P.F. McBrien

Electrical properties of electron and ion beam irradiated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//

An electron beam with sufficient energy can be used to create a high quality Josephson junction in a single layer of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//. The number of junctions thus produced is severely limited by the serial nature of the technique. An alternative method to create similar high quality Josephson junctions without such a serious throughput limitation is possibly the combination of high resolution masking and ion irradiation. For this reason we have studied the electrical properties of both electron and proton irradiated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// in some detail. It was found that the resistivity of electron beam irradiated barriers of intermediate length (200 nm) are strongly influenced by a proximity effect when the irradiated material has a finite T/sub c/. At higher electron doses the superconducting properties are fully suppressed and the electrical behaviour is dominated by a Variable Range Hopping mechanism.

Capacitance as a probe of high angle grain boundary transport in oxide superconductors

We report a series of studies of grain boundary (GB) capacitance for YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films grown on SrTiO/sub 3/ (STO) bicrystal substrates. By varying the film thickness and the width of the track containing the GB, we find that the substrate makes no contribution to the capacitance measured using Fiske resonances or hysteresis in most cases. This is due to the frequency dependence of the dielectric properties of SrTiO/sub 3/. We have also found that GB capacitance per unit area c/sub GB/ correlates with the resistance-area product R/sub n/A. For our own GBs and GBs reported in the literature the data is is consistent with c/sub GB//spl prop/(R/sub n/A)/sup -1/. We attribute this to variations in GB barrier properties, which reduce the active area, whilst maintaining locally the transport mechanism as tunneling.

The capacitance of grain boundaries in superconducting films with strontium titanate and other substrates

Abstract Bicrystal Josephson junctions have been fabricated with coplanar shunt capacitors on a strontium titanate (SrTiO 3 ) substrate in order to investigate the origin of the junction capacitance. None of the external structures, which varied in length from 5 μm to 2 mm, appeared to have any effect on the I – V characteristic. However, a correlation was observed between the junction resistance and the capacitance obtained from both the hysteresis in the I – V characteristic and the internal junction (Fiske) resonances. This suggests that at least at low temperatures the junction capacitance is intrinsic to the grain boundary and the SrTiO 3 substrate does not contribute. The correlation between the junction resistance and the capacitance is a common feature of many grain boundaries in high T c superconductors reported in the literature, artificial or otherwise.

Transfer function and noise properties of YBa2Cu3O7−δ direct-current superconducting-quantum-interference-device magnetometers with resistively shunted inductances

We have investigated resistively shunted direct-coupled high-Tc dc superconducting quantum interference device (SQUID) magnetometers with four different inductances (50, 100, 150, and 200 pH). The SQUIDs were based on 200 nm thick YBa2Cu3O7−δ films deposited on bicrystal substrates with a 24° misorientation angle, and the shunt resistors were fabricated by masked ion damage. At T=77 K, good quantitative agreement was observed between the measured maximum voltage modulation depth ΔV and calculated values based on the theoretical predictions by Enpuku et al., whereas the white magnetic flux noise SΦ at 10 kHz of all four devices was found to be a factor of 2.3 higher than predicted. The lowest white magnetic field noise of 153 fT/Hz was obtained for the magnetometer with a SQUID inductance of 100 pH with an outer dimension of the pickup loop of just 2 mm.

Performance of high-T/sub c/ dc SQUID magnetometers with resistively shunted inductances compared to "unshunted" devices

We have investigated resistively shunted direct-coupled high-T/sub c/ dc SQUID magnetometers with different inductances. At T=77 K, good quantitative agreement was observed between the measured maximum voltage modulation depth and calculated values based on the theoretical predictions by Enpuku et al. (1995) whereas the white magnetic flux noise at 10 kHz of all four devices was found to be a factor of 2.3 higher than predicted. The lowest white magnetic field noise of 153 fT/Hz/sup 1/2/ was obtained for the magnetometer with an inductance of 100 pH and an outer dimension of the pickup loop of just 3 mm. The combined theoretical and experimental results suggest that although similar magnetic field noise values can he obtained for inductances up to 200 pH, the minimum value of the unshunted case cannot be improved upon. However, the maximum voltage modulation depth could be increased significantly at little cost to the noise compared to an unshunted device.

Measurements of the dielectric properties of strontium titanate at submillimetre wavelengths using Josephson junction driven oscillators

Measurements of the dielectric constant R of thin-film SrTiO3 made using Josephson junction driven oscillators are reported. R for a 100 nm SrTiO3 film was found to vary between 120 and 245 in the temperature range from 4.2 K to 65 K and to be independent of frequency from 50 GHz to 340 GHz.

Josephson junctions with hysteretic current voltage characteristics at high temperatures

The properties of bicrystal Josephson junctions with external capacitors are reported. It was found that the hysteresis in the current voltage (IV) characteristic was very sensitive to the wavelength of the Josephson oscillation in the dielectric and thermal noise suppression of the critical current. A McCumber parameter /spl beta//sub c/ of 1.2 at 65 K has been achieved. In addition intrinsic grain boundary capacitance has been found to give a large /spl beta//sub c/ value of 9.6 at 4.2 K.

Ferroelectric characterisation using Josephson junctions

Measurements of the permittivity of a series of strontium titanate films of various thicknesses at frequencies from 100 to 900 GHz are reported. The permittivity was measured using Josephson junctions coupled to external resonators. The permittivity was found to decrease with decreasing film thickness and was frequency independent. On application of a dielectric bias voltage, the permittivity of a 200 nm film was tunable between 245 and 112 at 30 K.

Device Fabrication and Optimisation for Josephson Broadband Spectroscopy of Ferroelectric Thin Films

Conventional methods of detecting dielectric properties of ferroelectric thin films like microwave techniques, are difficult to operate in a frequency range higher than around 50 GHz. Optical methods based on measurement of refractive indices of materials operate in the THz range. In this work, use of a simple and convenient technique to investigate the dielectric properties of ferroelectric thin films in the millimetre wave-band (50–1000 GHz) is discussed. Here we report measurements of ϵ R and the tunability of ϵ R of thin films of STO.

Is it possible to fabricate a relaxation oscillation SQUID, using high temperature superconductors and grain boundary junctions?

We discuss whether it would be possible to fabricate Relaxation Oscillation SQUIDs (ROSs) using high temperature superconductors (HTSs). This would require HTS junctions with significant hysteresis at high temperatures and we have attempted to induce this using external capacitive shunts. We achieved McCumber parameter values as large as /spl beta//sub c/=4.5 at 40 K and were able to maintain /spl beta//sub c/>1 to 72 K. However we found that small junction resistances, thermal noise associated with high temperature operation and transmission line resonances in the shunt capacitor limit the value of /spl beta//sub c/. Using the parameters we obtained, we have simulated the performance of a ROS at 60 K and found that voltage pulses were generated. However, the small time averaged voltage across the ROS, would make it impossible to construct a working D-ROS. Despite this, it may be possible to detect the pulses and use them to construct a digital SQUID.