F. Kahlmann

Vortex channeling and the voltage–current characteristics of YBa2Cu3O7 low-angle grain boundaries

We have performed (voltage–current) V–I measurements on a thin film YBa2Cu3O7 4° [001] tilt low-angle grain boundary over an extensive range of temperatures and fields, verifying the presence of a linear characteristic. We report on the occurrence of the linear characteristic in its basic form and on the observation of V–I kinking into several, and in some cases numerous, linear segments. We interpret these findings in terms of a variation in the dissipative width at the grain boundary. Kinking from one linear V–I section to another of different gradient is described in terms of a change in the number of vortex rows being viscously channeled along the boundary.

Critical current of YBa(2)Cu(3)O(7-delta) low-angle grain boundaries.

Transport critical current measurements have been performed on 5 degrees [001]-tilt thin film YBa(2)Cu(3)O(7-delta) single grain boundaries with the magnetic field rotated in the plane of the film, phi. The variation of the critical current has been determined as a function of the angle between the magnetic field and the grain boundary plane. In applied fields above 1 T the critical current j(c) is found to be strongly suppressed only when the magnetic field is within an angle phi(k) of the grain boundary. Outside this angular range the behavior of the artificial grain boundary is dominated by the critical current of the grains. We show that the phi dependence of j(c) in the suppressed region is well described by a flux cutting model.

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.

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.

Angular hysteresis in the critical current of YBa2Cu3O7 low-angle grain boundaries

The dependence of the critical current density, J c , on the orientation of an applied magnetic field has been measured for two YBa 2 Cu 3 O 7 [001]-tilt low-angle grain boundaries. As the field is rotated in the boundary plane, a marked hysteresis in the variation of J c with angle is observed. The effect is visible as a clear shift in the position of the intrinsic pinning peak as a function of the direction of field rotation (θ + or θ - ). In addition, we observe a switch over from J c (θ + ) to J c (θ - ) in just a few degrees of field rotation and find angular hysteresis to be increasingly pronounced at both low field and low temperature. Similar measurements taken on intragranular tracks displayed no hysteresis, suggesting the effect is associated with the presence of the grain boundary. We explore one possible model, involving flux trapping within the sample, to explain this hysteretic behaviour.

Optical emission spectroscopy and Langmuir probe characterisation of the sputtering plasma and its influence upon YBCO thin film properties

Abstract YBa 2 Cu 3 O 7− x (YBCO) thin films have become the most commonly used material in HTS-based electronics applications. In order to achieve reliable performance of devices, the deposition technology must yield reproducible high-quality film growth. During high-pressure d.c. on-axis sputtering the condensing species are thermalized, whilst the biased target is under continuous bombardment. Previous investigations have shown that the state of the target (its temperature and level of oxidation) influence the composition of the plasma and hence of the film. We have used the non-intrusive plasma characterisation method of optical emission spectroscopy, as well as Langmuir probes, for a systematic study of the plasma at different deposition conditions, including small gas additions to the main gas mixture. YBCO films were characterised by X-ray diffraction and Rutherford baskscattering.

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.

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.