S.W. Goodyear

Effect of small changes in composition on the electrical and structural properties of YBa2Cu3O7 thin films

Epitaxial thin films of YBa2Cu3O7 have been grown in situ by evaporation onto (001) MgO substrates. The composition was varied systematically to investigate the effects of changes in Cu content and Ba/Y ratio on the film properties. The results demonstrate that deviations from stoichiometry at the limit of resolution of most analytic techniques can have a large effect on structural and transport properties, as well as causing marked changes in surface morphology. The best properties (Jc≳3×106 A/cm2 at 77 K) are only found for a narrow range of compositions, which can be readily identified from the surface morphology.

Orientation control of YBa2Cu3O7 thin films on MgO for epitaxial junctions

A low‐temperature argon ion milling process has been used to induce growth of c‐oriented YBa2Cu3O7 on (001) MgO substrates with the in‐plane film axes rotated 45° with respect to those of the substrate, and with respect to those of films grown on untreated regions of the substrate. This process is compatible with standard photolithographic processing, and has been used to produce defined areas of the two orientations on the same substrate. Josephson junctions and dc SQUIDs have been fabricated using the resulting grain boundaries.

Low‐noise YBa2Cu3O7 ‐PrBa2Cu3O7 multiturn flux transformers

Epitaxial thin‐film YBa2Cu3O7 flux transformers with multiturn input coils have been constructed on 1 cm2 MgO substrates using PrBa2Cu3O7 as an insulator. By coupling these to high‐temperature superconducting quantum interference devices (SQUIDs) using a flip‐chip technology, several low‐noise magnetometers and a gradiometer have been constructed. The yield of fully working flux transformers on 1 cm2 substrates exceeds 80%, and field sensitivities at 77 K of (520±30)fT/√Hz at 1 Hz and (165±8)fT/√Hz above 10 Hz have been achieved. No evidence for excess low‐frequency noise above that of the uncoupled SQUID has been found in the flip‐chip magnetometers.

OPTIMISATION OF YBa2Cu3O7 THIN FILMS FOR MULTILAYERS

Abstract Thin films of YBa2Cu3O7 have been grown “in situ” using e-beam evaporation. The electrical and structural properties of the films have been correlated with composition, oxygen environment, temperature and substrate type and orientation. When the composition is controlled with sufficient precision ( 90K, Jc > 3 106 A/cm2 at 77K) and smooth surfaces are reproducibly obtained. Fully epitaxial SIS trilayers of YBa 2 Cu 3 O 7 Y 2 O 3 have been grown.

Critical currents and rapid measurements of magnetic relaxation in superconducting thin films

Abstract A fast DC magnetisation technique using a Hall effect probe is described. It is used for measuring thin film critical currents and their relaxation as functions of temperature. At fixed temperature it has been used to take current relaxation data over 6 decades of timescale in under 3 h, much faster than standard magnetisation techniques. Using a thin film ring geometry we deduce E − J characteristics extending down to 10 −14 V/cm. Magnetic relaxation measurements made on over 300 films consistently reveal a sharp divergence near to T c . At lower temperatures the relaxation rate is found to vary with film growth temperature and annealing conditions. The effect of field line curvature has been measured and its significance for our results is discussed.

High-temperature superconducting gradiometric SQUIDs and flux transformers

Abstract Gradiometric DC superconducting quantum interference devices (SQUIDs) have been constructed from thin-film YBa 2 Cu 3 O 7− x on MgO substrates. The SQUIDs were fabricated as forst-order gradiometers with PrBa 2 Cu 3 O 7− x insulator providing electrical isolation at the crossovers. Highly symmetrical first-order gradiometer flux transformers have also been fabricated. These couple to the SQUIDs and enhance the sensitivity to magnetic-field gradients. Each flux transformer has two 20 turn flat spiral SQUID input coils and a pair of larger-area balanced pickup coils. Flux transformers with both parallel and series configured pickup loops have been fabricated. When coupled to a SQUID as a flip chip, one gradiometer was operated at 77 K with a gradient sensitivity of (0.79±0.03) nT/m/ √Hz at 10 Hz and (2.0±0.1) nT/m/ √Hz at 1 Hz.

Laser tuning of HTS microwave filters

A laser-trimming system has been designed and built that can trim an HTS microstrip filter while it is cooled and connected to a network analyser. Software has been written to control the system and fit measured data. Laser-trimmed material has been found to show no measurable microwave loss. The system has been used to tune up a simple three-resonator filter.

Spatially resolved critical current limitations in YBa2Cu3O7 multilayer structures

Current flow in YBa2Cu3O7/PrBa2Cu3O7 multilayer structures has been studied using low temperature, scanning electron microscope, beam‐induced voltage contrast. Specialized data acquisition has been used to limit thermalization effects and improve resolution. For both via hole contacts and crossovers, the lowest critical current densities were found at steps where the superconductor tracks climb up or down over patterned edges of the insulator.

Spatially resolved electrical characterisation of YBa2Cu3O7 / PrBa2Cu3O7 multilayer structures

Abstract Spatially resolved electrical variations in YBa 2 Cu 3 O 7 / PrBa 2 Cu 3 O 7 multilayer structures have been imaged using low temperature scanning electron microscopy. In both via hole and crossover arrays the outer connecting track crossing an MgO / PrBa 2 Cu 3 O 7 step is often found to limit the critical current of the whole structure. Widening the track over such a step produced limiting behaviour in the crossover region itself rather than at the step.

Electrical properties of YBa2Cu3O7 thin films

Abstract Evaporated and ex situ annealed YBa 2 Cu 3 O 7 epitaxial thin films have been laser patterned to form a ring and a long (5cm) meander track. These samples have been used to study the dependence of the electric field on the applied current in the flux creep regime, using both magnetic and transport measurements.