Yukihiko Wada

Hysteretic Josephson Junction Behavior of Ba1-xKxBiO3 Grain Boundary Junctions Using SrTiO3 Bicrystal Substrates

We have fabricated Ba1-x Kx BiO3 artificial grain boundary junctions with low resistance on a SrTiO3 bicrystal substrate. These junctions show hysteretic junction behavior with a clear superconducting energy gap structure and Josephson current. The area resistance was low; it ranged from 5.2×10-8 to 2.0×10-5 Ω cm2. The I cR n product of the 20-µm-wide junction was 3.5 mV at 4.2 K. The gap voltage increased as the heater temperature increased and reached about 8 mV. A weak magnetic field applied normal to the substrate induced a Fraunhofer-type pattern.

Fabrication of Full High-Tc Superconducting YBa2Cu3O7-x Trilayer Junctions Using a Polishing Technique

We have successfully fabricated full high-Tc superconducting YBa2Cu3O7-x(YBCO)/PrBa2Cu3O7-x(PBCO)/YBCO trilayer junctions, which have a simple device structure, such as a Pb-alloy-based Josephson tunneling junction. It has been demonstrated that a polishing technique is extremely useful in the fabrication process: it is effective in smoothing a coarse surface and gentling the slopes of the edges, or decreasing the slope angles. Owing to the polishing technique, the PBCO barrier layer and the upper YBCO layer have been notably thinned: the thicknesses of these layers are 10 nm and 250 nm, respectively. Junctions with the dimensions of 5 µm × 5 µm showed resistively shunted junction-like current-voltage curves with a typical critical current density of 110 A/cm2 at 4.2 K. Furthermore, the operation of superconducting quantum interference devices has been demonstrated.

YBaCuO/PrBaCuO/YBaCuO Trilayer Junctions on Vicinal Substrates.

Trilayer junctions with c-axis oriented YBaCuO/PrBaCuO/YBaCuO films were fabricated. We used vicinal substrates in order to improve the surface morphology of YBaCuO base electrodes. Junctions with PrBaCuO barrier thickness of 15 nm showed resistively shunted junction (RSJ)-like I–V curves with a small excess current of below 10% at 4.2 K. The spreads (1σ) of Ic and Rn were 11% and 8.8%, respectively, for 14 junctions on a chip. These values are markedly small considering the trilayer junctions. The flattening of the interfaces between YBaCuO and PrBaCuO layers may improve the uniformity of the barrier thickness.

Significant Improvement in Ba 1-xKxBiO 3 Grain Boundary Junctions on MgO Bicrystal Substrates by Minimal BaBiO 3 Sputtering

High quality Ba1-xKxBiO3 (BKBO) artificial grain boundary junctions were fabricated on MgO (100) bicrystal substrates. We found that extremely-short-time sputtering of BaBiO3 (BBO) onto MgO substrate prior to BKBO deposition results in a marked improvement in the quality of the BKBO film. Utilizing this technique, a junction with J c=6 kA/cm2 and ΔV g≈1 mV at 4.2 K was fabricated. In addition, a junction with a high critical current density of J c=78 kA/cm2 was obtained.

Observation of two in-plane epitaxial states in (001) Bi2Sr2CaCu2Ox films formed on (001) MgO

Abstract X-ray and ICP-MS characterizations are performed in c -axis oriented Bi 2 Sr 2 CaCu 2 O x (2212) films on (001) MgO single-crystal substrates. The chemical composition adjustment enables us to realize two distinctly different in-plane epitaxial states, namely 2212 [100] ‖ MgO [100] or 2212 [100] ‖ MgO [110].

Improvement in Ba 1-xKxBiO 3 Grain Boundary Junctions by Ar + Beam Irradiation

To control the normal resistance of Ba1-xKxBiO3 (BKBO) artificial grain boundary junctions, the influence of Ar+ beam irradiation on BKBO junctions on MgO (100) bicrystal substrates was investigated. Contrary to our expectation, the properties of all junctions were found to be improved. In the most marked changes, the critical current density increased from 2.3 kA/cm2 to 31 kA/cm2, while the normalized junction resistivity decreased from 1.4 × 10-6 Ωcm2 to 1.4 × 10-7 Ωcm2. No change was observed in the gap voltages and the critical current density of the BKBO films.

Bi-Sr-Ca-Cu-O Thin Films Grown by Laser Ablation

Superconducting Bi-Sr-Ca-Cu-O thin films were grown on SrTiO3(100) substrates in oxygen atmosphere by laser ablation using an ArF excimer laser and Bi2Sr2Ca2Cu3O x composition sintered targets. X-ray diffraction patterns showed that the films consist of the mixture of 2223 and 2212 phases, whereas inductively coupled plasma (ICP) data indicated their compositions are almost 2223 ratio, especially that deposited in higher oxygen pressure. The highest critical temperture (Tc0) of the films is 70 K.

100 GHz Band Planar-Type SIS Mixer Using BaKBiO Grain Boundary Junctions

BaKBiO (BKBO) grain boundary junctions were fabricated on a MgO bicrystal substrate with a misorientation angle of 24°. These junctions showed a superconductor-insulator-superconductor (SIS) behavior with a wide gap voltage (2Δ ≈ 8 mV). A 100 GHz band planar-type SIS mixer consisting of microwave integrated circuit elements such as a power splitter, directional couplers, matching circuits, band rejection filters and resistors (Ta2N) was fabricated.