Quasiparticles are disturbances in a material that behave like particles, with momentum and position, and so can be regarded as particles. When they occur in condensed matter, they are generally long-lived and non-interacting. Magnon excitations in ordered magnetic lattices are a prototypical example, and understanding the conditions under which they break down will inform understanding of the low energy properties in a variety of systems including superconductivity, frustrated magnets and quantum liquids.

In work published in Physical Review Letters, an international team of researchers used neutron spectroscopy on MERLIN to demonstrate the breakdown of these excitations in RbFe²⁺Fe³⁺F₆, a classical magnet with noncollinear magnetic order in which spin geometry is key to establishing quasiparticle stability. They found that the separation of different Fe²⁺ and Fe³⁺ chains results in an orthogonal spin arrangement on the two magnetic sites, as well as separate spin-wave branches. Their results demonstrate that multi magnon processes can occur in magnets with a noncollinear spin arrangement.

Related publication: “Anharmonic Magnon Excitations in Noncollinear and Charge-Ordered RbFe²⁺Fe³⁺F₆", Phys. Rev. Lett. 121, 087201, DOI: 10.1103/PhysRevLett.121.087201

Authors: M. Songvilay (University of Edinburgh), E. E. Rodriguez (University of Maryland), R. Lindsay (University of Kent), M. A. Green (University of Kent), H. C. Walker (ISIS), J. A. Rodriguez-Rivera (NIST Center for Neutron Research; University of Maryland), and C. Stock (University of Edinburgh).