Rung Kuang Lo

DEVELOPMENT OF A RELIABLE MATERIALS BASE FOR SUPERCONDUCTING ELECTRONICS

Careful studies of the corrosion, redox, galvanic and oxygen evolution/uptake reactions associated with YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} and related compounds have been completed. These studies have led to an understanding of the many factors that contribute to the poor material characteristics exhibited by these popular high-T{sub c} phases. With knowledge of the structure-reactivity relationships, a powerful crystal engineering approach has been developed that is capable of producing cation substituted versions of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}; the resulting compounds therefrom produced exhibit markedly improved processability, oxygen stability, and durability characteristics. These materials have been combined in thin film structures so as to make prototype SNS junctions and SQUID sensors which exhibit promising device performance characteristics. {copyright} {ital 1997 Materials Research Society.}

Environmental degradation properties of YBa2Cu3O7−δ and Y0.6Ca0.4Ba1.6La0.4Cu3O7−δ thin film structures

Abstract Utilization of the high temperature superconductor, YBa 2 Cu 3 O 7−δ , in commercial applications is becoming increasingly feasible. Before full advantage of this material can be taken, however, the lifetime, oxygen stability and processability of this ambient reactive superconductor must be improved. Corrosion resistance of YBa 2 Cu 3 O 7−δ and a cation substituted compound, Y 0.6 Ca 0.4 Ba 1.6 La 0.4 Cu 3 O 7−δ , were studied and their lifetimes in aqueous environments were determined. Results indicate a dramatic enhancement in the stability against environmental degradation for the cation substituted phase. Important mechanistic factors responsible for the enhanced corrosion resistance of the substituted phase over the parent compound are discussed.

Conductive polymer/high-Tc superconductor bilayer structures

Abstract Electrochemical techniques are exploited to fabricate conductive polymer/high-T c superconductor bilayer structures. Scanning electron microscopy, x-ray fluorescence and electrochemical techniques are utilized to characterize the electrodeposition of polypyrrole layers grown onto oriented YBa 2 Cu 3 O 7 thin films. In such hybrid polymer/superconductor systems, it is found that when the polymer is oxidized to its conductive state, the transition temperature (T c ) and critical current (J c ) of the underlying superconductor film are suppressed. Here described are new methods for the fabrication of superconductor thin film layers which possess multiple points of weak electrical contact. Such weak links are very susceptible to the influence of external forces making them well suited for the creation of hybrid polymer/superconductor structures. Moreover, measurements recorded as a function of temperature of the polymer/superconductor interface resistance show dramatic decreases in resistivity at T c . Collectively, these results are consistent with observations of conductive polymer proximity effects.

Electrochemical and optical devices based on molecule/high-Tc superconductor structures

Through the use of electrochemical and vacuum deposition techniques, molecular materials are combined with high-T c superconductors in order to fabricate two new classes of hybrid devices. In the first class of devices, electrochemical methods are utilized to deposit polypyrrole onto YBa 2 Cu 3 O 7-δ structures. With such systems, experiments are conducted which explore electron transfer events which occur between the conductive polymer and the high-T c superconductor at temperatures both above and below T c . Here it is found that the transition temperature (T c ) of the superconductor is depressed dramatically when polypyrrole is oxidized to its conductive form and that it returns to a value which is close to the original value upon reduction of the polypyrrole layer to the neutral, nonconductive form. In the second class of devices, the ability of molecular dye layers to affect the optical properties of YBa 2 Cu 3 O 7-δ photodetectors is demonstrated. By examining changes in the optical and electronic properties of these systems, energy transfer processes which occur between molecules and high-T c superconductors are explored. Here it is found that dye layers are able to act as antenna for optical radiation at those wavelengths which the dye layer absorbs light most strongly.

Conductive polymer/superconductor thin film assemblies

Abstract Electrochemical procedures are utilized to deposit conductive polymer layers directly onto thin film samples of YBa2Cu3O7-δ and to cycle the polymeric structures between their neutral (non-conductive) and oxidized (conductive) forms. Through changes in the polymer doping level, the superconducting properties of the high-Tc component can be modulated in a controllable fashion. In this regard, large changes in the superconducting critical temperature and critical current values are noted as a result of the doping of the polymer layer. This paper focuses on the role of superconductor weak link structures and how such features influence the magnitude of the modulation of the superconducting properties.

Molecular engineering of organic conductor / high-Tc superconductor assemblies

Abstract Organic monolayers on high-T c superconductors have allowed for the precise structural control of composite organic conductor / high-Tc superconductor assemblies. These hybrid systems display a number of novel electronic and structural properties. This paper discusses the utility of the monolayer self-assembly procedure for the preparation of both conductive polymer and crystalline charge transfer salt layers which are supported directly on top of cuprate superconductor thin film samples.

Molecule/High-Tc Superconductor Structures as Optical Sensors

Abstract By combining patterned high-temperature superconductor thin films with overlayers of molecular materials, a new generation of hybrid optical sensors is obtained. Here, the ability of molecular dye layers to influence the light sensing properties of a high-Tc superconducting bolometer is described. These dye-coated devices are shown to display selective optical responses which are dependent on the properties of the molecular layer. Two different types of molecular absorbing layers prepared from molecular dyes and conductive polymers are examined in this paper Furthermore, the prospects for producing wavelength tunable detectors from hybrid conducting polymer/superconductor structures are addressed.