Metal-Insulator and Superconductor-Insulator Transitions in Correlated Electron Systems

Quantum transitions between the Mott insulator and metals by controlling filling in two-dimensional square lattice are characterized by a large dynamical exponent $z=4$ where the origin of unusual metallic properties near the Mott insulator are ascribed to the proximity of the transition. The scaling near the transition indicates the formation of flat dispersion area due to singular momentum dependence of the single-particle renormalization. The flat dispersion controls critical properties of the Mott transition. An instability of the flat dispersion to the d-wave superconducting order is discussed. We also discuss a case of the Mott transition for a model of Mn perovskite compounds with orbital degeneracy where orbital correlation length shows critical divergence toward the metal-insulator transition.