Quantum valence criticality and superconductivity

The nature of the quantum valence transition is studied on the basis of the periodic Anderson model with Coulomb repulsion between f and conduction electrons. The density matrix renormalization group calculation for the ground state shows that the first-order valence transition emerges with the quantum critical point with diverging valence susceptibility. Instead of the phase separation in the mean-field result, quantum fluctuations generate a wide region of crossover between the Kondo and mixed valence states. It is found that the superconducting correlation is developed in the Kondo regime near the quantum critical point of the valence transition. The origin is ascribed to the enhanced coherent motion of electrons with valence fluctuation. Remarks on the valence transition are given in connection with Ce compounds and Ce metal.