Kondo resonant spectra in coupled quantum dots

The Kondo effect in coupled quantum dots is investigated from the viewpoint of transmission spectroscopy using the slave-boson formalism of the Anderson model. The antiferromagnetic spin-spin coupling J between the dots is taken into account. Conductance G through the dots connected in a series is characterized by the competition between the dot-dot tunneling coupling $V_C$ and the level broadening $\Delta$ in the dots (dot-lead coupling). When $V_C/\Delta <1,$ the Kondo resonance is formed between each dot and lead, which is replaced by a spin-singlet state in the dots at low gate voltages. The gate voltage dependence of G has a sharp peak of ${2e}^2/h$ in height in the crossover region between the Kondo and spin-singlet states. The sharp peak of G survives when the energy levels are different between the dots. When $V_C/\Delta >1,$ the “molecular levels” between the Kondo resonant states appear; the Kondo resonant peaks are located below and above the Fermi level in the leads at low gate voltages. The gate voltage dependence of G has a broad peak, which is robust against J. The broad peak splits into two peaks when the energy levels are different, reflecting the formation of the asymmetric molecular levels between the Kondo resonant states.