Supercurrent from the imaginary part of the Andreev levels in non-Hermitian Josephson junctions

TL;DR

This paper explores how non-Hermitian effects influence supercurrent in Josephson junctions, revealing a new phase-dependent contribution linked to the imaginary part of Andreev levels, especially near exceptional points.

Contribution

It introduces a novel current component arising from the phase derivative of level broadening in non-Hermitian Josephson junctions, and proposes an experimental detection protocol.

Findings

A new supercurrent contribution proportional to the phase derivative of level broadening.

Identification of spectral configurations where the new contribution is prominent.

An experimental protocol to detect non-Hermitian effects independent of exceptional points.

Abstract

We investigate the electronic transport properties of a superconductor-quantum dot-superconductor Josephson junction coupled to a ferromagnetic metal reservoir in the presence of an external magnetic field. The device is described by an effective non-Hermitian Hamiltonian, whose complex eigenvalues encode the energy (real part) and the broadening (imaginary part) of the Andreev quasi-bound states. When extending the Andreev current formula to the non-Hermitian case, a novel contribution arises that is proportional to the phase derivative of the levels broadening. This term becomes particularly relevant in the presence of exceptional points (EPs) in the spectrum, but its experimental detection is not straightforward. We identify optimal Andreev spectrum configurations where this novel current contribution can be clearly highlighted, and we outline an experimental protocol for its detection. We point out that the phase dependence in the levels imaginary part originates from the breaking of a time-reversal-like symmetry. In particular, spectral configurations in the broken phase of the symmetry and without EPs can be obtained, where this novel contribution can be easily resolved. The proposed protocol would allow to probe for the first time a fingerprint of non-Hermiticity in open junctions that is not strictly related to the presence of EPs.