A new twist to the interplay between superconductivity and Kondo correlation

A new twist to the interplay between superconductivity and Kondo correlationRecently, Deng et al. [1] found a new twist to the intriguing physics resulting from the interplay between Kondo correlation and superconductivity.Kondo correlation [2,3] and superconductivity [4,5] are two fundamental phenomena in condensed matter physics, and various novel physics have been discovered from their interplay.Superconductivity occurs when electrons in the material form Cooper pairs and can thus move through the material without resistance.In a primary s-wave superconductor, Cooper pairs can be disrupted by the presence of magnetic impurities or other interactions.Kondo correlation is a type of interaction between localized magnetic moments and conduction electrons.This correlation can significantly modify the electrical conductivity of a normal state quantum-dot system.When a quantum dot coupled to superconductor leads, superconductivity and Kondo correlation may compete [6].Depending on the ratio ∆/T k , where ∆ is the superconductor gap parameter and T k is the Kondo temperature, the system can be at the 0-phase (singlet ground state) or π-phase (doublet ground state) or some phases in between (namely, 0′-or π′-phase) [7].When the Kondo correlation is sufficiently strong, i.e., ∆<T k , the system is at the 0-phase and Cooper pairs can transfer through the quantum dot via the Kondo resonance, leading to the appearance of a zero-bias conductance peak.In the opposite case, ∆>T k , the system is at the π-phase and Cooper pairs can pass through the quantum dot dominantly via co-tunneling processes, leading to a low-conductance valley in the differential conductance spectrum.In the intermediate regime, the system exhibits complex new physics.However, this is the regime that has rarely been explored.Deng and his colleagues fabricated devices of an InSb nanowire quantum dot coupled with superconductor leads, and experimentally studied the conductance spectra in a wide range of the relative strengths of the superconductivity and Kondo correlations.They found a Cooper pair tunneling-induced zero-bias conductance peak emerging in the intermediate regime, where the superconductivity and Kondo correlations are in comparable strengths.They discovered a remarkable phenomenon: the height of this zero-bias peak increases sharply when a weak magnetic field is applied, i.e., the system exhibits negative magneto-resistance (NMR).Similar counter-intuitive NMR behavior has been observed in previous studies and was attributed to the phase transition caused by the change of ∆/T k [8].However, Deng et al. carefully checked the anomalous NMR and found it strongly depends on the formation of the superconductor vortices in the leads when a weak magnetic field is applied, rather than the quantum phase transition due to a change in ∆/T k .These vortices can serve as drains for unpaired electrons or, in other words, quasiparticles.Such quasiparticle trapping results in a decrease in current dissipation and thus gives rise to an enhancement in the zero-bias conductance peak value.The work by Deng and colleagues provides a new insight into the rich physics that can emerge in a strongly correlated semiconductor quantum dot-superconductor hybrid system.The finding also reminds the researchers that quasiparticle dynamics can play an important role in analyzing the physical properties of advanced quantum devices consisting of superconductors, including superconductor qubits.