Non-Hermitian nodal-line semimetals with an anomalous bulk-boundary correspondence

TL;DR

This paper explores three-dimensional non-Hermitian nodal-line semimetals with gain-and-loss perturbations, revealing the splitting into exceptional rings, novel topological invariants, and a breakdown of conventional bulk-boundary correspondence, including the emergence of the non-Hermitian skin effect.

Contribution

It introduces the topological characterization of non-Hermitian nodal-line semimetals using vorticity and winding numbers, and uncovers the breakdown of bulk-surface correspondence due to non-Hermitian effects.

Findings

Nodal ring splits into two exceptional rings under perturbation.

Topological invariants take half-integer or integer values depending on ERs.

Bulk-boundary correspondence breaks down, leading to non-Hermitian skin effect.

Abstract

Recently, topological quantum states of non-Hermitian systems, exhibiting rich new exotic states, have attracted great attention in condensed-matter physics. As for the demonstration, most of non-Hermitian topological phenomena previously focused on are in one- and two-dimensional systems. Here, we investigate three-dimensional non-Hermitian nodal-line semimetals in the presence of a particle gain-and-loss perturbation. It is found that this perturbation will split the original nodal ring into two exceptional rings (ERs). The topological nature of the bulk electronic structure is characterized by two different topological invariants, namely, the vorticity and the winding number defined for a one-dimensional loop in momentum space, both of which are shown to take half-integer (integer) values when an odd (even) number of ERs thread through the loop. The conventional bulk-surface correspondence in non-Hermitian nodal-line semimetals is found to break down, where the surface zero-energy flat bands are no longer bounded by projections of bulk ERs. Alternatively, a macroscopic fraction of the bulk eigenstates can be localized near the surface, thus leading to the so-called non-Hermitian skin effect.