Studies of High-Tc and Buckminsterfullerene Superconductors under High Hydrostatic Pressure

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w:\awards\awards96\*.doc 9803820 Schilling This experimental research project is concerned with the role of defects in setting the transition temperature of two interesting types of superconductor, the cuprates and fullerides. The PI in previous work has shown that in Tl-2201 and YBCO the redistribution of oxygen defects can alter Tc by as much as 40K. The present project will make use of unique apparatus including a pressure cell incorporating a SQUID detector, to measure the time dependence of relaxational changes in transition temperature following application of hydrostatic pressure. The goals of the project are: (a) establish in which oxides and at what stoichiometries important relaxation effects occur, and in these cases to measure the T and P dependences of the relaxation times; (b) to determine through diffraction studies and NMR measurements the nature of the defects involved; (c) through correction of the measured pressure dependence of Tc for these relaxation processes, to determine at varying doping levels the intrinsic dependence of Tc on lattice parameter. This project also includes high pressure magnetization studies on several fullerene superconductors. The goal here is to clarify anomalous properties which include pressure-induced monomeric-to-polymeric transitions, orientational ordering effects, and covalency effects in the bonding. This research program is interdisciplinary in nature and involves one or more graduate students, who receive excellent training in preparation for careers in industry, government laboratories or academia. %%% This experimental research project is concerned with the role of crystalline defects in setting the transition temperature of two interesting types of superconductor, the cuprates and fullerides. The cuprates are the highest transition temperature s uperconductors known, in some cases superconducting up to 160K. The fullerides, also known as Buckminsterfullerenes, are recently discovered superconductors up to about 30K, formed by adding alkali metals such as Na or Rb to carbon-sixty molecules. The PI in previous work has shown that in certain cuprate cases the redistribution of oxygen defects can alter Tc by as much as 40K. The present project will make use of unique apparatus including a pressure cell incorporating a SQUID detector, to measure the time dependence of changes in transition temperature following application of hydrostatic pressure. The time dependence is observed in some cases even at very low temperatures, which strongly indicates that crystalline defects are involved. For example, the oxygen in the cuprate can transfer from one location to another, following the application of a change in hydrostatic pressure. Two goals of the project are: (a) establish in which oxides and at what compositions important relaxation effects occur, and in these cases to measure the temperature and pressure dependences of the relaxation times; and, (b) to determine through diffraction studies and NMR measurements the nature of the defects involved. These studies are important for applications of cuprate superconductors, because in most applications it is desirable to select compositions for which the transition temperature will be stable over time. This project also includes high pressure magnetization studies on several fullerene superconductors. The goal here is to clarify anomalous properties which include pressure-induced monomeric-to-polymeric transitions, orientational ordering effects, and covalency effects in the bonding. This research program is interdisciplinary in nature and involves one or more graduate students, who receive excellent training in preparation for careers in industry, government laboratories or academia. ***