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.
