  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.
