Studies of Alkali Metal and Low-z Superconductors under Extreme Hydrostatic Pressure

技术：碱金属是典型的自由电子系统，根据传统智慧，在压力下应该变得更像自由电子一样。因此，值得注意的是，理论工作预测，在强压缩下，碱金属和碱土金属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，从而可能导致向低对称性晶体结构的转变和/或超导的出现。对于相对简单的开始过渡金属Sc、Y和Lu，对于碱金属Li和Cs，预计也会有类似的结果，它们在高压缩下表现出反常的超导电性。本项目将深入研究部分碱金属、它们的合金和化合物以及Sc、Y和Lu中的超高压超导相图和临界磁场行为。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一个学习和开发重要实验室技术的绝佳机会，并与美国和国外的小组合作。非技术：如果施加高压使原子更紧密地靠近原子，从而增加外层电子的轨道重叠，非金属固体可能会变成金属。在最简单的情况下，施加高压应该把绝缘体变成金属，或者把好金属变成更好的金属。相反，实验表明，已知的最简单的金属，即碱金属Li、Na、K、Rb和Cs，在非常高的压力下变得越来越复杂，对碱土如Ca、Sr或Ba也有类似的结果。在足够的压力下，Cs甚至转变为过渡金属，如Ta或Nb，并成为超导，Li和Ca在高达15-25K的转变温度下也是如此，理解这些相对简单的系统在压力下的变化加深了我们对金属和超导电性的总体理解。这些研究提供了对数百万大气压下金属氢在室温附近可能的超导状态的洞察。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。

Technical: 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 theoretical work predicts that under strong compression the nearly free electrons in the alkali and alkaline earth metals should become markedly less free and begin to couple strongly with the lattice, leading to possible transformations to lower symmetry crystal structures and/or the appearance of superconductivity. Similar results are anticipated for the relatively simple beginning transition metals Sc, Y, and Lu which, as for the alkalis Li and Cs, show anomalous superconductivity under high compression. This project will study in depth to ultrahigh pressures the superconducting phase diagram and critical magnetic field behavior of selected alkali metals, their alloys and compounds, as well as in Sc, Y, and Lu. 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 and collaborate with groups both in the US and abroad.Non-Technical:Solids which are not metallic may become metals if high pressures are applied to bring the atoms closer together, thus increasing the orbital overlap of the outer electrons. In the simplest picture, applying high pressures should turn insulators into metals or good metals into better ones. To the contrary, experiment shows that the simplest metals known, the alkali metals Li, Na, K, Rb, and Cs, become increasingly complex under very high pressures, with similar results for the alkaline earths like Ca, Sr, or Ba. Under sufficient pressure Cs even turns into a transition metal, like Ta or Nb, and becomes superconducting, as do Li and Ca at transition temperatures as high as 15-25 K. Understanding the changes under pressure in these relatively simple systems deepens our understanding of metals and superconductivity in general. These studies 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.