Weyl magnons in breathing pyrochlore antiferromagnets.

Fei-Ye Li,Yao-Dong Li,Gang Chen,Yue Yu,Leon Balents,Yong Baek Kim

doi:10.1038/ncomms12691

Abstract

Frustrated quantum magnets not only provide exotic ground states and unusual magnetic structures, but also support unconventional excitations in many cases. Using a physically relevant spin model for a breathing pyrochlore lattice, we discuss the presence of topological linear band crossings of magnons in antiferromagnets. These are the analogues of Weyl fermions in electronic systems, which we dub Weyl magnons. The bulk Weyl magnon implies the presence of chiral magnon surface states forming arcs at finite energy. We argue that such antiferromagnets present a unique example, in which Weyl points can be manipulated in situ in the laboratory by applied fields. We discuss their appearance specifically in the breathing pyrochlore lattice, and give some general discussion of conditions to find Weyl magnons, and how they may be probed experimentally. Our work may inspire a re-examination of the magnetic excitations in many magnetically ordered systems.

Highlights

Frustrated quantum magnets provide exotic ground states and unusual magnetic structures, and support unconventional excitations in many cases
In contrast to the other three categories of systems, the band structure of magnons in antiferromagnets is highly tunable in situ by application of readily available magnetic fields, which is a consequence of the spontaneous symmetry breaking of the antiferromagnet ground state and the relatively low-energy scale for magnetic interactions in most solids
We have explicitly shown the presence of Weyl nodes in a simple and physically relevant model for the breathing pyrochlore lattice antiferromagnet

Summary

Introduction

Frustrated quantum magnets provide exotic ground states and unusual magnetic structures, and support unconventional excitations in many cases. We consider a concrete magnetic system, namely, the Cr-based breathing pyrochlore, and explicitly demonstrate that it supports Weyl magnon excitations with a linear band touching in the spin-wave spectrum of the magnetic ordered phase. We consider below a spin model for the breathing pyrochlore, which generalizes and includes the uniform limit, and displays Weyl points even in the uniform case.

Results

Conclusion