Shinji Inoue

Analysis of I/sub c/ spreads in interface engineered junctions

We have investigated the electrical properties of interface engineered junctions fabricated under various process conditions to clarify the possible origin of the spreads in I/sub c/ and R/sub n/ values. We found that in most cases, the I/sub c/R/sub n/, values of junctions on a wafer scaled with the square root of I/sub c/. The distribution of both the junction I/sub c/ and R/sub n/ were expressed well by the log-normal distribution function with their /spl sigma/ values being different by a factor of two. These results indicate that Cooper pairs and quasiparticles in the junctions transfer through different transport channels, i.e., direct tunneling for the former and resonant tunneling for the latter, and that a variation in the tunnel barrier thickness among junctions dominates the spreads in the I/sub c/ and R/sub n/ values. In contrast, we could not observe a universal scaling relation between I/sub c/R/sub n/, and I/sub c/ among junctions processed under different conditions. This indicates that the I/sub c/ variation due to process conditions originates from something other than a simple barrier-thickness variation.

Josephson Effect in YBa2Cu3O7/Au-Ag/Pb Junctions

A comprehensive study was carried out on the Josephson effect in YBCO/Au-Ag/Pb SNS junctions prepared on YBCO thin films with various crystal orientations. Most of the fabricated junctions showed a dc Josephson current with a clear Fraunhofer pattern dependence on the applied magnetic field, which ensured the uniformity of the fabricated junction structure. Experimental results were quantitatively compared with the conventional junction theory based on the Usadel equation. The junction characteristics were confirmed to be reasonably explained by a junction model in which a thin insulating or highly resistive layer was assumed to exist at the YBCO-Au interface.

Design and fabrication of a voltage divider utilizing high-T/sub c/ ramp-edge Josephson junctions with a ground plane

Circuit operation of a voltage divider utilizing YBa/sub 2/Cu/sub 3/O/sub x//Co-doped PrBa/sub 2/Cu/sub 3/O/sub y//YBa/sub 2/Cu/sub 3/O/sub x/ ramp-edge Josephson junctions with a ground plane was demonstrated for the voltage region of 0-400 /spl mu/V at 12.5 K, where the maximum voltage corresponds to /spl sim/200 GHz internal Josephson oscillation. This voltage was around the critical current-normal resistance (I/sub c/R/sub n/) product of 420 /spl mu/V. In the design of the voltage divider, a transmission line model was adopted for an inductance loop because of the large dielectric constant of the insulation layer interposed between the ground plane and the wiring serving as a loop inductance. In the fabrication of the circuit, a SrTiO/sub 3//Co-doped PrBa/sub 2/Cu/sub 3/O/sub y//SrTiO/sub 3/ multilayer insulation and four-directional ramp-edge junction process using a photoresist reflow technique were adopted.

Transport processes in ramp-edge-type Josephson junctions

Abstract Current transport mechanisms in ramp-edge-type junctions with a Co-doped PrBa2Cu3O7−x (PBCO) barrier were discussed. We confirmed that the quasiparticle current in the junctions was predominated by resonant tunneling via localized states with a radius of around 1 nm in the barrier. In contrast, when we took the spatial variation of the barrier-layer thickness into account, the Josephson critical current was found to be explained quantitatively by a direct tunneling mechanism alone. Electric properties of interface-engineered junctions were also investigated. The junctions were considered to be of the superconductor–normal metal–insulator–normal metal–superconductor (SNINS) type. The localized states in the barrier seemed to play a less important role in determining the junction characteristics compared with the Co-doped PBCO barrier junctions.

Characterization of ramp-type Josephson junctions with a Co-doped PrBaCuO barrier

Current transport mechanism in Go-doped PrBaCuO barriers was investigated for ramp-type junctions. The junction characteristics were extremely sensitive to a slight variation in the substrate temperature and the oxygen atmosphere during the heating process for the subsequent barrier layer deposition. Such sensitivity was related to the thermodynamic stability of YBaCuO. The conductance of junctions fabricated under the optimized conditions exhibited an exponential dependence on the barrier layer thickness at low temperatures, and the decay length was estimated to be around 1 nm. Characteristic power law dependence of junction conductance on temperature was confirmed for these junctions, indicating that resonant tunneling and hopping conduction via a small number of localized states were predominant. Clear Josephson characteristics were observed for junctions with a barrier thinner than 11 nm. We found that the experimental I/sub c/ versus barrier thickness relation was also explained well by resonant tunneling models.

Interface fabrication of YBa2Cu3Oy ramp-edge junction with PrBa2(Cu1−xCox)3Oy barrier layer in enhanced oxidizing atmosphere

Abstract Thin film growth and fabrication process for YBa 2 Cu 3 O y /PrBa 2 (Cu 1− x Co x ) 3 O y /YBa 2 Cu 3 O y ramp-edge type junctions have been studied. Preferable conditions to maintain a stable ramp surface were investigated by comparative measurements of the surface morphology of ramp-edge structures of epitaxial bilayers of SrTiO 3 /YBa 2 Cu 3 O y annealed at various conditions of temperature and oxygen partial pressures. The stability of the ramp-edge surfaces was discussed in terms of the equilibrium reaction of YBa 2 Cu 3 O y , and the degradation of surfaces by damages through the fabrication process was concluded to be the main factor for the decomposition at high temperatures. A two-step etching technique and annealing with atomic oxygen activated by microwave enabled the ramp surfaces to be maintained quite stable even at the deposition temperature of barrier layers. The ramp surface of YBa 2 Cu 3 O y annealed under 4×10 14 cm −2 s −1 activated oxygen flux gave no evidence of decomposition at 800°C except for a slight decrease in thickness in the YBa 2 Cu 3 O y layer. In ECR (electron cyclotron resonance)-processed junctions, resistively shunted junction (RSJ) type current–voltage characteristics were observed and the magnetic field dependence of the critical current showed a Fraunhofer pattern.

Proximity junctions on YBCO thin films

Abstract A comprehensive study has been made on YBCO/AuAg/Pb SNS junctions fabricated on YBCO films with various crystal orientations. Most of the junctions, except those on (001) YBCO, exhibitted a dc Josephson current with a clear Fraunhofer pattern dependence on the magnetic field. The junction characteristics have been reasonably explained by an SINS junction model.

Interface-engineered junctions with YbBaCuO as the counter-electrode

The electric properties of interface-engineered junctions with YbBa/sub 2/Cu/sub 3/O/sub 7/ as the counter-electrode were investigated. The junctions exhibited excellent Josephson characteristics with the critical current density (J/sub c/) ranging from 10/sup 2/ A/cm/sup 2/ to more than 10/sup 6/ A/cm/sup 2/, and the normal resistance (R/sub n/) ranging from 10/sup -6/ /spl Omega/cm/sup 2/ to 10/sup -9/ /spl Omega/cm/sup 2/. The R/sub n/ values varied approximately in accordance with J/sub c//sup -p/, where p was close to 0.25 for low-J/sub c/ junctions and increased gradually up to 0.75 for high-J/sub c/ junctions. The junctions with R/sub n/ exceeding 10/sup -7/ /spl Omega/cm/sup 2/ exhibited dI/dV profiles peculiar to tunneling processes via localized states. The dI/dV profiles of the junctions with lower R/sub n/ were characterized by reproducible fine structures below 15 mV, probably due to multiple Andreev reflections. These results indicate that the crossover from the tunneling regime to metallic weak-links takes place in these junctions depending on the process conditions.

The Effect of Spatial Variation of Barrier Layer Thickness on Rampedge-type Josephson Junction Characteristics

We have developed a new model for ramp-edge-type Josephson junctions with a Codoped PrBa2Cu3O7-x barrier in which the spatial variation of the barrier layer thickness within a junction is taken into account. The reexamination of experimental data based on this new model has clarified that the barrier thickness dependence of I c of our junctions can be explained quantitatively by a direct tunneling mechanism alone.

Analysis of capacitance in interface-engineered junctions based on an equivalent circuit model

Abstract The capacitance, C , in interface-engineered junctions (IEJs) has been evaluated by comparing experimental hysteresis parameters ( α c ) with theoretical calculations based on an equivalent circuit model, in which the excess current incidental to Josephson junctions, made of high-temperature superconductors, is incorporated by a simple empirical formula. We have confirmed that the ( α c ) versus critical current ( I c ) relationship observed for an IEJ, by varying the I c value with an applied magnetic field, is reproduced well by the model. The analyses for a number of junctions have revealed that 1/ C in IEJs scales approximately with log( I c ), reflecting a variation in the tunnel-barrier thickness among junctions.