Quantum dot Josephson junctions in the strong-coupling limit

Luka Pavešič, Ramón Aguado, Rok Žitko

Published 2023-04-24Version 1

We consider an interacting quantum dot embedded in a Josephson junction between two superconducting leads. By defining symmetry-adapted superpositions of states from the leads, we formulate an effective model with a single orbital directly coupled to the dot and three additional indirectly coupled orbitals. This minimal basis set allows to account for the quasiparticles in the vicinity of the dot that experience the Kondo exchange interaction, as well as quasiparticles far away in the leads. This formulation reveals the changing nature of the spin-doublet state in the strong coupling limit. We show that the local moment in the quantum dot is strongly screened in the doublet state as well, binding a quasiparticle in the same way as in the singlet state. This explains the physical origin of the "doublet chimney" in the phase diagram: the doublet and singlet states are nearly degenerate, with the doublet stabilized by the residual coupling of an ``almost free'' quasiparticle resulting from the Cooper-pair splitting by the quantum dot local moment.