Suppression of Kondo screening by the Dicke effect in multiple quantum dots

The interplay between the coupling of an interacting quantum dot to a conduction band and its connection to localized levels has been studied in a triple quantum-dot arrangement. The electronic Dicke effect, resulting from quasiresonant states of two side-coupled noninteracting quantum dots, is found to produce important effects on the Kondo resonance of the interacting dot. We study in detail the Kondo regime of the system by applying a numerical renormalization-group analysis to a finite-U multi-impurity Anderson Hamiltonian model. We find an extreme narrowing of the Kondo resonance, as the single-particle levels of the side dots are tuned toward the Fermi level and "squeeze" the Kondo resonance, accompanied by a strong drop in the Kondo temperature, due to the presence of a supertunneling state. Further, we show that the Kondo temperature vanishes in the limit of the Dicke effect of the structure. By analyzing the magnetic moment and entropy of the three-dot cluster versus temperature, we identify a different local singlet that competes with the Kondo state, resulting in the eventual suppression of the Kondo temperature and strongly affecting the spin correlations of the structure. We further show that system asymmetries in couplings, level structure or due to Coulomb interactions, result in interesting changes in the spectral function near the Fermi level. These strongly affect the Kondo temperature and the linear conductance of the system.