Magnon waveguide with nanoscale confinement constructed from topological magnon insulators

Topological magnon insulators host spatially confined edge magnons brought about by the Dzyaloshinskii-Moriya interaction. Bringing two topological magnon insulators into contact results in topologically protected unidirectional interface magnons. These interface modes decay rapidly toward the bulk regions of the sample. As a result, heat and spin currents associated with these magnons are as well unidirectional and strongly confined to a few-nanometer-wide strip along the interface. On top of this, these interface currents follow any geometry owing to the topological nature of the magnons. In this theoretical study, we propose and analyze two recipes for composing magnon waveguides with nanoscale confinement, one from topologically different phases, another from identical phases. We further identify material classes to construct these magnon waveguides and propose an experiment to verify their topological nature.

Figure 1

Sketch of a magnon waveguide. Two kagome ferromagnets, phases $\mathsf{A}$ (green) and $\mathsf{B}$ (red), share a common interface (dashed line) along the $y$ direction. As a result, a topological interface magnon moves unidirectionally along this interface (blue arrow). It decays rapidly toward the two bulk regions of the sample, i.e., in both positive and negative $x$ direction. Consequently, the heat current $j_q^y\left(x\right)$ associated with this interface magnon is strongly confined to a few-nanometer-wide strip along the interface.

Figure 2

Heat current profile in a kagome slab with two topological phases. Phase $\mathsf{A}$ has a Chern triple of $(-1,0,1)$ and exchange parameters of $J_{\mathrm{N}}={}^{25}/{}_{6}D=7.5\text{meV}$ as well as $J_{\mathrm{NN}}=0\text{meV}$; it occupies the left 48 atomic layers. Phase $\mathsf{B}$ has $(-1,2,-1), J_{\mathrm{N}}={}^7/{}_3{J}_{\mathrm{NN}}={}^7/{}_{6}D=\text{3.5}\text{meV}$ and occupies the right 48 layers. (a) Heat current vs atomic layer. The black arrows represent the propagation directions of the edge and interface magnons. (b)–(d) Spectral density of the left edge, interface, and right edge, respectively. Temperature $T=40\text{K}$.

Figure 3

Energy-resolved contribution to the local heat current density. Upper panels show the current profile for the slab in Fig. 2. The layers marked in red around the interface [(a), atomic layers 45–50] and within the $\mathsf{B}$ region [(b), atomic layers 65–70] were considered for the energy-resolved current. Lower panels show the spectral density and the energy-resolved current density $j_q^y(\varepsilon ,x)$ of said layers. Temperature $T=40\text{K}$, energy interval $0.52\text{meV}$. The “wriggling” of $j_q^y(\varepsilon ,x)$ about 0 is attributed to numerics.