Topological Magnon-Plasmon Hybrids

TL;DR

The paper analyzes magnon-plasmon coupling in effectively two-dimensional vdW bilayers and shows that magnetic dipole-mediated hybridization endows the bands with Berry curvature and nontrivial Chern numbers, enabling intrinsic thermal Hall and spin-Nernst responses. It presents explicit FM and AFM models: FM hybrids carry Chern numbers C^± = ±1, while AFM hybrids exhibit zero net Berry curvature but nonzero spin Berry curvature, leading to a spin-Nernst effect. Extending to skyrmion crystals, it predicts chiral edge states that bridge magnon and plasmon branches, supported by a minimal two-band model and edge LDOS. The work discusses damping, continuum limitations, and experimental routes, highlighting the potential for nonreciprocal devices and new topological magnonics platforms.

Abstract

We study magnon-plasmon coupling in effectively two-dimensional stacks of van der Waals layers in the context of the band structure topology. Invoking the quasiparticle approximation, we show that the magnetic dipole coupling between the plasmons in a metallic layer and the magnons in a neighboring magnetic layer gives rise to a Berry curvature. As a result, the hybrid quasiparticles acquire an anomalous velocity, leading to intrinsic anomalous thermal Hall and spin-Nernst effects in ferromagnets and antiferromagnets. We propose magnetic layers supporting skyrmion crystals as a platform to realize chiral magnon-plasmon edge states, inviting the notion of topological magnon-plasmonics.

Figures (4)

• Figure 1: Sketch of topological chiral edge magnon-plasmons in a bilayer of a metal (top layer) and a magnetic skyrmion crystal (bottom layer). The magnons of the skyrmion crystal are coherently coupled to the plasmons by magnetic dipole coupling, causing a topologically non-trivial spectral anticrossing in the bulk, which supports the chiral hybrid excitations at the edges of the sample (indicated by the water-wave-like feature and the yellow wave packet with an arrow).
• Figure 2: Magnon-plasmon coupling in a ferromagnet-metal bilayer. (a) Quasiparticle dispersion with color indicating magnon and plasmon character. The Chern number $C_n$ of the upper and lower band is indicated. (b) Thermal Hall conductivity $\kappa_{xy}$ as a function of temperature $T$. Here $\Delta=\unit[0.2]{meV}$ is the ferromagnetic magnon gap.
• Figure 3: Magnon-plasmon coupling in an antiferromagnet-metal bilayer. (a) Quasiparticle dispersion, with color indicating the magnon and plasmon character. (b) Thermal Hall conductivity $\kappa_{xy}$ (blue) and spin Nernst conductivity $\alpha_{xy}$ as a function of temperature $T$. Here $\hbar\omega_0=\unit[1.5]{meV}$ is the exchange-enhanced antiferromagnetic magnon gap.
• Figure 4: Topological magnon-plasmon polariton in a heterostructure of SkXs and graphene. (a) Three-dimensional magnon band structures of the metastable zero-field SkX. The energy and wave vector are rescaled from the dimensionless spin-lattice model as detailed in the SM SM. (b,c) Local density of states (LDOS) of (b) magnons and (c) plasmons at an edge of the semi-infinite lattice. We set the damping rate of plasmons and magnons at $\kappa_\textrm{pl}/\omega_\textrm{pl}=\kappa_\textrm{mag}/\omega_\textrm{mag}=10^{-4}$.