Quantum critical “opalescence” around metal-insulator transitions

Divergent carrier-density fluctuations equivalent to the critical opalescence of gas-liquid transition emerge around a metal-insulator critical point at a finite temperature. In contrast to the gas-liquid transitions, however, the critical temperature can be lowered to zero, which offers a challenging quantum phase transition. We present a microscopic description of such quantum critical phenomena in two dimensions. The conventional scheme of phase transitions by Ginzburg, Landau and Wilson is violated and an unconventional universality appears. It offers a clear insight into the criticalities of metal-insulator transitions associated with Mott or charge-order transitions. Fermi degeneracy involving the diverging density fluctuations generates emergent phenomena near the endpoint of the first-order transition and must shed new light on remarkable phenomena found in correlated metals like unconventional cuprate superconductors. It indeed accounts for otherwise puzzling criticality of the Mott transition recently discovered in an organic conductor. We propose to measure enhanced dielectric fluctuations at small wavenumbers by accurate probes.