Violation of the magnonic Wiedemann-Franz law in the strong nonlinear regime

TL;DR

This paper investigates the breakdown of the magnonic Wiedemann-Franz law in the strong nonlinear regime, revealing deviations from the universal linear response behavior in topologically trivial insulating magnets.

Contribution

It demonstrates that the magnonic Wiedemann-Franz law does not hold in the nonlinear regime, providing new insights into magnon transport beyond linear response.

Findings

The ratio of thermal to spin transport deviates from the universal law in the nonlinear regime.

The breakdown of the WF law occurs in topologically trivial insulating magnets.

Experimental verification is feasible with current technology.

Abstract

The celebrated Wiedemann-Franz (WF) law which governs the relation between charge and heat transport traces back to the experimental discovery in 1853 by Wiedemann and Franz. Despite the fundamental difference of the quantum-statistical properties between fermions and bosons, the linear-in-$T$ behavior of the WF law at low temperatures has recently been found to be the universal property by the discovery of the WF law for magnon transport. However, the WF law is for the linear response, and whether or not the universal law is valid even in the nonlinear regime of Bose systems remains an open issue. Here we provide a solution to this fundamental challenge. We show that the ratio of the thermal to spin transport coefficient of magnons in topologically trivial insulating magnets exhibits a different behavior from the linear response and the universal law breaks down in the strong nonlinear regime. This finding is within experimental reach with current device and measurement technologies. Our discovery is the key ingredient in magnon-based spintronics, in the evaluation of the figure of merit for thermomagnetic conversion elements of spintronics devices.