Magnons, magnon bound pairs, and their hybrid spin-multipolar topology

Abstract

The fully polarized ferromagnetic ground state is arguably the least quantum of all magnetic ground states. Nonetheless, the authors show that it can support topological quantum spin excitations with exotic spin-multipolar properties if spin conservation is broken. These excitations are superpositions of dipolar magnons and quadrupolar two-magnon bound states. As a result of their topology, such quantum ferromagnets exhibit both chiral edge excitations, akin to edge states in Chern insulators but at finite energy, and an intrinsic thermal Hall effect.