Studies of Alkali Metal and High-Tc Oxide Superconductors and Molecular Materials under Extreme Hydrostatic Pressure

极端静水压下碱金属和高温氧化物超导体和分子材料的研究

• 批准号: 0404505
• 负责人: James Schilling
• 金额: $ 36万
• 依托单位: Washington University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-03-01 至 2007-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0404505&HistoricalAwards=false
• 关键词: Studies; Alkali; Metal; Tc; Oxide

Alkali metals are canonical free-electron systems which, according to conventional wisdom, should become even more free-electron-like under pressure. It is, therefore, remarkable that recent theoretical work predicts that under strong compression the nearly free electrons in Li and Na should become markedly less free and begin to couple strongly with the lattice. Thereby leading to possible transformations to lower symmetry crystal structures and/or the appearance of superconductivity. Building on the recent discovery of type-I superconductivity in Li and multiple phase transitions in the pressure range to 70 GPa, this project will study the superconducting phase diagram, Tc(P), and critical magnetic field behavior, Tc(H), of Li and selected Li alloys to ultrahigh pressures. In addition, there will be a search for pressure-induced superconductivity in Na and other low-z elements, alloys, and compounds, as well as in the heavier alkali metals K and Rb. The systematics derived from this study should shed some light on the properties of hydrogen under pressure which has been predicted to become a metal at multi-Mbar pressures with possible superconductivity near room temperature. The proposed experiments give undergraduate and graduate students an excellent opportunity to both learn and develop important laboratory techniques; these students will also benefit from collaborations with groups both in the US and abroad.All metals are not created equal: some are complex and some are simple. The alkali metals Li, Na, K, Rb, Cs are widely accepted as being the simplest metals of all since each contains valence electrons which are free to roam about the lattice of well separated ion cores. The alkali metals are extremely compressible. Under very high pressures the ion cores come close together. Since the valence electrons must avoid the ion cores, they are forced to follow tortuous catacomb-like paths around the ion cores. For this reason they are no longer free but couple strongly with the lattice. This generates potential changes in crystal structure and/or the appearance of an unusual superconducting state. Exactly this is observed in the alkali metal Li under strong compression: multiple phase transitions occur and superconductivity at relatively high temperatures (15 K or -258 C) suddenly appears for pressures above 200,000 atmospheres. In this project the anomalous superconducting state of Li and selected Li alloys will be studied in depth both under varying pressure and magnetic field. A search will also be conducted for pressure-induced superconductivity in the alkali metals Na, K, and Rb. These experiments should provide insight into a possible superconducting state near room temperature in metallic hydrogen under millions of atmospheres of pressure. The proposed experiments give undergraduate and graduate students the opportunity to learn and develop a multitude of essential experimental techniques; these students will also benefit from collaborations with groups both in the US and abroad.

碱金属是典型的自由电子系统，根据传统的智慧，在压力下应该变得更像自由电子。因此，值得注意的是，最近的理论工作预测，在强压缩下，Li和Na中的近自由电子将变得明显不那么自由，并开始与晶格强烈耦合。从而导致可能的低对称性晶体结构的转变和/或超导的出现。基于最近在Li中发现的i型超导性和在70 GPa压力范围内的多相转变，本项目将研究Li和选定Li合金在超高压下的超导相图Tc(P)和临界磁场行为Tc(H)。此外，还将研究Na和其他低z元素、合金和化合物以及较重的碱金属K和Rb中压力诱导的超导性。从这项研究中得出的系统分类应该对氢在压力下的性质有一些启示，氢被预测在多毫巴压力下成为一种金属，在室温附近可能具有超导性。提出的实验为本科生和研究生提供了学习和发展重要实验室技术的绝佳机会；这些学生还将受益于与美国和国外团体的合作。并非所有的金属都是一样的：有些复杂，有些简单。碱金属Li、Na、K、Rb、Cs被广泛认为是最简单的金属，因为每一种碱金属都含有价电子，这些价电子可以在分离良好的离子核晶格中自由漫游。碱金属是极可压缩的。在非常高的压力下，离子核靠得很近。由于价电子必须避开离子核，它们被迫沿着弯曲的地下墓穴般的路径绕过离子核。由于这个原因，它们不再是自由的，而是与晶格强烈耦合。这会产生晶体结构的潜在变化和/或不寻常的超导状态的外观。这正是在强压下的碱金属Li中观察到的：在相对较高的温度下（15 K或-258 C），在超过200,000个大气压的压力下，会突然出现多个相变和超导性。在本项目中，将深入研究不同压力和磁场下锂和选定的锂合金的异常超导状态。还将对碱金属Na、K和Rb的压力诱导超导性进行研究。这些实验将有助于深入了解金属氢在数百万大气压下接近室温的超导状态。提出的实验为本科生和研究生提供了学习和发展多种基本实验技术的机会；这些学生还将受益于与美国和国外团体的合作。