Chemical potential of magnon polarons

TL;DR

This paper rigorously defines the chemical potential of magnon-polaron quasiparticles in ferromagnets and antiferromagnets, revealing their chiral coupling and transport properties through a microscopic thermodynamic and Boltzmann framework.

Contribution

It introduces a microscopic, symmetry-based definition of magnon-polaron chemical potential and develops a Boltzmann transport theory applicable to high-symmetry magnetic materials.

Findings

Magnon-polarons in FMs and AFMs are governed by a single chemical potential linked to angular momentum.

Chiral selectivity manifests in the coupling between magnons and phonons, depending on polarization.

Derived transport equations interpolate between coupled and decoupled regimes, aligning with previous spin Seebeck models.

Abstract

Using a rotationally invariant formulation of spin-lattice coupling, we derive a rigorous definition of the chemical potential for magnon-polaron quasiparticles in collinear ferromagnets (FMs) and antiferromagnets (AFMs), valid when magnetoelastic scattering equilibrates magnons and acoustic phonons on timescales much shorter than those associated with quasiparticle-nonconserving relaxation processes. While our microscopic framework applies to generic magnon-phonon interactions, here we focus on high-symmetry crystals where the two transverse acoustic modes form a degenerate doublet. This doublet can combine into circularly polarized phonons, making the chiral selectivity of the coupling manifest: the FM magnon mode hybridizes only with the co-rotating phonon, whereas in collinear AFMs each magnon branch of opposite handedness couples to the phonon of the same chirality. We show that, in both FM and AFM systems, the nonequilibrium magnon-polaron gas is governed by a single chemical potential conjugate to the conserved axial angular momentum. In FMs, the two hybrid branches in the co-rotating sector share this chemical potential, weighted by their magnonic fractions; in AFMs, the four magnon-polaron branches split into two chiral sectors that carry opposite angular momenta and couple with opposite sign to the same chemical potential. Building on this microscopic thermodynamic framework, we formulate a Boltzmann transport theory for magnon-polarons and derive compact expressions for angular-momentum and heat currents that interpolate continuously to the decoupled regime and reproduce the phenomenological magnon-polaron transport framework underlying previous spin Seebeck analyses.