Faculty Bibliography

The stability of double-side-coated Bi2Sr2CaCu2O8 (Bi-2212) tapes on Ag substrates in a water environment has been studied. A phase analysis of the as-processed tape conductors was performed and indicated that aqueous precipitates are formed in the tapes exposed to aerated water solution. The nominal composition of the precipitates was found to be Sr1-xCaxCuO2 (x similar or equal to 0.4). This phase is present in the tapes before the corrosive damage ensues. In order to evaluate the stability properties of the Sr0.6Ca0.4CuO2 phase in a water environment, the pure compound was synthesized and treated under the same conditions as the tapes. Studies reveal that similar corrosion occurs in the Sr0.6Ca0.4CuO2 phase. Thus, the presence of this impurity phase, which appears upon heat treatment of the tape assemblies, is found to be an important factor in the degradation behaviour of the Bi-2212 tape conductors.

The substitutions of rare earths for La on the magnetic properties of the perovskites La(0.7-x)R(x)A(0.3)MnO(3) (0 < x < 0.7), R=Pr or Gd and A=Ca, Sr, or Ba have shown that small substitution of Pr slightly increases the coercive field H-c and magnetization M, and strongly improves the magnetoresistance (MR) while lowering T-c. On the other hand, the substitution of Gd lowers H-c, M and T-c, however, increases MR of the system La0.7-xGdxSr0.3MnO3 at temperature < T-c. Large differences in the A-site ionic radii r(A) of the AMnO(3) perovskites proved detrimental. The optimal composition has been discussed for the half-metallic ferromagnet of a spin-switch device based on the manganese oxides. (C) 1999 American Institute of Physics. [S0003-6951(99)02634-0].

Critical-current (J(c)) measurements for YBa2Cu3O7-delta (YBCO), Gd0.6Ca0.4Ba1.6La0.4Cu3O7-delta (TX-GBCO) and TX-CBCO/YBCO bilayers were performed different applied magnetic fields. rue film samples were prepared by pulsed laser deposition and patterned as microbridges. The highest J(c) (1.5 x 10(6) A/cm(2)) at 75 K in 5 Tesla (Hllab) was observed for TX-GBCO/YBCO bilayers, which showed a 40 % increase in J(c) over YBCO under the same conditions. The bilayer samples possess a T-c at 90 Ii and display superconducting properties similar to YBCO. A possible mechanisms for the high J(c) of bilayer YBCO films is discussed. Development of these bilayer and multilayer structures represents a promising new direction to improve the superconducting properties of YBCO since the TX-GBCO layer provide both protection against corrosion and a significant improvement in J(c).

By using a cation-modified and corrosion-resistant compound of (PryGd0.6-y)Ca0.4Ba1.6La0.4Cu3O7 (y = 0.4, 0.5, and 0.6) as normal-metal barrier materials, high-temperature superconducting Josephson junctions have been fabricated in a ramp-edge superconductor/normal-metal/superconductor (SNS) configuration. We have tuned the Pr substitution level in order to achieve the optimal electrical resistivity of the barrier layer for high-performance SNS junctions. The junctions fabricated with these normal-metal barriers show well-defined RSJ-like current vs voltage characteristics at liquid-nitrogen temperature. The junction performance is mainly controlled by the N-layer instead of the interface. We have also fabricated de superconducting quantum interference devices based on ramp-edge SNS technology with these normal-metal barriers. The ratio of peak-to-peak voltage modulation of the superconducting quantum interference devices to the IcRn product is more than 30%.

Composite dye / superconductor sensors which can discriminate different wavelengths of light in the visible and near-infrared regions have been fabricated. By lithographically patterning 1500 Angstrom thick films of the superconductor YBa2Cu3O7(-delta) on MgO substrates, arrays of microbridges have been created. A layer of dye dispersed in a polymeric matrix is deposited on top of each bridge to create the wavelength selective light absorbing element. Each meandering path bridge is similar to 20 mu m wide and similar to 10 mm long. The device functions in a different manner to traditional semiconductor-based light sensing technologies in which a dye structure serves the role of a filtering agent. Here, the response of the hybrid dye/superconductor clement is amplified at wavelengths strongly absorbed by the dye layer. Such devices represent the initial steps towards a larger structure capable of simultaneously sensing wavelength bands from the visible through to the infrared. In addition to dye sensitization method, another approach to adding wavelength selectivity to detectors is described here in which interference effects in micromachined microbolometers are exploited.

The thermal diffusivity of a thin organic dye layer deposited atop thin films of the high temperature superconductor YBa2Cu3O7-delta is measured using a pulsed laser flash method. Here, the underlying superconductor acts as a highly sensitive temperature transducer after appropriate conversion of the transient voltage response from 7 ns optical pulses. Film surface temperature decays for several thicknesses of the dye layers were evaluated; these decays exhibited a linear dependence of the time at half temperature maximum versus thickness squared. Three dimensional finite difference modeling was used to study and extract the thermal diffusivity values of the thin organic layers as well as to investigate the transient temperature distributions within the dye and superconductor areas. (C) 1998 American Institute of Physics. [S0003-6951(98)01145-0].