Quantum Phase Transition in a Multilevel Dot

We discuss electronic transport through a lateral quantum dot close to the singlet-triplet degeneracy in the case of a single conduction channel per lead. By applying the numerical renormalization group, we obtain rigorous results for the linear conductance and the density of states. A new quantum phase transition of the Kosterlitz-Thouless–type is found, with an exponentially small energy scale $T^{*}$ close to the degeneracy point. Below $T^{*}$, the conductance is strongly suppressed, corresponding to a universal dip in the density of states. This explains recent transport measurements.