Magnonic Floquet Quantum Spin Hall Insulator in Bilayer Collinear Antiferromagnets

TL;DR

This paper predicts a new Floquet topological magnon phase in irradiated bilayer antiferromagnets, characterized by a $ ext{Z}_2$ invariant, leading to spin-filtered edge states and a magnon spin Nernst effect, with potential experimental implications.

Contribution

It introduces a magnonic Floquet quantum spin Hall insulator in bilayer antiferromagnets, revealing a $ ext{Z}_2$ topological invariant and associated edge states under circularly polarized light.

Findings

Identification of a $ ext{Z}_2$ Floquet topological invariant in bilayer antiferromagnets.

Prediction of spin-filtered magnon edge states in the Floquet regime.

Observation of a non-zero Floquet magnon spin Nernst effect.

Abstract

We study irradiated two-dimensional insulating bilayer honeycomb ferromagnets and antiferromagnets coupled antiferromagnetically with a zero net magnetization. The former is realized in the recently synthesized bilayer honeycomb chromium triiodide CrI$_{\bf 3}$. In both systems, we show that circularly-polarized electric field breaks time-reversal symmetry and induces a dynamical Dzyaloshinskii-Moriya interaction in each honeycomb layer. However, the resulting bilayer antiferromagnetic system still preserves a combination of time-reversal and space-inversion ($\mathcal{PT}$) symmetry. We show that the magnon topology of the bilayer antiferromagnetic system is characterized by a $\pmb{\mathbb{Z}_2}$ Floquet topological invariant. Therefore, the system realizes a magnonic Floquet quantum spin Hall insulator with spin filtered magnon edge states. This leads to a non-vanishing Floquet magnon spin Nernst effect, whereas the Floquet magnon thermal Hall effect vanishes due to $\mathcal{PT}$ symmetry. We study the rich $\pmb{\mathbb{Z}_2}$ Floquet topological magnon phase diagram of the system as a function of the light amplitudes and polarizations. We further discuss the great impact of the results on future experimental realizations.