  We study the Kondo effect in side-coupled double quantum dots with particular
focus on the crossover between two distinct singlet ground states, using the
numerical renormalization group. The crossover occurs as the quantized energy
level of the embedded dot, which is connected directly to the leads, is varied.
In the parameter region where the embedded dot becomes almost empty or doubly
occupied, the local moment emerging in the other dot at the side of the path
for the current is screened via a superexchange process by the conduction
electrons tunneling through the embedded dot. In contrast, in the other region
where the embedded dot is occupied by a single electron, the local moment
emerges also in the embedded dot, and forms a singlet bond with the moment in
the side dot. Furthermore, we derive two different Kondo Hamiltonians for these
limits carrying out the Schrieffer-Wolff transformation, and show that they
describe the essential feature of the screening for each case.
