A Second Critical Point in First Order Metal-Insulator Transitions
For more than a century metal insulator transitions were identified by a rapid resistance drop as the fraction of metal is increased. Here we show that there can be a second separate critical point, where the dielectric constant turns negative and the metallic reflectivity sharply increases. The rapid resistance change is associated with the vanishing of metallic conductance paths, while the second critical point occurs at a higher metallic fraction when the metal is sufficient to close off dielectric tunnels through the material. The intermediate state between these two critical points is characterized by two coexisting infinite clusters of both dielectric and metal. The second critical point may be seen in changes in the polarization, eddy current losses, and the presence of two sound velocities. The figure shows a calculation of the dielectric response as a function of metal fraction and frequency. The peak corresponds to the conductivity transition while the region below the horizontal blue plane indicates the transition to metallic reflectivity as seen in recent experiments under pressure on magnesium oxide. A similar phase separation can occur in superconductors with insulating inclusions.
***
EV10459
The finite size of ions yields unexpected correlation effects in liquid
In this paper, we upgrade the classical Poisson-Boltzmann theory which describes the electrostatic properties of ionic liquids in such a way to account for the finite size of ions. Each ionic species now obeys a Fermi-like distribution formulated with the ionic volume fractions in the bulk solution. Near a charged interface for example, the ions can no longer exceed the close packing density as it is the case in the classical theory. As a result, when large counterions with different valences are present, they tend to phase-separate into layers enriched in ions of same valence. The theory can be readily implemented into existing software packages computing the electrostatic of charged objects in solution and in particular, it should help biophysicists understand better the interactions of biomolecules with their ion atmosphere.
