J. C. Garland

The critical current of Ag/YBa2Cu3O7−δ random bulk composites

We have measured the current‐voltage characteristics of bulk percolating Ag/YBa2Cu3O7−δ composites ranging in composition from 0.10 to 1.00 volume fraction superconductor at temperatures from 40 to 100 K. The dependence of the normal state resistivity on volume fraction is consistent with the percolationtheory. The critical current densities of the composites exhibit a temperature dependence consistent with superconductor‐normal metal‐superconductor junction behavior. Our data suggest that the critical currents in these composites are limited by proximity coupling through Ag between the high‐T c grains.

Nonlinear conductivity of an In-Y1Ba2Cu3O7−x interface

We report dynamic resistance (dV/dI) measurements of a large, macroscopic interface between In metal and polycrystalline Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-//sub x/. The interface was prepared by pressing a 3-mm/sup 2/ In pellet onto a freshly prepared Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-//sub x/ polycrystalline ceramic surface. Although this interface covered about 10/sup 5/ crystallites of the superconductor, the electrical characteristics were very similar to those observed in point contact measurements on both single-crystal and polycrystalline specimens. A slowly varying bias-dependent background resistivity, narrow resistance peaks, and a zero-bias resistance anomaly were all features of the dV/dI spectra. Both the temperature and magnetic field dependencies of the interface conductivity are reported.We report dynamic resistance (dV/dI) measurements of a large, macroscopic interface between In metal and polycrystalline Y1Ba2Cu3O7−x. The interface was prepared by pressing a 3‐mm2 In pellet onto a freshly prepared Y1Ba2Cu3O7−x polycrystalline ceramic surface. Although this interface covered about 105 crystallites of the superconductor, the electrical characteristics were very similar to those observed in point contact measurements on both single‐crystal and polycrystalline specimens. A slowly varying bias‐dependent background resistivity, narrow resistance peaks, and a zero‐bias resistance anomaly were all features of the dV/dI spectra. Both the temperature and magnetic field dependencies of the interface conductivity are reported.

Collective response of YBa2Cu3O7/Ag weak-link junction arrays

The collective response of superconducting thin-film arrays of 300×300 coupled YBa2Cu3O7/Ag weak-link junctions is investigated as a function of external magnetic fields. Our highTc arrays, which were fabricated using electron-beam and photolithography techniques, are based on a substrate-induced defect method. In this method, a square array of thin-film Ag lines produces a two-dimensional lattice of junctions in an overlaying thin-film array of highTc lines. We find that the zero-field resistive transition of the array is governed by a Kosterlitz-Thouless-type transition. Below the super-conducting transition, we observe quantum oscillations in magnetoresistance curves at both integer and half-integer multiples of the flux quantum. These results suggest that long-range phase coherence may be established in large highTc arrays.

Mechanism of the interlayer dissipation in Bi2Sr2CaCu2O8−x single crystals

We have measured both the c-axis and the ab-plane resistivities on carefully fabricated Bi2Sr2CaCu2O8−x single crystals in a magnetic fieldH‖c. Low temperature data in the mixed state for both directions show a thermally activated behavior with the same field dependence of the activation energy,U∝H−1/3. The results were analyzed in terms of a theoretical model that was recently proposed.

Vortex lattice melting and interlayer coupling in Bi2Sr2CaCu2O8+δ crystals

We find that resistive hysteresis accompanies vortex lattice melting in Bi2Sr2CaCu2O8+° single crystals, together with a sharp drop of the resistance with decreasing temperature. The melting transition temperatureTm was current dependent, indicating that current drives the vortex system out of equilibrium. AboveTm, we find a second sharp feature in the data which we interpret to signify the decoupling transition of the vortex line liquid. Our measurements show nonlocal conductivity in the vortex liquid phase, which correlates with the strength of interlayer vortex coupling. Finally, we review existing literature on vortex lattice melting in BSCCO. While the melting curve varies significantly for different samples, the measurements appear to probe the same physical process in all cases.

Dimensionality crossover and magnetic field frustration in disordered proximity coupled arrays

We have fabricated site-diluted disordered arrays of proximity coupled superconducting junctions. We have measured the critical current superconducting phase boundary, I c (H), as a function of disorder and found that the higher order frustration-induced structure is significantly reduced with the introduction of as little as 10% disorder. Lower order structure, however, persists out to higher disorder than indicated by simulations. We see no evidence of a dimensionality crossover in the phase boundary for our disordered arrays, in agreement with recent data on percolating superconducting wire networks.

Thermoelectric Effects in Superconductors

The first measurements of thermoelectricity in superconductors were made by Meissner (1927) who found the Seebeck coefficient S (thermopower) in a superconducting thermocouple to be at least an order of magnitude below the equivalent normal state value. This result was subsequently verified by Borelius et al., (1931); Burton et al., (1935); Keesom and Matthijs (1938); and Casimir and Rade-maker s (1947). In 1953, Pullan showed unequivocally that the thermopower of a super-conductor decreased at Tc by at least four decades.

Thermoelectric charge imbalance in superconducting aluminum

The charge imbalance voltage produced in superconducting aluminum by the presence of a temperature gradient and an electric current has been studied over the temperature range 0.5–1.2 K. Measurements were obtained of the magnitude and temperature dependence of the charge imbalance voltage of seven samples, two of which contained magnetic impurities. The data are compared with recent theoretical models of the effect.

Fabrication and measurement of high Tc weak links

Abstract We have fabricated Y1Ba2Cu3O7−δ (YBCO) superconducting weak links by depositing a perpendicular stripe of YBCO film on top of a 0.25μm–1.5μm wide line of normal metal (e.g. Ag) previously deposited onto a SrTiO3 substrate. The c-axis orientation of the YBCO film is interrupted over the metal line, creating a disordered grain boundary junction. Measurements of the resistive transition, critical current and current-voltage (IV) characteristics of these structures show definite weak link behavior: the critical current is significantly reduced from that of samples with no metal line present and the IV curves show qualitative agreement with the Ambegaokar-Halperin (AH) model of Josephson weak links with thermal noise.

Modulation of low—frequency shapiro steps of superconducting proximity-effect junctions*

We observe a modulation of Shapiro step heights in superconducting Nb-Au-Nb (SNS) Josephson junctions at an rf frequency v = 20 MHz (hv/2eRIc ≈ 0.17). In addition to the expected steps at Vn = nhv/2e, n = 0,1,2,etc., we record an odd sequence of steps at an rf current amplitude Irf/Ic ≈ 3.81, and an even sequence of steps at Irf/Ic ≈ 3.96. For Irf/Ic ≈ 2.22, both even and odd steps are present after the 4th step. The measured rf power and frequency dependences of the step heights are in excellent agreement with the Resistively-Shunted Junction (RSJ) model predictions.