Exceptional Points in a Parallel Double-Quantum-Dot Josephson Junction Coupled to a Ferromagnetic Reservoir

Abstract

We investigate exceptional points (EPs) in a parallel double-quantum-dot Josephson junction coupled to a dissipative reservoir. By integrating out the leads, we obtain a non-Hermitian Bogoliubov-de Gennes description wherein the superconducting phase difference and orbital flux govern the complex Andreev spectrum. For spin-independent dissipation, the second-order EPs identified within the infinite superconducting gap limit are eliminated when the finite superconducting gap is properly incorporated. In contrast, spin-dependent dissipation originating from a ferromagnetic reservoir, in conjunction with magnetic flux, gives rise to second-order EPs that persist in superconducting leads with finite gap. Moreover, flux tuning enables the coalescence of two second-order EPs into a third-order EP, whose eigenvalue splitting exhibits cubic-root scaling behavior. A many-body parity analysis establishes the connection between the contrasting finite-gap behavior and the spectral relationship between the even- and odd-parity sectors. Finally, Josephson currents calculated from both the free-energy derivative and the surrogate-model density matrix demonstrate consistency and remain continuous across the EPs. These findings establish spin-selective dissipation and interferometric flux as effective control parameters for robust non-Hermitian singularities in superconducting nanostructures.