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

Washington University, Saint Louis MO

Investigators

Abstract

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.