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One of the important problems of high-temperature superconductivity is to understand and ultimately to control fluxoid motion. We present the results of a new technique for measuring the pressure dependence of the transition to superconductivity in a diamond anvil cell. By measuring the third harmonic of the ac susceptibility, we determine the onset of irreversible flux motion. This enables us to study the effects of pressure on flux motion. The application of pressure changes interplanar spacing, and hence the interplanar coupling, without significantly disturbing the intraplanar superconductivity. Thus we are able to separate the effects of coupling from other properties that might affect the flux motion. Our results directly show the relationship between lattice spacing, effective-mass anisotropy, and the irreversibility line in Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+/spl delta//. Our results also demonstrate that an application of 2.5 GPa pressure causes a four-fold decrease in the effective-mass anisotropy.

The authors examine the modification of the microwave surface resistance (R/sub s/), the critical current (J/sub c/), and the critical temperature (T/sub c/) resulting from 2-MeV H/sup +/ and 12-MeV Si/sup 4+/ irradiation of high-quality thin films of YBa/sub 2/Cu/sub 3/O/sub 7- delta / on LaAlO/sub 3/ substrates. For both ions, the change in T/sub c/ (R=0) and J/sub c/ (77 K and 4.2 K) was roughly linear with fluence. The change in these properties with fluence was quantitatively similar once corrected for differences in the rate of energy loss to atomic displacements. R/sub s/ was also measured as a function of temperature for the same films at 36 GHz using the cylindrical cavity end-wall replacement technique. High quality c-axis oriented films exhibited sharp transitions in R/sub s/ near T/sub c/, followed by a relatively temperature-independent R/sub s/ with a value of approximately 6 m Omega at 20 K. For irradiation with H/sup +/ and Si/sup 4+/ ions, the sharp transition in R/sub s/ was shifted to lower temperatures, although below the transition temperature R/sub s/(T) was unchanged. The loaded quality factor, or Q, of a 5.3-GHz ring resonator, patterned from a 500-nm film on MgO substrate, was measured as a function of temperature for 2-MeV H/sup +/ irradiation. While Q(T) was observed to decrease as a function of fluence, Q(T/T/sub c/) was invariant, as expected from the insensitivity of R/sub s/ to particle fluence below approximately 0.9 T/sub c/.

Tunneling studies have been made using sputtered Cu/sub 2/O as an insulating barrier between an EBCO thin film and a normal metallic layer. Cu/sub 2/O was selected for its low potential for chemical reaction with the superconducting layer and for its photoconductive properties. Having a band gap in the visible (2 eV), Cu/sub 2/O is a suitable candidate for photosensitive tunneling experiments. EBCO thin films were deposited in situ onto MgO substrates using an off-axis sputtering technique. T/sub c/'s of as-deposited films were between 80 and 85 K. Inductively measured J/sub c/'s ranged between 10/sup 6/ and 10/sup 7/ A/cm/sup 2/ at 4 K. A 5-20-nm layer of Cu/sub 2/O was sputtered directly on top of the superconducting film. Room-temperature resistivities of the Cu/sub 2/O layer were typically greater than 10/sup 6/ Omega -cm. A normal metal layer (typically gold) was then deposited onto the Cu/sub 2/O layer. Transport properties of the structure were measured at temperatures ranging above and below the T/sub c/ of the superconducting layer. Measurements of structures fabricated to date showed no tunneling effects. Measurements of the I-V characteristic of a Cu/EBCO bilayer exhibited nonohmic behavior at 85 K and 4 K, indicating the existence of an extraneous interfacial layer. Preliminary results on one such structure indicate possible tunneling effects.