Non-Hermitian Higher-Order Weyl Semimetals

TL;DR

This paper explores non-Hermitian higher-order Weyl semimetals, revealing unique surface states, exceptional Fermi structures, and stability phenomena, advancing understanding of topological phases in non-Hermitian systems.

Contribution

It introduces the concept of intrinsically non-Hermitian phases with unique topological features, including exceptional Weyl nodes and Fermi arcs, and demonstrates their stability and deformability.

Findings

Discovery of non-Hermitian Weyl nodes (NHWNs) and their deformation into line nodes.

Identification of exceptional helicity and stability of Fermi arcs in non-Hermitian systems.

Introduction of the intrinsically non-Hermitian phase called exceptional HOWSM.

Abstract

We study non-Hermitian higher-order Weyl semimetals (NHHOWSMs) possessing real spectra and having inversion $\mathcal{I}$ ($\mathcal{I}$-NHHOWSM) or time-reversal symmetry $\mathcal{T}$ ($\mathcal{T}$-NHHOWSM). When the reality of bulk spectra is lost, the NHHOWSMs exhibit various configurations of surface Fermi Arcs (FAs) and Exceptional Fermi Rings (EFRs), providing a setup to investigate them on an equal footing. The EFRs only appear in the region between 2nd-order WNs. We also discover Weyl nodes originating from non-Hermicity, called non-Hermitian Weyl nodes (NHWNs). Remarkably, we find T-NHHOWSMs which host only 2nd-order NHWNs, having both surface and hinge FAs protected by the quantized biorthogonal Chern number and quadrupole moment, respectively. We call this intrinsically non-Hermitian phase exceptional HOWSM. In contrast to ordinary WNs, the NHWNs can instantly deform to line nodes, forming a monopole comet. The NHWNs also show exceptional tilt-rigidity, which is a strong resistance towards titling due to attachment to exceptional structures. This phenomenon can be a promising experimental knob. Finally, we reveal the exceptional stability of FAs called exceptional helicity. Surface FAs having opposite chirality can live on the same surface without gapping out each other due to the complex nature of the spectrum. Our work motivates an immediate experimental realization of NHHOWSMs.