Renormalized and iterative formalism of the Andreev levels within large multi-parametric space

Abstract

We attain a renormalized and iterative expression of the Andreev level in a quantum-dot Josephson junction, which is bound to have significant implications due to several significant advantages. The renormalized form of the Andreev level not only allows us to extend beyond the limitations of small tunnel coupling, quantum dot energy, magnetic field, and mean-field Coulomb interaction but also enables the capturing of subgap levels that leak out of the superconducting gap into the continuous spectrum. Furthermore, the iterative form of the Andreev level provides an intuitive understanding of the spin-split and superconducting proximity effects of the superconducting leads. We find a singlet-doublet quantum phase transition (QPT) in the ground state due to the intricate competition between the superconducting and spin-split proximity effects, that differs from the typical QPT arising from the competition between the superconducting proximity effect (favoring singlet phase) and the quantum dot Coulomb interaction (favoring doublet phase). This QPT has a diverse phase diagram owing to the spin-split proximity effects which favors the doublet phase akin to the quantum-dot Coulomb interaction but can be also enhanced by the tunneling coupling like the superconducting proximity effect. Unlike the typical QPT, where tunnel coupling prefers singlet ground state, this novel QPT enables strong tunnel coupling to suppress the singlet ground state via the spin-split proximity effect, allowing a singlet-doublet-singlet transition with increasing tunnel coupling. Our renormalized and iterative formalism of the Andreev level is crucial for the electrostatic gate, external flux, and magnetic field modulations of the Andreev qubits.