Electronic and structural ground state of heavy alkali metals at high pressure

TL;DR

This study explores how high pressure induces electronic localization and structural phase transitions in heavy alkali metals K, Rb, and Cs, revealing pressure-enhanced correlations and hybridization effects.

Contribution

It combines experimental x-ray techniques with ab initio calculations to elucidate the electronic and structural behavior of heavy alkali metals under high pressure, highlighting localization and hybridization phenomena.

Findings

Pressure induces similar phase transitions in K, Rb, and Cs, except for Cs oC84 phase.

Valence electrons become more localized with pseudo-gap formation near the Fermi level.

Localization suggests these alkali metals approach strongly correlated electron states.

Abstract

Alkali metals display unexpected properties at high pressure, including emergence of low symmetry crystal structures, that appear to occur due to enhanced electronic correlations among the otherwise nearly-free conduction electrons. We investigate the high pressure electronic and structural ground state of K, Rb, and Cs using x-ray absorption spectroscopy and x-ray diffraction measurements together with ab initio theoretical calculations. The sequence of phase transitions under pressure observed at low temperature is similar in all three heavy alkalis except for the absence of the oC84 phase in Cs. Both the experimental and theoretical results point to pressure-enhanced localization of the valence electrons characterized by pseudo-gap formation near the Fermi level and strong spd hybridization. Although the crystal structures predicted to host magnetic order in K are not observed, the localization process appears to drive these alkalis closer to a strongly correlated electron state.