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

碱金属是典型的自由电子系统，根据传统智慧，在压力下，它应该变得更像自由电子一样。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中的近自由电子应该变得明显不那么自由，并开始与晶格强烈耦合。从而可能导致向较低对称性晶体结构的转变和/或超导电性的出现。基于最近在Li中发现的I型超导电性和在70 Gpa压力范围内的多次相变，本项目将研究Li和部分Li合金在超高压下的超导相图T_c(P)和临界磁场行为T_c(H)。此外，还将在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中寻找压力诱导超导电性。从这项研究中得出的系统学应该会对氢在压力下的性质提供一些启示，因为有人预测氢在几兆巴的压力下会变成一种金属，在室温附近可能具有超导电性。拟议中的实验为本科生和研究生提供了学习和开发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的研究小组的合作。所有金属都不是平等的：有些很复杂，有些很简单。碱金属Li，Na，K，Rb，Cs被广泛认为是所有金属中最简单的，因为每种金属都含有价电子，这些电子可以自由地在分离良好的离子核心的晶格中漫游。碱金属具有极强的可压缩性。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，它们被迫沿着离子核周围曲折的地下洞状路径运行。由于这个原因，它们不再是自由的，而是与晶格强烈耦合。这会导致晶体结构的潜在变化和/或出现不寻常的超导态。在强压缩下的碱金属Li中确实观察到了这一点：在超过20万个大气压的压力下，发生了多个相变，在相对较高的温度(15K或-258℃)下突然出现超导电性。在本项目中，我们将深入研究Li及部分Li合金在变压强和磁场作用下的反常超导态。还将研究碱金属Na、K和Rb中的压力诱导超导电性。这些实验应该可以洞察在数百万个大气压下，金属氢在室温附近可能存在的超导状态。拟议的实验为本科生和研究生提供了学习和开发大量基本实验技术的机会；这些学生还将从与美国和国外的小组合作中受益。

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.