Spatially Resolving the Pre-Thermal Anatomy of a Driven Bosonic Fluid

Abstract

Understanding how coherently driven quantum many-body systems redistribute energy prior to thermal equilibrium remains a central challenge in many-body physics. Here, we utilize nitrogen-vacancy (NV) magnetometry to perform micron-scale spatial imaging of room-temperature magnon dynamics in a yttrium iron garnet (YIG) thin film. We resolve a hierarchy of discrete parametric scattering events that serve as deterministic stepping stones toward thermalization. By applying a two-tone wave-mixing protocol, we first isolate the elementary four-magnon interaction and extract its coupling strength via the spatial growth of the scattering product. We then drive the system with an intense single-frequency excitation near ferromagnetic resonance, revealing that magnon-magnon interactions trigger a spontaneous, multi-generation scattering cascade. We demonstrate that in each generation, the dominant scattering channels correspond to one of the out-scattered magnons being in the slow magnon regime, reminiscent of the enhancement of optical nonlinearities in slow light systems. We capture this dynamics quantitatively using a near field magnonics framework and extract the cascade order and nonlinear coefficients directly from power-dependent frequency shifts. By revealing the multi-stage dynamical process through which monochromatic injected magnons evolve toward equilibrium, our work establishes spatially resolved magnonics as a powerful platform for visualizing non-equilibrium many-body kinetics.