Y. Ochiai

The gadolinium ordering in superconducting and non-superconducting GdxY1−xBa2Cu3Oy compounds

Abstract In the 90 K superconductor GdxY1−xBa2Cu3Oy compounds, the antiferromagnetic ordering is observed below about 2.3 K by means of measurements of dc-susceptibility. The Neel temperatures of the compounds are proportional to the gadolinium concentrations, for x > 0.80. This result indicates that the gadolinium-gadolinium interaction in the compounds is of long range. In the non-superconducting GdxY1−xBa2Cu3Oy compounds, Neel temperatures are also proportional to gadolinium concentrations. No difference in the concentration dependence of Neel temperatures is observed between the superconducting and the non-superconducting compounds. It is concluded that the gadolinium ordering is almost independent of superconductivity.

Low temperature magnetoresistance in a narrow wire of GaAs/AlGaAs

Abstract Aperiodic magnetoresistance (MR) fluctuations in narrow GaAs/AlGaAs wires are studied up to 8 T by means of analyzing their Fourier transform. It is found that decay of the spectrum shifts toward low frequencies with increasing the magnetic field region where Fourier transformation is done. This suggests that the effective area for the quantum interference may be changed by the magnetic field. Shubnikov-de Haas (SdH) oscillations are also described.

Magnetic interactions in GdBa2Cu3Oy compounds

Abstract Electron-Spin-Resonance (ESR) measurements are performed for GdBa 2 Cu 3 O y compounds. The change of g values is observed in compounds of having different oxygen contents (6.1 2 (Cu 1−x M x ) 3 O y (M = Ni and Co) compounds are also observed. A broadening of the linewidth is observed with increasing x. Interesting temperature dependence of the linewidth is reported in these compounds. We will discuss the interactions among Gd ions.

Size effect in the low temperature magnetoresistance of narrow GaAs/AlGaAs wires

We have measured the low temperature magnetoresistance of narrow wires formed as constrictions in the two-dimensional electron gas of a GaAs/AlGaAs heterojunction. In the quasi-ballistic regime, two relaxation times, at high and low magnetic fields, are observed in the amplitude analysis of the Shubnikov-de Haas oscillations. The boundary field which distinguishes these relaxation times is found to be determined by a simple relationship between the conducting width and the corresponding cyclotron radius. We discuss these features in terms of the effect of the magnetic field on sidewall scattering in a quasi-ballistic wire.

The Gd ESR measurements in GdBa2(Cu1−xMx)3Oy (M= Ni, Co, and Fe) compounds

Electron-Spin-Resonance measurements are performed in GdBa2(Cu1−xMx)3Oy (M= Ni, Co, and Fe) compounds by using X, K, and Q-band measurements. The frequency dependence of the linewidths are observed. For GdBa2Cu3Oy compounds, the interactions among Gd ions are explained by the indirect exchange interaction of 6 kOe and the dipolar interaction of 3 kOe. In the Ni doped compounds, the linewidth broadening due to the decreasing of the exchange fields is observed. The same broadening is also observed in the Fe doped and quenched compounds. In this report, the magnetic behaviors of doped ions are discussed from the Gd ESR measurements.

Low-temperature magnetic properties of high Tc Gdx Y1−xBaCu oxide

Abstract Low-temperature magnetic susceptivilities of high T c compound of Gd x Y 1−x Ba Cu oxide have been studied down to 0.3 K and at the magnetic fields up to 8 T. For the case of x=1.00, the Neel temperature of antiferromagnetic ordering is about 2.3 K. The decrease of the Neel temperature is almost proportional to that of the Gd concentration. However, in the case of x=0.60, the Neel temperature becomes lower and deviates from the linear dependence in the high Gd concentrations. While the linear dependence of the Neel temperature can be expected from a decrease of the dipole-dipole interaction between Gd ions, above drastic decrease of the Neel temperature can not be explained by such a simple magnetic interaction.

The low-temperature magnetoresistance of Si/Nb superlattices

Abstract We have studied the correlation between localisation and superconductivity in two-dimensional thin films of amorphous Si/Nb. Si/Nb superlattice films, which exhibit superconducting transitions below 2 K, and a Si/Nb/Si sandwich film, which exhibits no superconducting transition down to 0.3 K, were fabricated in this experiment. In order to clarify their mesoscopic properties, we have measured the low-temperature magnetoresistance and analysed this using the transport parameters of weak localisation theory. The analysis suggests that the onset of superconductivity occurs once the superconducting coherence length becomes shorter than the inelastic scattering length, and we deduce that the onset of superconductivity in the disordered films is controlled by these characteristic length scales.

Low temperature conduction in GdBa2Cu3Oy(y<6.4) films

Abstract Low temperature magnetoresistances of high- T c compound film of GdBa 2 Cu 3 O y ( y

Magnetic Field Dependence of Aperiodic Conductance Fluctuations in Narrow GaAs/AlGaAs Wires

The magnetic field dependence of aperiodic fluctuations has been studied by means of analysing the Fourier transform of the magnetoresistance in narrow GaAs/AlGaAs wires. Near the critical field of ωcτ=1, the decay of aperiodic fluctuations, which is observed on the low frequency side of the Fourier spectra, shifts to low frequencies. The shift can be explained by reduction of the effective area for the interference.

Conductance Fluctuations in GaAs/AlGaAs Narrow Wires in Quasi-Ballistic Regime

Conductance Fluctuations due to the quantum interference of electrons have been studied in GaAs/AlGaAs narrow wires in the quasi-ballistic transport regime. Narrow channels of length 2 µm and 6 µm were confined by metallic split gates formed by electron beam lithography and a lift off technique. In the shorter wire, the amplitude of the fluctuations first increased as a function of increasing width, and finally approached to the constant value of e2/h. On the other hand, it was almost independent on channel width in the longer wire, as expected for the universal conductance mechanism in the diffusive transport regime. We suspect that the non universal width dependence in the shorter wire might result from averaging among the individual transmission channels, caused by the small number of scattering events.