Nonlocal pairing as a source of spin exchange and Kondo screening

Abstract

We show that the Kondo screening in a correlated double quantum dot structure may be caused solely by the proximity of a superconductor, which induces nonlocal pairing by Andreev reflection processes. This leads to an effective exchange interaction, which we estimate perturbatively and corroborate the analytical predictions by the numerical renormalization group calculations, using an effective model for the superconductor-proximized nanostructure. We determine the dependence of the relevant Kondo temperature on the coupling to superconductor and predict a characteristic modification of conventional low-temperature transport behavior, which can be used to experimentally distinguish this phenomenon from other Kondo effects. The occurrence of nonlocal pairing exchange does not depend on details of the proposed setup, therefore it can be also of relevance for the bulk materials, such as heavy-fermion compounds. arXiv:1809.06415v2 [cond-mat.str-el]

Majorana-Kondo interplay in T-shaped double quantum dots

“…( 13) one can conclude that the coupling to the Majorana wire slightly decreases the effective exchange interaction between the quantum dots. Interestingly, this effect is opposite to what happens in the presence of a conventional superconductor [58,61]. Nevertheless, as shown in the following by accurate NRG calculations, the decrease of bare exchange interaction becomes overwhelmed by strong electron correlations.…”

“…In the T-shaped double quantum dot in the presence of normal leads the low-energy states relevant for the two-stage Kondo regime are those consisting of two singly-occupied quantum dots, organized into singlet and triplet, split by the effective antiferromagnetic exchange interaction J eff ≈ 4t 2 /U [41]. This structure remains untouched when the device is proximized by the conventional superconductor, only the value of J eff increases (irrespective of the geometry) or even J eff can arise due to the coupling to a BCS-like superconductor [58]. However, the situation qualitatively changes when the superconductor is topological.…”

Hallmarks of Majorana mode leaking into a hybrid double quantum dot

“…1), where the Majorana mode can affect QD 1 only indirectly, via the second quantum dot. For efficiency of this two-stage Majorana leakage, one should also take into account the Kondo effect developed in QD 2 [15,87,88]. With lowering the energy scale below the Kondo temperature T K , the Kondo effect first develops in the first quantum dot, giving rise to enhanced conductance [89].…”

Transport properties of proximitized double quantum dots

“…where ī = 2 for i = 1 and ī = 1 for i = 2, Γ Si (Γ Si ī) is the direct (cross) coupling between i-dot and the SC substrate. We assume that Γ S12 = Γ S21 = √ Γ S1 Γ S2 [20,25,42].…”

“…Other options of DQD with SC and N leads connection are also considered, e.g. connection of the first dot to two metallic leads and the second dot to the SC lead [11,12,19,23,25]. In such systems, there are possible both the normal electron transfers (when the single electron transfers between both normal metallic electrodes) and direct (DAR) and crossed (CAR) Andreev reflections (when electrons of the Cooper pair tunnel into SC lead and the holes tunnel to the same (DAR) or the second (CAR) metallic lead).…”