Magnonic Weyl semimetal and chiral anomaly in pyrochlore ferromagnets

Topological states of matter have been a subject of intensive studies in recent years because of their exotic properties such as the topologically protected edge and surface states. The initial studies were exclusively for electron systems. It is now known that topological states can also exist for other particles. Indeed, topologically protected edge states have already been found for phonons and photons. In spite of active searching for topological states in many fields, the studies in magnetism are relatively rare although topological states are apparently important and useful in magnonics. Here we show that the pyrochlore ferromagnets with the Dzyaloshinskii-Moriya interaction are intrinsic magnonic Weyl semimetals. Similar to the electronic Weyl semimetals, the magnon bands in a magnonic Weyl semimetal are nontrivially crossing in pairs at special points (called Weyl nodes) in momentum space. The equal energy contour around the Weyl node energy is made up of the magnon arcs on sample surfaces due to the topologically protected surface states between each pair of Weyl nodes. Additional Weyl nodes and magnon arcs can be generated in lower energy magnon bands when an anisotropic exchange interaction is introduced. Finally, magnonic chiral anomaly is realized under mutually perpendicular inhomogeneous electric and magnetic fields. The magnonic chiral anomaly results in linear electric spin and heat conductances that are experimentally detectable.