Linear level repulsions near exceptional points of non-Hermitian systems

TL;DR

This paper uncovers a new universal linear level repulsion distribution near exceptional points in non-Hermitian systems, contrasting with the cubic repulsion in Ginibre ensembles, and characterizes it for different symmetry phases.

Contribution

It introduces two universal level distribution functions near exceptional points of non-Hermitian Hamiltonians, revealing linear level repulsion and its dependence on localization properties.

Findings

Identified two universal distributions, $P_{SP}(s)$ and $P_{SB}(s)$, near exceptional points.

Discovered linear level repulsion ($ ext{s}$ proportionality) contrasting Ginibre ensembles.

Described the distributions with a generalized exponential form depending on localization.

Abstract

The nearest-neighbor level-spacing distributions are a fundamental quantity of disordered systems and universal. It is well-known that extended and localized states of random Hermitian systems follow the Wigner-Dyson and the Poison distributions, respectively, while the Ginibre distributions describe random non-Hermitian systems with complex eigenvalues. However, the level distribution of systems of neither Hermitian nor non-Hermitian with full complex eigenvalues is still unknown. Here we show a new class of universal level distributions in the vicinity of exceptional points of non-Hermitian Hamiltonians. Two universal distribution functions, $P_{\text{SP}}(s)$ for the symmetry-preserved phase and $P_{\text{SB}}(s)$ for the symmetry-broken phase, are needed to describe the nearest-neighbor level-spacing distributions near exceptional points. Surprisingly, both $P_{\text{SP}}(s)$ and $P_{\text{SB}}(s)$ are proportional to $s$ for small $s$, or a linear level repulsion, in contrast to the cubic level repulsions of the Ginibre ensembles. For non-Hermitian disordered tight-binding Hamiltonians, $P_{\text{SP}}(s)$ and $P_{\text{SB}}(s)$ can be well described by $P_{\text{SP(SB)}}(s)=\tilde{c}_1s\exp[-\tilde{c}_2s^{\tilde{\alpha}}]$ in the thermodynamic limit (of infinite systems) with a constant $\tilde{\alpha}$ that depends on the localization nature of states at exceptional points.