Plasmons in Sodium under Pressure: Increasing Departure from   Nearly-Free-Electron Behavior
  We have measured plasmon energies in Na under high pressure up to 43 GPa using inelastic x-ray scattering (IXS). The momentum-resolved results show clear deviations, growing with increasing pressure, from the predictions for a nearly-free electron metal. Plasmon energy calculations based on first-principles electronic band structures and a quasi-classical plasmon model allow us to identify a pressure-induced increase in the electron-ion interaction and associated changes in the electronic band structure as the origin of these deviations, rather than effects of exchange and correlation. Additional IXS results obtained for K and Rb are addressed briefly. 
I. Loa, K. Syassen, G. Monaco, G. Vanko, M. Krisch, M. Hanfland
cond-mat.mtrl-sci cond-mat.other