Electronic phase transitions in expanded metals Dielectric catastrophe or anomaly?

Abstract
A simple criterion is presented to assess whether to expect an insulator-metal (MNM) transition or an insulator-excitonic insulator (EI) transition as we increase the density of a one-component disordered dielectric insulator. The density dependence of the real part of the frequency-dependent dielectric constant is examined. For a system which undergoes an EI (MNM) transition, anomalous dielectric behaviour (a dielectric catastrophe) is found as we approach the EI (MNM) transition density from the insulating side. Expanded fluid Li is predicted to undergo an EI transition.