Coulomb blockade and Non-Fermi-liquid behavior in quantum dots

The non-Fermi-liquid properties of an ultrasmall quantum dot coupled to a lead and to a quantum box are investigated using a new variant of Wilson's numerical renormalization group. Below the charging energy of the quantum box, a second screening channel is dynamically generated. Tuning the ratio of the tunneling amplitudes to the lead and box, we find a two-channel Kondo fixed point for arbitrary Coulomb repulsion on the dot. For an asymmetric dot, we find a continuous transition from a spin to a charge two-channel Kondo effect. At $T = 0$, a step-like structure is found in the conductance of a two-lead setting, the height of which depends on the dot occupancy. The temperature scale below which the two-channel Kondo effect sets in is greatly enhanced away from the local-moment regime, making this exotic effect accessible in realistic quantum-dot devices.