NHSE-Driven Coalescence of Topological Defect States in Non-Hermitian Systems

In this work, we describe a novel localization phenomena, the so-called topological defect accumulation, occurring in a non-Hermitian chain with an arbitrary number of defect sites. Specifically, it refers to the localization and coalescence of multiple defect eigenstates at a single defect site closest to the localization edge of the bulk non-Hermitian skin modes. This phenomenon is distinct from the conventional topological defect states in Hermitian systems, where the defect states are separately localized at their respective defect sites. The requirement for the onset of topological defect accumulation is the presence of non-reciprocal coupling which renders the chain non-Hermitian, as well as imaginary onsite potentials at the defect nodes. This allows the defect state accumulation and distribution to be modulated by both the defect site distribution and their corresponding onsite potentials paving the way for possible applications. Furthermore, this phenomenon is realizable in various synthetic and experimentally realizable platforms, such as topolectrical and photonic systems.