Alan F. Clark

Evidence for two superconducting components in oxygen-annealed single-phase YBaCuO☆

Abstract The complex susceptibility of a sintered YBaCuO superconductor is strongly dependent on a.c. field amplitude, h . Very small values of h must be used for the real part of susceptibility, χ′, to reach a value corresponding to bulk diamagnetism just below the critical temperature, T c . The imaginary part, χ″, represents hysteresis loss in the sample. Thus, χ″ versus temperature becomes positive when h exceeds the lower critical field, H cl of the superconductor. Annealing the material in oxygen gives rise to two distinct components, a relatively high- T c , high- H cl superconductor (denoted as ‘G’ or ‘good’) and a relatively low-T c , low- H cl superconductor (denoted as ‘B’ or ‘bad’). Curves of susceptibility versus increasing temperature reflect the dual nature of the annealed sample: χ′ has an inflection point at T c of the B component and approaches zero at T c of the G component, while χ″ has a peak at each T c . Both critical temperatures decrease linearly with increasing h , though at very different rates. H cl of the G component is considerably greater than H cl of the B component. The lower critical fields are linearly decreasing functions of temperature. Two models might explain the susceptibility data. In the grain model, the G component consists of superconducting grains and the B component is either intergranular material, unfavourably orientated anisotropic grains, or oxygen-depleted grain boundaries. In the surface model, the G component is in the interior of the sample and the B component is at the samples surface. This condition could arise if there was oxygen depletion at the surface subsequent to total enrichment during annealing.

Half‐integral constant voltage steps in high‐Tc grain boundary junctions

A novel effect from microwave radiation near 9.3 GHz applied to high‐TcYBa2Cu3O7−δ single grain boundary junctions was observed. In addition to the usual Shapiro steps resulting from the ac Josephson effect, constant voltage steps with voltages halfway between the voltages of the Shapiro steps were present. The widths of these ‘‘half‐integral’’ steps were measured as a function of microwave power, and the influence of a magnetic field was investigated. From previous results on high‐Tc grain boundary junctions and a comparison of the results presented here with single‐ and multiple‐junction effects in low‐Tc materials, we conclude that the half‐integral steps are likely to be a result of grain boundaries being composed of multiple junctions in parallel.

Mechanical, thermal, and electrical properties of selected polymers

Abstract An extensive compilation has been completed on the mechanical, thermal, and electrical properties of six commercially available polymers. These data are discussed and summarized here as a function of temperature, radiation, and frequency. A brief description and characterization of each polymer is included.

Application of single electron tunneling: Precision capacitance ratio measurements

A metrological application is reported of the single electron tunneling (SET) phenomena: a precise measurement of the ratio of two cryogenic capacitors. The measurement used a superconducting SET electrometer as the null detector for a capacitance bridge. A 3‐ppm level of imprecision has been achieved in the measurement of the capacitance ratio from 100 to 1000 Hz. Further improvements can be made in the attempt to obtain an imprecision of 10−8 at lower frequencies, sufficient for the metrological measurement of capacitance or the fine‐structure constant using a SET pump.

Current transfer in multifilamentary superconductors. II. Experimental results

Measurements are reported of the current‐transfer effect in a 0.58×0.68‐mm NbTi (180 filament) copper‐matrix wire, and in a 0.33×0.66‐mm Nb3Sn (3553 filament) bronze‐matrix wire at magnetic fields from 2 to 8 T. With a voltage sensitivity of 100 nV, the effect of current transfer in the copper : NbTi wire was too small to be measured at a distance 1 cm from the current contact. In the bronze : Nb3Sn wire, however, the effect was relatively large and resulted in voltage‐current characteristics which had an extensive linear region. The slope of the linear region decreased with distance x from the current contact as x−2±0.3 and reached a value of 10−12 Ω cm, for example, at a distance of 3±0.5 cm. Both the magnitude and functional dependence of the measured current‐transfer effects correspond closely to that predicted by theory.

Definitions of terms for practical superconductors. 2. Critical parameters

Abstract The definitions of terms used in describing the phenomenology and measurement practices of practical superconductive materials are proposed. The definitions cover the subject categories of: 1. Fundamental states and flux phenomena, 2. critical parameters, 3. Fabrication, stabilization, and transient losses, and 4. Josephson phenomena. It is intended that these terms will become the basis for the development of standard measurement practices and responses are invited.

Electron tunneling measurement of the energy gap in a LaSrCuO superconductor

Abstract We have used the break junction technique to determine the energy gap of lanthanum—strontium—copper—oxide, one of the new high critical temperature superconductors. The current—voltage characteristics demonstrated a variety of tunneling behaviours. The best characteristic indicating quasiparticle tunneling between superconducting electrodes implied an energy gap of 7.0 ± 0.1 meV. Derivatives of other characteristics showed weak structure indicating possible energy gaps up to 9 meV.

The thermal expansion of several materials for superconducting magnets

The thermal expansion of several materials used in the construction of high field superconducting magnets has been measured from 4 K to room temperature. The materials were a NbTi and two Al5 multifilamentary conductors and several nonmetallic composites made from linen/phenolic, fiberglass/epoxy and superconducting wire/epoxy. The conductor expansions are typical of metals and the composite expansions are highly anisotropic. Both graphic and tabular values are provided by a computer fitting of the experimental data. The importance of thermal expansion differences in critical current measurement apparatus and superconducting magnet design are discussed.

Dynamics of overdamped Josephson junctions driven by a square-wave pulse

The periodic solutions of an overdamped Josephson junction driven by a square-wave pulse were found. Unlike those driven by a sinusoidal ac current, the integer step widths of a Josephson junction driven by a square-wave pulse train are reduced with increasing ac current frequency. The maximum value of any non-zero step width approaches 2Ic, where Ic is the critical current, by decreasing either the pulse repetition rate or the pulse duration time. The characteristic features of any integer step are almost identical and we give an explanation of this.

Josephson effect above 77 K in a YBaCuO break junction

We have observed the Josephson effect in a YBaCuO break junction. Critical currents as high as 10 mA were measured at 4 K for break junctions with a point contact within the fracture of a sample. The junction was susceptible to microwave radiation showing Shapiro steps with the ratio of V/f of 2.04±0.05 μV/GHz compared to the pair tunneling value of h/2e=2.068 μV/GHz. These steps were clearly visible in the current‐voltage characteristics at tempertures up to 85±5 K.