Coulomb blockade and quantum critical points in quantum dots

An ultrasmall quantum dot coupled to a lead and to a quantum box (a large quantum dot) is investigated. Tuning the tunneling amplitudes to the lead and box, we find a line of unstable non-Fermi-liquid fixed points as a function of the gate potentials of the quantum dots, extending to arbitrary charging energies on the small and large quantum dots. These quantum-critical fixed points possess a finite residual entropy. They govern the cross-over from one Fermi-liquid regime to another, characterized by distinct (high and low) conductance values.