Breaking Down the Magnonic Wiedemann-Franz Law in the Hydrodynamic Regime.

Recent experiments have shown an indication of a hydrodynamic magnon behavior in ultrapure ferromagnetic insulators; however, its direct observation is still lacking. Here, we derive a set of coupled hydrodynamic equations and study the thermal and spin conductivities for such a magnon fluid. We reveal the drastic breakdown of the magnonic Wiedemann-Franz law as a hallmark of the hydrodynamics regime, which will become key evidence for the experimental realization of an emergent hydrodynamic magnon behavior. Therefore, our results pave the way toward the direct observation of magnon fluids.