Zero-frequency chiral magnonic edge states protected by non-equilibrium topology

TL;DR

This paper demonstrates that non-equilibrium conditions can bring topologically-protected magnonic edge states to zero frequency, enabling their direct experimental detection and potential use in magnonic devices.

Contribution

It introduces a method to lower magnonic edge state energies to zero frequency using non-equilibrium magnetization, facilitating their experimental observation.

Findings

Zero-frequency chiral edge states can be stabilized in non-equilibrium.

Edge states can be excited with microwave fields in simulations.

Direct detection of edge states is feasible via spin wave spectroscopy.

Abstract

Topological bosonic excitations must, in contrast to their fermionic counterparts, appear at finite energies. This is a key challenge for magnons, as it prevents straightforward excitation and detection of topologically-protected magnonic edge states and their use in magnonic devices. In this work, we show that in a non-equilibrium state, in which the magnetization is pointing against the external magnetic field, the topologically-protected chiral edge states in a magnon Chern insulator can be lowered to zero frequency, making them directly accessible by existing experimental techniques. We discuss the spin-orbit torque required to stabilize this non-equilibrium state, and show explicitly using numerical Landau-Lifshitz-Gilbert simulations that the edge states can be excited with a microwave field. Finally, we consider a propagating spin wave spectroscopy experiment, and demonstrate that the edge states can be directly detected.