Toshinari Goto

Properties of Nb3Ge variable‐thickness bridges

Variable‐thickness bridges (VTB’s) using Nb3Ge film have been investigated. In order to form a very short length bridge in the direction of the current flow and cause little damage to the superconducting properties, slant evaporation and anodization technique was used. The current–voltage characteristics, temperature dependence of critical current response to microwave irradiation were measured. One of the VTB’s whose length is estimated to be less than 100 nm showed microwave induced steps over a wide temperature range, from 6 to 18.3 K. The resistively shunted Josephson junction model is found to explain the behavior of the device well.

Preparation of YBCO films on sapphire with CeO/sub 2/ deposited by ion beam sputtering

Although sapphire has superior high frequency properties, an adequate buffer layer is required to form a high-Tc film. We deposited a CeO/sub 2/ buffer layer by ion beam sputtering in high oxygen pressure, and investigated the influence of the deposition conditions on the superconducting properties of YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) films. CeO/sub 2/ films deposited at higher than 600/spl deg/C were c-axis oriented and YBCO films deposited on the top at 700/spl deg/C were also c-axis oriented. T/sub /spl alpha//(R=0)=89 K and J/sub c/>10/sup 6/A/cm/sup 2/ were obtained for a 120 nm thick YBCO film deposited on 50 nm thick CeO/sub 2/ layer. High oxygen pressure /spl sim/1 Pa is desirable. Grain boundary Josephson junctions were also successfully fabricated on bicrystal sapphire substrate. It was found that high pressure ion beam sputtering is a promising method for superconductive electron devices.

Submillimeter wave response of a high Tc superconducting microbridge

We observed the ac Josephson effect, the ‘‘Shapiro step,’’ at the high frequency of 584 GHz in an YBa2Cu3O7−δ grain boundary Josephson junction at 4.2 K for the first time. This Shapiro step grew to about 8% of the critical current without irradiation. We also noted that the critical current was gradually suppressed with increasing power. The critical current with irradiation was suppressed to about 97% of the critical current without irradiation. Detailed investigation revealed the consistency between the above two phenomena.

Fabrication of Variable-Thickness Bridge Using YBCO Thin Film

In-situ-grown YBCO thin films were prepared on a MgO substrate by rf magnetron sputtering with a dc-biased substrate holder using a stoichiometric Y1Ba2Cu3Oy target. The variable-thickness bridges with the bridge length of 80 nm-200 nm were fabricated in these films by means of photolithography and slant evaporation techniques. The current-voltage characteristics of the bridge in rf radiation show the structure of the rf-induced steps over a wide temperature range.

Fabrication of a high-T/sub c/ superconducting field effect transistor by ion beam sputtering

We have fabricated Au/SrTiO/sub 3/(STO)/YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) trilayer structures and superconducting field effect transistors (SuFETs). Insulating STO films were deposited by the ion beam sputtering (IBS) method at high oxygen pressure (/spl sim/10/sup -2/ Torr). (l00) peaks were observed in the X-ray diffraction (XRD) patterns of STO films deposited at temperatures higher than about 400/spl deg/C. The leakage current characteristics of a 400-nm-thick STO film were almost symmetric at different polarities and low leakage current of the order of 10/sup -9/ A was obtained up to around /spl plusmn/25 V at 4.2 K. The dielectric constant electric field product /spl epsiv//sub rS/E/sub BD/ of the STO film was estimated to be about 2.2/spl times/10/sup 8/ V/cm. In the SuFET fabrication process, a 10-nm-thick YBCO film covered with the STO layer was hardly degraded. The modulation of the normal state resistance of a sample almost corresponds to that of the induced carrier at positive bias. When a voltage of +20 V was applied to the gate electrode at 4.2 K, the current decreased by about 5% and the mutual conductance was 3.8 /spl mu/S at a drain voltage of 10 mV.

Josephson Effects in Niobium Variable-Thickness Bridges

Oblique deposition and anodization technique have been used to fabricate niobium variable-thickness bridges of length ~0.2 µm. Very sharp microwave-induced steps are observed and the height changes periodically as the microwave power increases. The bridges are found to show Josephson effects in a wide temperature range and have large IcRn products which are favorable in practical Josephson devices.

Fabrication and properties of YBCO tunnel junctions by ion beam sputtering

Au/SrTiO/sub 3//YBCO tunnel junctions were fabricated by ion beam sputtering. The electrodes of YBCO and SrTiO/sub 3/ barrier layer were deposited on MgO [100] substrate at 700/spl deg/C and 550/spl deg/C respectively. The electrodes of YBCO had a T/sub c/ of 70/spl sim/80 K. SrTiO/sub 3//YBCO bilayers have been examined by X-ray diffraction and the results show that both the YBCO and barrier layer were well oriented. The current-voltage (I-V) and dV/dI-V characteristics of the junctions were measured. In a pin hole free junction, the resistance dip, corresponding to the density of states, was obtained at voltage of 22 mV.<<ETX>>

Josephson effects in A-axis oriented YBa2Cu3O7-y SIS tunnel junctions with double layer barrier

Abstract In this study, we have fabricated vertical SIS tunnel junctions with a-axis oriented YBa 2 Cu 3 O 7−y (YBCO) electrodes. In order to prevent the pinhole in the barrier layer, double layer consists of PrGaO 3 (PGO) and CeO 2 was used. The standard lithography technology and ion milling were employed to define the junctions with area of 3 μ m × 5 μ m. The junctions with barriers 5nm thick PGO and 2.5nm thick CeO 2 showed supercurrents and RSJ like voltage-current(I–V) characteristics. Fraunhofer like magnetic field dependence of Ic were observed, and the results proved the existence of Josephson effects in these junctions.

Properties of Bi-Sr-Ca-Cu-O (2212) films deposited by sputtering on tilted substrates

Bi-Sr-Ca-Cu-O (2212) thin films have been deposited on tilted SrTiO/sub 3/ substrates using a dc sputtering system. These films have been deposited at 710-790/spl deg/C. The thickness of these films is about 170 nm. The films deposited at 730, 750 and 770/spl deg/C exhibit superconductivity, and the highest value of critical temperature is about 70 K for the film deposited at 750/spl deg/C. The crystal structure of this film has also been investigated by X-ray diffraction with c-axis oriented 2212 phase.

Preparation of Y-Ba-Cu-O films by ion beam sputtering with a hollow cathode oxidation

Abstract Deposition of as-grown high Tc superconducting films by ion beam sputtering was investigated. In order to improve the superconductivity, oxygen should be activated. For this purpose, we developed a hollow cathode activation system to deposit superconducting YBCO films. When the current of hollow cathode was 100mA, we obtained the Tc of 87K for YBCO film on (100) SrTiO 3 substrate at 730°C. YBCO targets with varying composition were used and resulting YBCO films were examined by X-ray diffraction. The result showed optimum target to obtain pure YBCO film was Ba and Cu rich one (YBa 2.2 Cu 4 O x ). IBS are suitable to deposit various materials such as insulator, the YBCO-Y 2 O 3 -YBCO trilayer structures were also succecfully formed.