Symmetry protected topological phases characterized by isolated exceptional points

TL;DR

This paper explores symmetry-protected isolated exceptional points in a 2D non-Hermitian lattice, revealing their topological nature and how they characterize different phases through vortex-like defects and winding numbers.

Contribution

It introduces symmetry-protected isolated exceptional points in a 2D non-Hermitian system and links their topological properties to phase transitions.

Findings

Isolated EPs are protected by symmetry and only move, merge, or split in the Brillouin zone.

Topological defects of a real planar vector field define vortices associated with EPs.

Different topological phases correspond to distinct EP configurations.

Abstract

Exceptional point (EP) associated with eigenstates coalescence in non-Hermitian systems has many exotic features. The EPs are generally sensitive to system parameters, here we report symmetry protected isolated EPs in the Brillouin zone (BZ) of a two-dimensional non-Hermitian bilayer square lattice; protected by symmetry, the isolated EPs only move, merge, and split in the BZ. The average values of Pauli matrices under the eigenstate of system Bloch Hamiltonian define a real planar vector field, the topological defects of which are isolated EPs associated with vortices. The winding number characterizes the vortices and reveals the topological properties of the non- Hermitian system. Different topological phases correspond to different EP configurations, which are unchanged unless topological phase transition occurs accompanying with the EPs merging or splitting.