Erasing odd-parity states in semiconductor quantum dots coupled to superconductors

Abstract

Quantum dots are gate-defined within InSb nanowires, in proximity to NbTiN superconducting contacts. As the coupling between the dot and the superconductor is increased, the odd-parity occupation regions in transport become nondiscernible (erased) both above and below the induced superconducting gap. Above the gap, conductance in the odd Coulomb-blockade valleys increases until the valleys are lifted. Below the gap, Andreev bound states undergo quantum phase transitions to Kondo-screened singlet ground states at odd occupancy. We investigate to what degree the apparent erasure of odd-parity regimes coincides at low and high biases. We complement experiments with numerical renormalization group simulations. We interpret the results in terms of a competition between Kondo screening and superconductivity. In the erased odd-parity regime, the quantum dot exhibits transport features similar to a finite-size Majorana nanowire, drawing parallels between even-odd dot occupations and even-odd one-dimensional subband occupations.