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

9803820先令这个实验研究项目是关于缺陷在设定两种有趣的超导体--铜酸盐和富勒化物--转变温度中的作用。以前工作中的等电点表明，在Tl-2201和YBCO中，氧缺陷的重新分布可以改变T_c高达40K。本项目将利用独特的仪器，包括装有SQUID探测器的压力室，测量施加静水压力后转变温度松弛变化的时间依赖性。该项目的目标是：(A)确定哪些氧化物和什么化学计量比发生重要的驰豫效应，并在这些情况下测量弛豫时间的T和P依赖关系；(B)通过衍射研究和核磁共振测量确定所涉及的缺陷的性质；(C)通过对测量的T_c对这些弛豫过程的压力依赖关系的修正，确定在不同掺杂水平下T_c对晶格参数的内在依赖关系。该项目还包括对几个富勒烯超导体的高压磁化研究。这里的目标是阐明异常性质，包括压力诱导的单体到聚合物的转变，取向有序效应，以及成键中的共价效应。这一研究项目本质上是跨学科的，涉及一名或多名研究生，他们接受了为在工业、政府实验室或学术界就业做准备的出色培训。这个实验研究项目涉及晶体缺陷在两种有趣的超导体--铜酸盐和富勒化物--转变温度设定中的作用。铜酸盐是已知的S超导体中转变温度最高的，在某些情况下超导体可达160K。富勒烯，也被称为巴克明斯特富勒烯，是最近发现的高达30K的超导体，通过在碳60分子中添加碱金属Na或Rb形成。先前工作中的等电点已经表明，在某些铜酸盐情况下，氧缺陷的重新分布可以改变T_c高达40K。本项目将利用独特的仪器，包括装有SQUID探测器的压力室，测量施加静水压力后转变温度变化的时间依赖性。在某些情况下，即使在很低的温度下也能观察到这种时间依赖性，这强烈地表明晶体缺陷涉及其中。例如，在施加静水压力变化后，铜酸盐中的氧气可以从一个位置转移到另一个位置。该项目的两个目标是：(A)确定在哪些氧化物和什么成分中发生重要的驰豫效应，并在这些情况下测量驰豫时间的温度和压力依赖关系；以及(B)通过衍射研究和核磁共振测量确定所涉及的缺陷的性质。这些研究对铜酸盐超导体的应用很重要，因为在大多数应用中，人们希望选择转变温度随时间稳定的成分。该项目还包括对几个富勒烯超导体的高压磁化研究。这里的目标是阐明异常性质，包括压力诱导的单体到聚合物的转变，取向有序效应，以及成键中的共价效应。这一研究项目本质上是跨学科的，涉及一名或多名研究生，他们接受了为在工业、政府实验室或学术界就业做准备的出色培训。***

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. ***