Topological magnons and the non-Hermitian topology of spontaneous magnon decay

 

 

Recently, magnon excitations have been explored as a potential route to realizing analogues of the topological band structures that have become ubiquitous in electronic systems.  However, magnons are not necessarily stable quasi-particles, with spontaneous magnon decay a generic feature of anisotropic magnets, or magnets with non-colinear order. In this talk I will discuss two aspects of this physics: First, I will discuss a case where sharp topological magnon bands exist in spite of the presence allowed decays, as realized in a Kitaev model in a large magnetic field. Second, I will discuss a scenario where decay in fact drives the appearance new kinds of intrinsically non-Hermitian topological physics. This can lead to new kinds of spectral features, such as exceptional points or lines and bulk "arcs" connecting them. I will illustrate this physics through a concrete example: a honeycomb ferromagnet with Dzyaloshinskii-Moriya exchange, comparing interacting spin-wave calculations with an effective non-Hermitian model. Finally, I discuss more broadly how the zoo of known topological protected magnon band structures may serve as a nearly ideal platform for realizing non-Hermitian physics in solid-state systems.