high-Tc Josephson Junctions Research Articles

Intrinsic emission and mixing processes in high-T c Josephson junctions

AC Josephson effect and frequency conversion processes have been investigated in the different high-Tc junctions in the millimeter wave band. Intrinsic radiation up to 140 GHz was directly detected. A down-conversion mode with internal local oscillator was realized in high-Tc Josephson junction for the first time.

Electron beam lithography and ion implantation techniques for fabrication of high-Tc Josephson junctions

Established semiconductor process technologies are demonstrated to be suitable for the fabrication of high temperature superconductor (HTS) Josephson junctions. Single YBCO bridges were modified by local oxygen ion irradiation through a narrow slit in a PMMA mask which was formed by electron beam lithography. The influence of slit dimension and irradiation dose was investigated. The critical current and normal resistance ot the modified microbridges can be controlled by these two parameters. Proximity coupling across the modified region is observed up to a slit width of 250 nm. When exposed to microwave irradiation the microbridges exhibited Shapiro steps. In dc SQUIDs a voltage modulation as a function of an applied magnetic flux is observed.

Voltage divider based on submicron slits in a high T c superconducting film and two bicrystal grain boundaries

Experiments on a model of rapid single flux quantum (RSFQ) flip-flop cell, based on high-Tc (HTS) Josephson junctions show that it can operate as a voltage divider at frequency up to 400 GHz. The junctions were formed in YBaCuO film, deposited on novel Y–ZrO2 bicrystals with two asymmetric 32° grain boundaries, about 10 μm apart, and allow a new design of RSFQ logic based on a single HTS layer. Small inductances (≂10 pH) were made as narrow, submicron size slits. The junction widths were between 4 and 10 μm and for ten junctions located close to the tested circuits, the linear critical current densities at T=4.4 K were 10.7 μA/μm±50% for one grain boundary and 8.3 μA/μm±50% for the other one. IcRn was about 1 mV±50%. A current density of half the expected value meant that the test circuit did not act as an ideal flip–flop down to the lowest frequency. As a voltage divider it gave a half value division up to 0.82 mV at T=4.4 K and to 0.4 mV at 30 K.

YBCO step-edge SQUIDs by magnetron sputtering technique

YBCO step-edge junction d.c. SQUIDs have been realized by using the Inverted Cylindrical Magnetron Sputtering (ICMS) technique. This last represents a novel technology for high-T c Josephson junctions (HTSC). Steps are obtained by standard ion milling procedure on LaAlO3 (100) substrates using Nb-masks patterned by reactive ion etching. Measurements of currentvs. voltage, maximum d.c. Josephson currentvs. magnetic field and SQUID voltage response measurements have been performed, also as a function of the temperature. Operating temperature as high as 77K has been achieved. At 4.2K the SQUIDs show a maximum voltage of flux transfer function ∂V/∂φ max = 870μV/Φ 0and a good periodicity of theV-ϕ modulation up to 20Φ without any sign of hysteresis. The ratio between the step height (h) and the film thickness (d) seems to play a fundamental role in determining Josephson properties of the bridges, these conditions being more severe with respect to most of the data available in literature.

YBa2Cu3O7 Josephson junctions and dc SQUIDs by mechanically induced growth disorder

A simple and reproducible method for the fabrication of high-Tc Josephson junctions and SQUIDs is presented. A very weak disturbance is produced by pressing a diamond needle onto the substrate surface before film deposition. The junction parameters such as the critical current Ic, the IcRn product, or β=2IcL/Φ0 (with L the self-inductance of the SQUID loop) are accessible by controlling the film thickness and can be varied by several orders of magnitude. The junctions usually have critical current densities jc in the range of 5×104–1×105 A/cm2 at 77 K and show Shapiro steps when exposed to rf power. The current-voltage characteristics are resistively shunted junctionlike and nonhysteretic. In dc SQUIDs a maximum peak-to-peak voltage response to dc flux of up to 12 μV at 77 K is observed.

High T c Josephson junctions combining a grain boundary and local strain, using NdGaO3 bicrystal substrates

The bicrystal substrate approach to high Tc Josephson junctions has previously used substrates with cubic (isotropic) structure. NdGaO3, which has orthorhombic structure, has excellent lattice match to YBa2Cu3O7−x and good dielectric properties. When bicrystals are made with NdGaO3 in the usual (110) orientation, the anisotropy means that there will be thermally induced local strain at the grain boundary in any overlaid film. Several junctions have been made with this novel variation on the bicrystal technique, showing values of IcRN around 400 μV at 77 K.

High-Tc thin film Josephson junctions and DC-SQUIDs

Abstract Different types of thin film Josephson junctions with high temperature superconductors are investigated. The textured or single-crystalline films are prepared by laser ablation or sputtering and patterned by direct laser writing, inhibit-layer technology or ion beam etching. Josephson junctions are characterized by current-voltage characteristics, dc and ac Josephson effects and noise measurements. DC-SQUIDs with these junctions are investigated up to 77K, especially the influence of the SQUID inductance, the noise behavior and different junction types. The experimental results are compared to theoretical ones out of computer simulation.

High quality YBa2Cu3O7 Josephson junctions made by direct electron beam writing

High-Tc Josephson junctions have been fabricated by direct electron beam writing over YBa2Cu3O7 thin-film microbridges, using scanning transmission electron microscope (STEM) with an accelerating voltage of 80–120 kV. Annealing at 330–380 K increases Tc and Ic of the junctions and makes them more stable. In the operating range of a few degrees below Tc, the junctions show 100% magnetic field modulation of the critical current, microwave-induced Shapiro steps oscillating according to the resistively shunted junction (RSJ) model, and RSJ current-voltage characteristics with IcRn product up to 0.5–0.6 mV at 75 K and 0.3 mV at 77 K.

Fabrication aspects of microwave devices, including ramp-type high-Tc Josephson junctions and log-periodic antennas

We describe the development of high-T c Josephson junction devices for applications at millimeter wave frequencies. These devices consist of ramp type YBCO/PBCO/YBCO Josephson junctions that are equipped with a noble metal log-periodic antenna. Growth conditions of all layers, as well as etching, cleaning and annealing procedures are being optimized, to guarantee well-defined device properties. Lowering the deposition temperature of the thick PBCO layer strongly improved its isolating properties, which is of extreme importance for good reproducability of junction fabrication. Special attention is being focused on the optimization of the contact of noble metal to YBCO as well its adhesion to the substrate. Best results are obtained using sputtered gold contacts, after a soft Ar ion sputter clean treatment of the damaged YBCO surface, followed by an anneal procedure.

Microwave-induced steps in high-Tc Josephson junctions

Starting with experimental results on break junctions with Y- and Bi-based high-T c superconductors the dependence of the height of microwave-induced steps on radiation power is investigated. Our simulations within the RSJ model with finite capacitance also give explanations of experimental results obtained by other groups on different types of high-T c Josephson junctions, for example deviations from the common Bessel behavior and rising of subharmonic steps.

Bi-epitaxial grain boundary junctions in YBa2Cu3O7

We have developed a new way of making grain boundary junctions in YBa2Cu3O7 thin films by controlling the in-plane epitaxy of the deposited film using seed and buffer layers. We produce 45° grain boundaries along photolithographically defined lines. The typical value of the critical current density of the junctions is 103–104 A/cm2 at 4.2 K and 102–103 A/cm2 at 77 K, while the rest of the film has a critical current density of 1–3×106 A/cm2 at 77 K. The current-voltage characteristics of the junctions show resistively shunted junction behavior and we have used them to fabricate dc superconducting quantum interference devices (SQUIDs) which show modulation at temperatures well above 77 K. This is the first planar high Tc Josephson junction technology that appears readily extendable to high Tc integrated circuits.

Millimeter wave detectors based on future thin film high Tc Josephson junctions

The properties of some millimeter wave detectors are evaluated by assuming the existence of (hypothetical) ideal BCS like thin film Josephson tunnel junctions made of high Tc superconductors. These extrapolations from the behaviour of low Tc junctions may or may not be justified depending on future progress in junction fabrication.

Millimeter wave detectors based on future thin film high Tc Josephson junctions

Millimeter wave detectors based on future thin film high Tc Josephson junctions

Synchronization of radiating intrinsic high-Tc Josephson junctions

Superconducting high Tc granular films radiate a pronounced electromagnetic energy up to millimeter wavelength range. Emission is very sensitive to the bias current, temperature and applied magnetic field. From the periodicity and shape of the emission dependence on external magnetic field the structure of radiating clusters was revealed. The most powerful emissions were ascribed to multi-link quantum interferometer clusters. Spectral properties of those emissions indicate that multijunction clusters get synchronized and radiate coherently.

Synchronization of radiating intrinsic high-Tc Josephson junctions

Superconducting high Tc granular films radiate a pronounced electromagnetic energy up to millimeter wavelength range. Emission is very sensitive to the bias current, temperature and applied magnetic field. From the periodicity and shape of the emission dependence on external magnetic field the structure of radiating clusters was revealed. The most powerful emissions were ascribed to multi-link quantum interferometer clusters. Spectral properties of those emissions indicate that multijunction clusters get synchronized and radiate coherently.