We have measured the current-voltage (I-V} characteristics of several high-temperaturesuperconducting materials with widely difkrent morphologies {bulk Ag/Pb-Bi-Sr-Ca-Cu-0 tapes, thin films of Y-Ba-Cu-O, and melt-textured, bulk Y-Ba-Cu-0 samples). The I-V curves were taken at several magnetic fields ranging from 0 to 8 T. The measurements were carried out at three temperatures (4.2, 27, and 77 K) where the samples were immersed in liquid cryogens to ensure good thermal equilibrium. We compared our experimental results to the predictions of dissipation in superconductors made by the following physical models: modified Ambegaokar-Halperin, flux creep, vortex glass, collective flux creep, and a power law. The fits were extremely good for the first model and were not nearly as good for the others. Using the modified Ambegaokar-Halperin model, the critical current I"the normal-state resistance R",and y, which is proportional to the pinning potential U(H, T), were obtained for each material. Since the Ambegaokar-Halperin model is the only one which uniquely defines I"we conclude that its use puts this parameter on a solid physical basis.

We have measured and interpreted the current-voltage characteristics of several thin YBa&Cu3069 films. They were prepared by metal-organic deposition and microscopy revealed that they had numerous stacking faults. The measurements were taken at temperatures of 77 and 4.2 K in applied magnetic fields varying from 0 to 6 T. Values of J, as large as 2 MA/cm were found at 77 K for films with T, =92 K, whereas values of J, as large as 80 MA/cm were found at 4.2 K for the same films. The data were compared with the predictions of a modified Ambegaokar-Halperin model, a conventional power-law model, and a vortex-glass model. The data were generally fit best by the Ambegaokar-Halperin model from which were obtained the resistance of the film, the critical current density, and the pinning potential as functions of the magnetic field and temperature. These parameters varied widely but systematically among the films and were interpreted in terms of differences in their imperfections. The critical current density and pinning potential obtained from this analysis were compared to the same quantities inferred from a more traditional method. The dependencies on the magnetic field were very similar, although the magnitudes were different. The advantage of defining the critical current density in terms of a physical model is emphasized.

Magnetization and resistivity measurements are reported for a series of radiation damaged La0.7Ca0.3MnO3 pulsed laser deposited thin films. When plotted as a function of activation energy, trends in the electrical transport properties are similar to those exhibited in the magnetic properties. A sharp drop in both Tc and Tp in samples with activation energies greater than ∼110 meV suggests a “decoupling” of the magnetic and transport properties. The results suggest the magnetic order is no longer sufficient to delocalize the system of the extra disorder induced by the radiation damage.

We have measured the electrical resistivity and thermoelectric power of commercial vapor-grown carbon fibers and fiber composites from 4 K to 300 K. Post-growth heat treatment decreased the resistivity and moderately increased the magnitude of the thermopower. Thermoelectric measurements on fiber composites showed that the host material altered the resistivity but had little effect on the thermopower. Thermopower data suggest that the fibers in the composite material have a smaller degree of graphitization than isolated fibers.

This paper reports the structure and thermoelectric properties of multilayered skutterudite thin films.In particular, the two skutterudite phases used were CoSb3 and IrSb3. The multilayers were constructed using pulsed laser deposition (PLD) from skutterudite targets containing excess antimony. The films are polycrystalline with small grains exhibiting columnar growth penetrating through the layers. The property measurements, while yielding absolute numbers that are inferior to bulk values, show a possible enhancement (relative to single alloy films) of the thermoelectric power factor for multilayers with a period near 25 nm. Thermal conductivity data for the multilayers show that they are higher than a single alloy film. However, the values are below that of bulk skutterudites and single films of CoSb3 and IrSb3.