Gate voltage effects in capacitively coupled quantum dots

– We study a system of two symmetrical capacitively coupled quantum dots, each coupled to its own metallic lead, focusing on its evolution as a function of the gate voltage applied to each dot. Using the numerical renormalization group and poor man’s scaling techniques, the low-energy Kondo scale of the model is shown to vary significantly with the gate voltage, being exponentially small when spin and pseudospin degrees of freedom dominate; but increasing to much larger values when the gate voltage is tuned close to the edges of the Coulomb blockade staircase where low-energy charge-fluctuations also enter, leading thereby to correlated electron physics on energy/temperature scales more accessible to experiment. This range of behaviour is also shown to be manifest strongly in single-particle dynamics and electron transport through each dot. Introduction. – Recent years have seen intense investigation of electron transport through semiconducting quantum dot devices [1], leading in turn to a strong resurgence of interest in basic Kondo physics [2]. The classic example—namely the spin-1/2 Kondo effect [3] in which the magnetic moment of an odd-electron dot is quenched by coupling to metallic leads—was observed in quantum dots almost a decade ago. Since then, a continuing goal has been to understand how coupled, multiple quantum dot systems may lead to variants of the Kondo effect involving both spin and orbital degrees of freedom. In this Letter we consider probably the simplest example of such—a symmetrical double quantum dot system in which the interdot coupling is capacitive. Experimental realisations of such devices have appeared in the literature [4–6], and various aspects of the rich inherent physics have been uncovered in a number of theoretical papers, see e.g. [7–13]. Here we elucidate theoretically a key underlying issue: the effect on transport of sweeping the dot energy levels through a wide range of values by means of a suitably applied gate voltage. The low-temperature transport through each dot is heavily dependent on the magnitude of the gate voltage: both the characteristic low-energy Kondo scale of the system, and the zerobias differential conductance, vary significantly as the energy levels of the dots are lowered from the Fermi level down. If the gate voltage is tuned in such a way that only low-energy spin, or spin/orbital-pseudospin, degrees of freedom are relevant, then the Kondo scale is