Uniform response theory of non-Hermitian systems: Non-Hermitian physics beyond the exceptional point

Abstract

Non-Hermitian systems display remarkable response effects that reflect a variety of distinct spectral scenarios, such as exceptional points where the eigensystem becomes defective. However, present frameworks treat the different scenarios as separate cases, following the singular mathematical change between the spectral decompositions from one scenario to another. This not only complicates the coherent description near the spectral singularities where the response qualitatively changes, but also impedes the application to practical systems. Here we develop a general response theory of non-Hermitian systems that uniformly applies across all spectral scenarios. We unravel this response by formulating uniform expansions of the spectral quantization condition and Green's function, where both expansions exclusively involve directly calculable data from the Hamiltonian. This data smoothly varies with external parameters as spectral singularities are approached, and nevertheless captures the qualitative differences of the response in these scenarios. We furthermore present two direct applications of this framework. Firstly, we determine the precise conditions for spectral degeneracies of geometric multiplicity greater than unity, as well as the perturbative behavior around these cases. Secondly, we formulate a hierarchy of spectral response strengths that varies continuously across all parameter space, and thereby also reliably determines the response strength of exceptional points. Finally, we demonstrate both generally and in concrete examples that the previously inaccessible scenarios of higher geometric multiplicity result in unique variants of super-Lorentzian response. Our approach widens the scope of non-Hermitian response theory to capture all spectral scenarios on an equal and uniform footing.