Multitude of exceptional points in van der Waals magnets

TL;DR

This paper explores the emergence and properties of exceptional points in the driven magnetization dynamics of van der Waals ferromagnetic bilayers, revealing extended regions of EPs and their topological edge states.

Contribution

It demonstrates the presence of exceptional points over large Brillouin zone regions and introduces a wavevector-dependent pseudo-Hermiticity in the magnon Hamiltonian.

Findings

Exceptional points appear over extended Brillouin zone regions.

The magnon Hamiltonian exhibits wavevector-dependent pseudo-Hermiticity.

Topological edge states can have complex or real spectra, with experimental implications.

Abstract

Several works have recently addressed the emergence of exceptional points (EPs), i.e., spectral singularities of non-Hermitian Hamiltonians, in the long-wavelength dynamics of coupled magnetic systems. Here, by focusing on the driven magnetization dynamics of a van der Waals ferromagnetic bilayer, we show that exceptional points can appear over extended portions of the first Brillouin zone as well. Furthermore, we demonstrate that the effective non-Hermitian magnon Hamiltonian, whose eigenvalues are purely real or come in complex-conjugate pairs, respects an unusual wavevector-dependent pseudo-Hermiticity. Finally, for both armchair and zigzag nanoribbon geometries, we discuss both the complex and purely real spectra of the topological edge states and their experimental implications.