E. E. Mitchell

Fabrication and characterisation of YBCO single grain boundary step edge junctions

We use ion beam etching techniques to fabricate YBCO step edge junctions (SEJ) on MgO substrates. Argon ion-beam etching (IBE) of the substrate at angles other than at normal incidence is used to define the step height and angle. Thin (/spl sim/300 nm) magnetron sputtered YBCO films are deposited over the step and patterned using microlithography and cold substrate ion-beam etching techniques. The critical current, I/sub c/ of these SEJs can be controlled by varying the angle of the step etched into the substrate. Fabrication techniques are described which produce one grain boundary at the top of the step and include a smooth return path thereby avoiding a second grain boundary at the bottom of the step. At 77 K, the current-voltage (I-V) characteristics show resistively shunted junction behaviour. These junctions routinely demonstrate reasonably large I/sub C/R/sub N/ products (0.1-0.6 mV), making them suitable for applications in high temperature SQUID devices.

YBCO step-edge junctions with highIcRn

Step-edge junctions represent one type of grain boundary Josephson junction employed in high-temperature superconducting junction technology. To date, the majority of results published in the literature focus on [001]-tilt grain boundary junctions (GBJs) produced using bicrystal substrates. We investigate the step morphology and YBCO (yttrium barium copper oxide) film structure of YBCO-based step-edge junctions on MgO [001] substrates which structurally resemble [100]-tilt junctions. High-resolution electron microscopy reveals a clean GBJ interface of width ~ 1?nm and a single junction at the top edge. The dependence of the transport properties on the MgO step-edge and junction morphology is examined at 4.2?K, to enable direct comparison with results for other junction studies such as [001]-tilt and [100]-tilt junctions and building on previously published 77?K data. MgO step-edge junctions show a slower reduction in critical current density with step angle compared with [001]-tilt junctions. For optimized step parameters, transport measurements revealed large critical current and normal resistance (IcRN) products (~3?5?mV), comparable with the best results obtained in other kinds of [100]-tilt GBJs in YBCO at 4.2?K. Junction-based devices such as SQUIDs (superconducting quantum interference devices) and THz imagers show excellent performance when MgO-based step-edge junctions are used.

Field trials using HTS SQUID magnetometers for ground-based and airborne geophysical applications

Since December 1992, CSIRO and BHP have been field trialing rf HTS SQUID magnetometers for mineral prospecting applications. Ten field trials in widely varying environments(from -16/spl deg/C to +40/spl deg/C ambient temperatures) in mostly remote locations saw the development of a system which can be operated in many configurations including ground based and airborne Transient ElectroMagnetics (TEM). The magnetometer system has been developed to a point where, at late times in TEM applications, the SQUID system has a higher signal-to-noise level than the competing traditional coil technology. In some trials, a SQUID magnetometer detected anomalies at later times than were observed with the coil system, indicating an enhanced ability to detect highly conductive targets. This paper reviews development of our 3-axis SQUID magnetometer. SQUID systems as B field sensors have advantages over coils which are dB/dt type sensors. We will discuss the importance of these advantages for mineral prospecting in regions with a conducting soil cover or overburden typical of the Australian landscape.

The drainage of a thin aqueous film between a solid surface and an approaching gas bubble

Abstract A new optical technique is used to measure the time-dependent shape of thin aqueous draining films between air bubbles and hydrophilic or hydrophobic quartz surfaces. The thickness is determined to a precision of ±0.2 nm to ±1 nm from the intensity of the reflected interference beam from a spot of laser light which is focussed on the thinning film and which is scanned radially. The film shape begins as a dimple which then flattens as it thins. Plots of the thickness as a function of time and of radial position are presented for films of pure water and of 0.23 mol m−3 NaCl solution at hydrophilic quartz surfaces. In the ring of closest approach, the pure water film is at all times thicker than that of the salt solution film. In the early phase of the approach, the pure water film thins more slowly at the centre, but after about 40 s it thins more rapidly. Both of these observations are consistent with the expected shorter range of the double layer repulsion in the salt solution. For hydrophobic surfaces, the drainage rate is more rapid, the film eventually collapsing.

Operation of a geophysical HTS SQUID system in sub-Arctic environments

Transient ElectroMagnetic geophysical prospecting using SQUID sensors has demonstrated potential for improved target detection at late response times compared to conventional coil sensors. We have developed a three-axis, rf SQUID sensor system which has been extensively operated in sub-Arctic conditions by a geophysics contractor. Due to the harsh environmental and operating conditions, the system is designed to operate in sub-zero temperatures (as low as minus 40/spl deg/C) and is ruggedly packaged whilst still remaining quite portable. Auto-tuning of the rf electronics has been implemented by adjusting the rf SQUID control parameters via a microprocessor controller. After a small amount of training, regular field crews have operated two of these systems in the field continuously for periods of months at a time. An example, comparing SQUID B field data to coil dB/dt data, is presented in this paper.

Fabrication and Characterisation of Series YBCO Step-edge Josephson Junction Arrays

Fabrication and characterisation of YBCO step-edge Josephson junction arrays in a series configuration are reported. The junction arrays were fabricated using CSIRO YBCO step-edge junction technology, which, owing to the flexibility of locating the junctions anywhere on the chip, makes it very attractive for implementation of multiple HTS devices and circuits. Arrays of 50 junctions in series were fabricated, and the DC current-voltage (I-V) characterisations were studied at temperatures ranging from 15 to 77 K; temperatures were obtained via either placing the samples in the vent gas of a liquid He cryostat or use of a commercial cryocooler. Eight fabricated and tested arrays, totalling 400 junctions, showed no electrically open or shorted junctions (i.e., a 100% yield of working Josephson junctions), demonstrating good fabrication reliability. Large junction normal resistances (several hundred ?s) were obtained from the series junction arrays, making them suitable for a range of high-frequency applications. A study of the junction critical current spread within an array, magnetic field dependence of the critical current, and microwave responses of the arrays is also described.

Issues relating to airborne applications of HTS SQUIDs

Airborne application of HTS SQUIDs is the most difficult environment for their successful deployment. In order to operate with the sensitivity required for a particular application, there are many issues to be addressed such as the need for very wide dynamic range electronics, motion noise elimination, immunity to large changing magnetic fields and cultural noise sources. This paper reviews what is necessary to achieve an airborne system giving examples in geophysical mineral exploration. It will consider issues relating to device design and fabrication, electronics, dewar design, suspension system requirements and noise elimination methods.

Experimental determination of theB−Tphase diagram ofYBa2Cu3O7−δto 150 T forB⊥c

The B-T phase diagram for thin film YBa_2Cu_3O_7-d with B parallel to the superconducting layers has been constructed from GHz transport measurements to 150T. Evidence for a transition from a high T regime dominated by orbital effects, to a low T regime where paramagnetic limiting drives the quenching of superconductivity, is seen. Up to 110T the upper critical field is found to be linear in T and in remarkable agreement with extrapolation of the longstanding result of Welp et al arising from magnetisation measurements to 6T. Beyond this a departure from linear behaviour occurs at T=74K, where a 3D-2D crossover is expected to occur.

Vortex penetration and hysteretic behaviour of narrow planar Josephson junctions in a magnetic field

Abstract We have studied narrow YBa2Cu3O7 (YBCO) step edge junctions (width w=0.8–18 μm) in applied magnetic fields up to Ba∼35 mT. At Ba=0, their I–V characteristics show good agreement with other YBCO grain boundary junctions. We use the magnetic field dependence of the junction critical current, Ic(Ba), to study the penetration of vortices. At 77 K, the junctions exhibit jumps in Ic(Ba) due to the creation, annihilation and motion of a small number of vortices close to the junction. The fields at which the jumps occur and the number of jumps measured for a particular Ba show a dependence on w. These data suggest that field enhancement affects the penetration of vortices and support a field enhancement factor which is proportional to w. The data also reveal information about the hysteretic trapping of vortices near the junction. The entire Ic(Ba) pattern demonstrates a shift along the Ba axis depending on the size and direction of the maximum applied field, Bmax. The magnitude of this hysteretic behaviour increases both with w and with Bmax until showing signs of saturation for Bmax greater than ∼15 mT.

2D SQIF arrays using 20 000 YBCO high R n Josephson junctions

Superconducting quantum interference filters (SQIFs) have been created using two dimensional arrays of YBCO step-edge Josephson junctions connected together in series and parallel configurations via superconducting loops with a range of loop areas and loop inductances. A SQIF response, as evidenced by a single large anti-peak at zero applied flux, is reported at 77 K for step-edge junction arrays with the junction number N = 1 000 up to 20 000. The SQIF sensitivity (slope of peak) increased linearly with N up to a maximum of 1530 V T−1. Array parameters related to geometry and average junction characteristics are investigated in order to understand and improve the SQIF performance in high temperature superconducting arrays. Initial investigations also focus on the effect of the SQUID inductance factor on the SQIF sensitivity by varying both the mean critical current and the mean inductance of the loops in the array. The RF response to a 30 MHz signal is demonstrated.