Haas (SdH) oscillations have been measured in multiterminal GaAs/AsxGa1-xAs wires, where the Schottky gate lies across the nonlocal path between the nominal current and voltage paths. The influence of the bulk and edge states on the nonlocal resistance has been investigated by cutting the bulk and edge current at the gate. With decreasing gate voltage and carrier concentration underneath the gate, the SdH oscillation peaks of the down-spin Landau levels diminish at lower gate voltage than those of the up-spin levels. This could be explained by the fact that ''the bulk current'' corresponding to the down-spin Landau-level peak of the SdH oscillation comes from the overlapping down-spin and up-spin states, while the up-spin Landau-level peak comes from mostly the up-spin state. © 1992 The American Physical Society.

The Shubnikov-de Haas (SdH) oscillations of multi-terminal GaAs/AlGaAs wires have been observed in the nonlocal geometry with very long separation length (0.5mm < DELTA-L < 1.5mm) between the current and voltage probes. The nonlocal effect seems to be related to the macroscopic quantum effect in connection with the bulk and edge currents, where energy relaxation length of edge currents is very long. Further, it is found that the signal of nonlocal SdH resistance becomes even negative, when the contact resistance is very large. This will be explained by nonequilibrium among chemical potentials of edge currents and by different transmission probabilities of edge currents to the probe due to the selective contact resistance for each edge state.

The nonlocal Shubnikov-de Haas (SdH) oscillations in GaAs/AlGaAs mesoscopic multi-terminal wires have been measured. The ampitude of the oscillations decreases in general with increasing nonlocal separation length (ΔL) between the nominal current path and the voltage probes. The amplitude also becomes weak, when there exist branches with electrodes between the current and the voltage probes which have ostensibly nothing to do with the voltage measurement. The behavior is interpreted by the combination of the edge and bulk current at high magnetic fields. © 1991.

The temperature dependence of Hall coefficient and electrical resistivity have been measured in BiSrCaCuO superconductor system. The Hall coefficient (RH) of the 110K phase is nearly proportional to 1/T, while the temperature dependence of RH in the 80K phase is smaller than those in the 110K phase. It seems that there is an optimum "carrier concentration" (1/eRH) to get the maximum Tc in the 80K phase. © 1989.