Ramp-edge Josephson Junctions Research Articles

Development of ramp-edge SNS Josephson junctions and SQUIDs

We have fabricated high performance Josephson junctions, dc SQUIDs, and directly-coupled SQUID magnetometers, where the junction uses a ramp-edge superconductor/normal-metal/superconductor (SNS) configuration. The devices are fully defined by the standard photolithography. We use Ag-doped YBa 2Cu 3O 7− x for the superconducting electrodes and PrBa 2Cu 3O 7− x as the N-layer barrier. The controllability of the processing and the reproducibility of the device performance was improved substantially by using Ag-doped YBa 2Cu 3O 7− x for the electrodes.

Properties of interface-engineered high Tc Josephson junctions

We have created YBCO thin film ramp edge Josephson junctions by modification of the edge surface prior to counterelectrode deposition. No deposited interlayer or barrier layer is employed. These devices are uniform and reproducible, and they display resistively shunted junction current-voltage characteristics with excellent magnetic field modulation. IcRn values over the range 0.5–3 mV and corresponding RnA values of 6×10−8–1.2×10−9 Ω cm2 at 20 K are easily attained by varying the process. We believe these junctions offer significant promise as the building blocks of a high Tc electronics technology.

Stability of dc superconducting quantum interference devices fabricated using ramp-edge superconductor/normal-metal/superconductor technology

The degradation with time was determined for high-temperature superconducting quantum interference devices (SQUIDs) with ramp-edge geometry Josephson junctions (Ag:YBa2Cu3O7−x/PrBa2Cu3O7−x/Ag:YBa2Cu3O7−x). The critical current of the devices tended to decrease with a mean percentage of less than 7% over a period of more than 500 days. The junction resistance, on the other hand, increased with a mean percentage of 5%. The voltage modulation of the SQUIDs was unchanged within experimental reproducibility, presumably due to the cancellation of the changes in critical current and resistance.

Supercurrent distributions and flux penetration in YBa2Cu3O7–YBa2C o0.15Cu2.85O7–YBa2Cu3O7 edge junctions

We report the measurement and analysis of critical current (Ic) modulation and microwave response in magnetic field for a set of YBa2Cu3O7-YBa2Co0.15Cu2.85O7-YBa2Cu 3O7 ramp-edge Josephson junctions. These junctions have been fabricated with various widths w to span the transition from the moderately small (w<2λJ) to the moderately large (w>2λJ) junction limit at 77 K. As w increases, both the Ic modulation and microwave absorption behavior independently suggest a growing contribution from edge supercurrents. The addition of a superconducting ground plane was shown to reduce the edge supercurrent effects.

Ramp-edge Josephson junctions with Nd 1.85Ce 0.15CuO 4− y barriers

We have fabricated YBa 2Cu 3O 7−δ ramp-edge Josephson junctions using Nd 1.85Ce 0.15CuO 4− y as the barrier material. In the fabrication process we introduced an in situ cleaning process for the ramp surface in the base electrode prior to the deposition of the barrier layer and the top electrode. In the cleaning process a low energy oxygen and/or argon ion beam is used. Varying the barrier thickness d, the critical current density J c and the normal resistance area ϱ n of the junctions were found to be proportional to exp(− d/ ξ n) and exp( d/ ξ r) with characteristics decay lengths ξ n = 3.8 nm and ξ r = 7.2 nm, respectively. For junctions with d > 10 nm, J c ϱ n products above 1 mV at 77 K were obtained. The spatial disribution of the critical current density was imaged directly by low-temperature scanning electron microscopy (LTSEM) and showed considerable inhomogeneties on a μm scale.

Properties of a YBCO/insulator/YBCO trilayer and its application to a multilayer Josephson junction

We have examined the properties of a trilayer high-temperature superconductor (HTS)/insulator/HTS structure and incorporated this structure with multilayer Josephson ramp edge junctions to form a SQUID with a ground plane. The lower and upper (YBCO) films in the trilayer structure showed zero-resistance temperatures of 89 K and 85 K respectively. A directly coupled ground-planed SQUID using ramp edge Josephson junctions with a normal barrier layer (PBCO) showed a voltage modulation of at 9 K and operated up to 50 K.

Distribution of the critical current density and flux trapping in YBa2Cu3O7−δ ramp-edge Josephson junctions

We have studied the spatial distribution of the critical current density in YBa2Cu3O7−δ ramp edge Josephson junctions using low-temperature scanning electron microscopy. Applying this technique allows the imaging of the critical current density distribution with a spatial resolution of about 1 μm. Our measurements show that the geometry of the ramp-edge junction eases the trapping of magnetic flux quanta in the YBa2Cu3O7−δ layer covering the ramp edge. These trapped flux quanta result in a spatially inhomogeneous magnetic field parallel to the barrier layer, which in turn results in a spatially modulated supercurrent density and an unusual magnetic field dependence of the critical current.

YBCO-based ramp-edge Josephson junctions and DC SQUIDs with a cubic-YBCO barrier layer

Recently, the discovery of a nonsuperconducting metastable cubic phase of YBa2Cu3O7 (YBCO) has been reported. Because of the structural, chemical, and depositional compatibility of cubic-YBCO with superconducting YBCO, it is an excellent candidate for use in heteroepitaxial structures. Such structures involving superconducting YBCO/cubic YBCO/superconducting YBCO ramp-edge proximity-effect Josephson junctions were fabricated on MgO substrates. Junctions having critical current densities of ≈8×104 A/cm2 and IcRn products as high as 0.8 mV at 15 K were produced. Shapiro steps were seen in the presence of RF radiation. DC SQUIDs were fabricated with these junctions and showed periodic critical current modulation with applied magnetic field to temperatures in excess of 77 K.