Dynamics of nonequilibrium magnons in gapped quantum magnets

Nonequilibrium dynamics in spin systems is a topic currently under intense investigation as it provides fundamental insights into thermalization, universality, and exotic transport phenomena. While most of the studies have been focused on ideal closed quantum many-body systems such as ultracold atomic quantum gases and one-dimensional spin chains, driven-dissipative Bose gases in steady states away from equilibrium in classical systems also leads to intriguing physics such as classical Bose-Einstein condensation of magnons in solids and is relevant for Floquet engineering in quantum magnets via tailored dissipation. In this work, we theoretically investigate out-of-equilibrium dynamics of magnons in a gapped Heisenberg quantum antiferromagnet based on Boltzmann transport theory. We show that, by treating scattering terms beyond the relaxation time approximation in the Boltzmann transport equation, energy and particle number conservation mandate that nonequilibrium magnons cannot relax to equilibrium, but decay to other nonequilibrium stationary states, partially containing information about the initial states. The only intrinsic decay channel for these stationary states back to equilibrium is through the interaction with phonons, which is a much slower process in a gapped spin system at low temperatures. We then propose that nonequilibrium steady states of magnons can be maintained and tailered using periodic driving at frequencies faster than relaxation due to phonon interactions. These findings reveal a class of classical material systems that are suitable platforms to study nonequilibrium statistical physics and macroscopic phonemona such as classical Bose-Einstein condensation of quasiparticles and magnon supercurrents that are relevant for spintronic applications.