Magnon Hall effect in AB-stacked bilayer honeycomb quantum magnets

TL;DR

This paper explores the magnon Hall effect in bilayer honeycomb quantum magnets, revealing topologically nontrivial magnon bands and edge states influenced by interlayer coupling and Dzyaloshinsky-Moriya interactions.

Contribution

It introduces the study of thermal Hall transport in bilayer magnetic systems with both ferromagnetic and antiferromagnetic couplings, highlighting topological magnon phenomena.

Findings

Magnon Hall and spin Nernst effects observed in bilayer systems.

Sign change in conductivities with magnetic field reversal.

Edge states propagate differently depending on interlayer coupling.

Abstract

Motivated by the fact that many bilayer quantum magnets occur in nature, we generalize the study of thermal Hall transports of spin excitations to bilayer magnetic systems. It is shown that bilayer magnetic systems can be coupled either ferromagnetically or antiferromagnetically. We study both scenarios on the honeycomb lattice and show that the system realizes topologically nontrivial magnon bands induced by alternating next-nearest-neighbour Dzyaloshinsky-Moriya interaction (DMI). As a result, the bilayer system realizes both magnon Hall effect and magnon spin Nernst effect. We show that antiferromagnetically coupled layers differ from ferromagnetically coupled layers by a sign change in the conductivities as the magnetic field is reversed. Furthermore, Chern number protected magnon edge states are observed and propagate in the same direction on the top and bottom layers in ferromagnetically coupled layers, whereas the magnon edge states propagate in opposite directions for antiferromagnetically coupled layers.