Nonequilibrium Kondo effect by the equilibrium numerical renormalization group method: The hybrid Anderson model subject to a finite spin bias

Abstract

We investigate Kondo correlations in a quantum dot with normal and superconducting electrodes, where a spin bias voltage is applied across the device and the local interaction U is either attractive or repulsive. When the spin current is blockaded in the large-gap regime, this nonequilibrium strongly-correlated problem maps into an equilibrium model solvable by the numerical renormalization group method. The Kondo spectra with characteristic splitting due to the nonequilibrium spin accumulation are thus obtained at high precision. It is shown that while the bias-induced decoherence of the spin Kondo effect is partially compensated by the superconductivity, the charge Kondo effect is enhanced out of equilibrium and undergoes an additional splitting by the superconducting proximity effect, yielding four Kondo peaks in the local spectral density. In the charge Kondo regime, we find a universal scaling of charge conductance in this hybrid device under different spin biases. The universal conductance as a function of the coupling to the superconducting lead is peaked at and hence directly measures the Kondo temperature. Our results are of direct relevance to recent experiments realizing negative-U charge Kondo effect in hybrid oxide quantum dots [Nat. Commun. 8, 395 (2017)].

Superior thermoelectric properties through triangular triple quantum dots (TTQD) attached to one metallic and one superconducting lead

“…On the other hand, although a lot of research has already been done on the transport properties of QDs coupled to superconducting leads, 30–43 the research on the thermoelectric properties of such systems has only just started. 15–17,44–47 Moreover, the Andreev transport regime was mainly investigated in three-terminal devices, where superconducting is just regarded as an additional property, and thermoelectric transport only appears between metallic–metallic lead.…”

Interplay of the Kondo effect with the induced pairing in electronic and caloric properties of T-shaped double quantum dots

“…This gives rise to an interesting interplay between the electrostatic attraction and pairing, which leads to suppression of the super-current through the device in the crossover region between the weak-coupling and strong-coupling unitary transmission regimes [66]. Moreover, unlike the spin Kondo effect, its charge counterpart may become enhanced under nonequilibrium spin bias [67]. Although in this paper we focus on the repulsive U case, our work shall contribute to the general understanding of the interplay between Kondo correlations with the superconducting proximity effect.…”