Breaking down the magnonic Wiedemann-Franz law in the hydrodynamic regime

TL;DR

This paper derives hydrodynamic equations for magnons in ferromagnetic insulators and predicts a breakdown of the Wiedemann-Franz law, providing a key signature for observing magnon hydrodynamics experimentally.

Contribution

It introduces a theoretical framework for magnon hydrodynamics and predicts the breakdown of the Wiedemann-Franz law as a hallmark of this regime.

Findings

Hydrodynamic equations for magnon fluids derived.

Magnonic Wiedemann-Franz law significantly breaks down.

Provides a theoretical basis for experimental detection of magnon hydrodynamics.

Abstract

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 towards the direct observation of magnon fluids.