Edge Modes, Degeneracies, and Topological Numbers in Non-Hermitian Systems

TL;DR

This paper investigates topological edge modes in non-Hermitian 2D Dirac systems, revealing their relation to exceptional points and classifying them into three families characterized by two winding numbers.

Contribution

It introduces a classification of non-Hermitian topological edge modes into three families based on their properties and links these modes to exceptional points and bulk spectra degeneracies.

Findings

Edge modes are classified into three families: Hermitian-like, non-Hermitian, and mixed.

Edge modes are characterized by two winding numbers related to exceptional points.

Realization of these topological modes in honeycomb lattices of ring resonators with gain/loss.

Abstract

We analyze chiral topological edge modes in a non-Hermitian variant of the 2D Dirac equation. Such modes appear at interfaces between media with different "masses" and/or signs of the "non-Hermitian charge". The existence of these edge modes is intimately related to exceptional points of the bulk Hamiltonians, i.e., degeneracies in the bulk spectra of the media. We find that the topological edge modes can be divided into three families ("Hermitian-like", "non-Hermitian", and "mixed"), these are characterized by two winding numbers, describing two distinct kinds of half-integer charges carried by the exceptional points. We show that all the above types of topological edge modes can be realized in honeycomb lattices of ring resonators with asymmetric or gain/loss couplings.