Kondo effect in double quantum dot systems

The tunneling conductance of the double dot systems is calculated by using numerical renormalization group method. In low temperature, the following states continuously appear as increasing the coupling between two dots: (1) Kondo singlet state, (2) local singlet state between spins on the dots, and (3) double occupancy in even orbital. The conductance shows peaks at the transition regions between these states. Especially, the peak at the boundary between (1) and (2) shows anomalous narrowing of the width in the strongly correlated cases. We notice this is related to the quantum transition suggested previously for the two impurity Kondo model.