Scaling of pseudospectra in exponentially sensitive lattices

Abstract

One of the important features of non-Hermitian Hamiltonians is the existence of a unique type of singularities, the so-called exceptional points (EPs). When the corresponding systems operate around such singularities, they exhibit ultrasensitive behavior that has no analog in conservative systems. An alternative way to realize such ultra-sensitivity relies on asymmetric couplings. Here we provide a comprehensive analysis based on pseudospectra, that shows the origin of exponential sensitivity, without relying on topological zero modes or the localization of all eigenstates (skin effect), but on the underlying extreme lattice non-normality. In particular, we consider four different types of lattices (Hatano-Nelson, Sylvester-Kac, non-Hermitian Su-Schrieffer-Heeger and a non-Hermitian random lattice) and identify the conditions for exponential sensitivity as a function of the lattice size. Complex and structured pseudospectra reveal the signatures of exponential sensitivity both on the eigenvalue spectra and on the underlying dynamics. Our study, may open new directions on studies related to the exploitation of non-normality for constructing ultra-sensitive systems that do not rely on the existence of EPs.