Transport through side-coupled double quantum dots: From weak to strong interdot coupling

We report low-temperature transport measurements through a double-quantum-dot device in a configuration where one of the quantum dots is coupled directly to the source and drain electrodes, and a second (side-coupled) quantum dot interacts electrostatically and via tunneling to the first one. As the interdot tunneling coupling increases, a crossover from weak to strong interdot tunneling is observed in the charge stability diagrams that present a complex pattern with mergings and apparent crossings of Coulomb blockade peaks. While the weak-coupling regime can be understood by considering a single level on each dot, in the intermediate- and strong-coupling regimes, the multilevel nature of the quantum dots needs to be taken into account. Surprisingly, both in the strong- and weak-coupling regimes, the double-quantum-dot states are mainly localized on each dot for most values of the parameters. Only in an intermediate-coupling regime does the device present a single dotlike molecular behavior as the molecular wave functions weight is evenly distributed between the quantum dots. At temperatures larger than the interdot coupling energy scale, a loss of coherence of the molecular states is observed.