Topological magnon amplification

Daniel Malz,Andreas Nunnenkamp,Johannes Knolle

doi:10.1038/s41467-019-11914-2

Daniel Malz, Andreas Nunnenkamp + Show 1 more

Open Access

https://doi.org/10.1038/s41467-019-11914-2

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Abstract

Topology is quickly becoming a cornerstone in our understanding of electronic systems. Like their electronic counterparts, bosonic systems can exhibit a topological band structure, but in real materials it is difficult to ascertain their topological nature, as their ground state is a simple condensate or the vacuum, and one has to rely instead on excited states, for example a characteristic thermal Hall response. Here we propose driving a topological magnon insulator with an electromagnetic field and show that this causes edge mode instabilities and a large non-equilibrium steady-state magnon edge current. Building on this, we discuss several experimental signatures that unambiguously establish the presence of topological magnon edge modes. Furthermore, our amplification mechanism can be employed to power a topological travelling-wave magnon amplifier and topological magnon laser, with applications in magnon spintronics. This work thus represents a step toward functional topological magnetic materials.

Highlights

Topology is quickly becoming a cornerstone in our understanding of electronic systems
While fermionic topological insulators have a number of clear experimental signatures accessible through linear transport measurements[1,2], noninteracting bosonic systems with topological band structure have a simple condensate or the vacuum as their ground state[3], making it more difficult to ascertain their topological nature
It has been predicted that topological magnon insulators (TMI) are realized, e.g., in kagome planes of certain pyrochlore magnetic insulators as a result of DzyaloshinskiiMoriya (DM) interaction[4,10,11]

Summary

Introduction

Topology is quickly becoming a cornerstone in our understanding of electronic systems. 1234567890():,; While fermionic topological insulators have a number of clear experimental signatures accessible through linear transport measurements[1,2], noninteracting bosonic systems with topological band structure have a simple condensate or the vacuum as their ground state[3], making it more difficult to ascertain their topological nature. We show that selective amplification of edge modes can be achieved while preserving the stability of the bulk modes and the magnetic order Another key experimental signature we predict is that applying a driving field gradient gives rise to a temperature gradient along the transverse direction, establishing what one might call a driven Hall effect (DHE). Our work on driving topological edge modes in magnetic materials complements previous investigations in ultracold gases[16,17], photonic crystals[9], and most recently arrays of semiconductor microresonators[14,15] and graphene[18]

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