$\mathcal{PT}$-symmetry breaking in a Kitaev chain with one pair of gain-loss potentials

TL;DR

This paper investigates the $ ext{PT}$-symmetry breaking threshold in a finite Kitaev chain with gain and loss potentials, revealing a complex phase diagram influenced by system parameters and band structure, including re-entrant phases.

Contribution

The study derives the $ ext{PT}$-threshold for a Kitaev chain with gain-loss potentials, combining numerical and analytical methods to reveal a rich phase diagram and re-entrant $ ext{PT}$-symmetric phases.

Findings

Identified the $ ext{PT}$-breaking threshold as a function of system parameters.

Discovered a re-entrant $ ext{PT}$-symmetric phase in even chains with zero on-site potential.

Mapped the phase diagram using band-structure analysis.

Abstract

Parity-time ($\mathcal{PT}$) symmetric systems are classical, gain-loss systems whose dynamics are governed by non-Hermitian Hamiltonians with exceptional-point (EP) degeneracies. The eigenvalues of a $\mathcal{PT}$-symmetric Hamiltonian change from real to complex conjugates at a critical value of gain-loss strength that is called the $\mathcal{PT}$ breaking threshold. Here, we obtain the $\mathcal{PT}$-threshold for a one-dimensional, finite Kitaev chain -- a prototype for a p-wave superconductor -- in the presence of a single pair of gain and loss potentials as a function of the superconducting order parameter, on-site potential, and the distance between the gain and loss sites. In addition to a robust, non-local threshold, we find a rich phase diagram for the threshold that can be qualitatively understood in terms of the band-structure of the Hermitian Kitaev mo del. In particular, for an even chain with zero on-site potential, we find a re-entrant $\mathcal{PT}$-symmetric phase bounded by second-order EP contours. Our numerical results are supplemented by analytical calculations for small system sizes.