Yu–Shiba–Rusinov screening of spins in double quantum dots

Abstract

A magnetic impurity coupled to a superconductor gives rise to a Yu–Shiba–Rusinov (YSR) state inside the superconducting energy gap. With increasing exchange coupling the excitation energy of this state eventually crosses zero and the system switches to a YSR ground state with bound quasiparticles screening the impurity spin by ħ/2. Here we explore indium arsenide (InAs) nanowire double quantum dots tunnel coupled to a superconductor and demonstrate YSR screening of spin-1/2 and spin-1 states. Gating the double dot through nine different charge states, we show that the honeycomb pattern of zero-bias conductance peaks, archetypal of double dots coupled to normal leads, is replaced by lines of zero-energy YSR states. These enclose regions of YSR-screened dot spins displaying distinctive spectral features, and their characteristic shape and topology change markedly with tunnel coupling strengths. We find excellent agreement with a simple zero-bandwidth approximation, and with numerical renormalization group calculations for the two-orbital Anderson model.

Spatially dispersing Yu-Shiba-Rusinov states in the unconventional superconductor FeTe0.55Se0.45

“…The superconducting single impurity Anderson model 57 involves an impurity level ϵ 0 with charging energy U coupled via a tunneling rate Γ s to a superconducting bath with energy gap Δ s 58 – 60 . We represent the sample by a simple s -wave Bardeen–Cooper–Schrieffer (BCS) superconductor, and use the zero-bandwidth approximation, including only a single spin-degenerate pair of quasiparticles at energy Δ s 59 , 61 . We further assume that the gating from the tip changes the impurity level ϵ 0 linearly with distance.…”

Giant superconducting proximity effect on spintronic anisotropy

“…Because strong pairing correlations tend to lower the Coulomb interactions in the molecule, these charge fluctuations become enhanced, which effectively gives rise to a large increase of the generated anisotropy. On the other hand, when the molecule crosses the parity changing transition as the superconducting pairing potential becomes increased further, the molecule enters the Yu-Shiba-Rusinov screened phase [28] and the effect of spintronic anisotropy becomes suppressed.…”

“…(4), as well as the renormalization of the molecular states and energies resulting from Γ S > 0. It also allows for calculation of Andreev bound states energies; see Appendix A, and even more importantly, reproduces Yu-Shiba-Rusinov impurity physics [28]. Next, the normal leads [q = L(eft), R(ight)] of the junction are described as reservoirs of spin-polarized, noninteracting itinerant electrons,…”

Nonlocal pairing as a source of spin exchange and Kondo screening

“…The proximity of SC induces pairing in QDs [41,42] and tends to suppress the Kondo effect if the superconducting energy gap 2∆ becomes larger than the relevant Kondo temperature T K [40,[43][44][45][46][47][48][49]. Moreover, the strength of SC pairing can greatly affect the Kondo physics in the sub-gap transport regime: For QDs attached to SC and normal contacts, it can enhance the Kondo effect [50][51][52], while for DQD-based Cooper pair splitters, it tends to suppress both the SU(2) and SU(4) Kondo effects [53].…”