Tunneling conductance through two quantum dots, which are connected in series to left and right leads, is calculated by using the numerical renormalization group method. As the hopping between the dots increases from a very small value, the following states continuously appear: (i) Kondo singlet state of each dot with its adjacent-site lead, (ii) singlet state between the local spins on the dots, and (iii) double occupancy in the bonding orbital of the two dots. The conductance shows peaks at the crossover regions between these states. The peak at the boundary between (i) and (ii) especially has the unitarity limit value of ${2e}^2/h$ because of coherent connection through the lead-dot-dot-lead. For the strongly correlated cases, the characteristic energy scale of the coherent peak shows anomalous decrease relating to the quantum critical transition known for the two-impurity Kondo effect. The two dot systems give a new realization of the two-impurity Kondo problem.

• Received 7 December 1999

DOI:https://doi.org/10.1103/PhysRevB.62.10260