Thermodynamics of free and bound magnons in graphene

TL;DR

This paper introduces a local measurement technique to analyze the thermodynamic properties of magnons in graphene's quantum Hall ferromagnetic phase, revealing magnon binding, chemical potential, and skyrmion formation.

Contribution

It presents a novel experimental approach to probe thermodynamic properties of magnons and skyrmions in graphene, advancing understanding of charge-neutral excitations in symmetry-broken phases.

Findings

Magnons bind to electrons and holes forming skyrmions.

Magnon chemical potential and density can be extracted from measurements.

The $ u=1$ gap reduces by up to 20% due to magnons.

Abstract

Symmetry-broken electronic phases support neutral collective excitations. For example, monolayer graphene in the quantum Hall regime hosts a nearly ideal ferromagnetic phase at filling factor $\nu=1$ that spontaneously breaks spin rotation symmetry. This ferromagnet has been shown to support spin-wave excitations known as magnons which can be generated and detected electrically. While long-distance magnon propagation has been demonstrated via transport measurements, important thermodynamic properties of such magnon populations--including the magnon chemical potential and density--have thus far proven out of reach of experiments. Here, we present local measurements of the electron compressibility under the influence of magnons, which reveal a reduction of the $\nu=1$ gap by up to 20%. Combining these measurements with estimates of the temperature, our analysis reveals that the injected magnons bind to electrons and holes to form skyrmions, and it enables extraction of the free magnon density, magnon chemical potential, and average skyrmion spin. Our methods furnish a novel means of probing the thermodynamic properties of charge-neutral excitations that is applicable to other symmetry-broken electronic phases.