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Linear level repulsions near exceptional points of non-Hermitian systems

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 $P_{\text{SP(SB)}}(s)=\tilde{c}_1s\exp[-\tilde{c}_2s^{\tildeα}]$ in the thermodynamic limit (of infinite systems) with a constant $\tildeα$ that depends on the localization nature of states at exceptional points.