Topological magnons in a kagome-lattice spin system with XXZ and Dzyaloshinskii-Moriya interactions

We study the phases of a spin system on the kagome lattice with nearest-neighbor $XXZ$ interactions with anisotropy ratio $\mathrm{\ensuremath{\Delta}}$ and Dzyaloshinskii-Moriya interactions with strength $D$. In the classical limit where the spin $S$ at each site is very large, we find a rich phase diagram of the ground state as a function of $\mathrm{\ensuremath{\Delta}}$ and $D$. There are five distinct phases which correspond to different ground-state spin configurations in the classical limit. We use spin-wave theory to find the bulk energy bands of the magnons in some of these phases. We also study a strip of the system which has infinite length and finite width; we find states which are localized near one of the edges of the strip with energies which lie in the gaps of the bulk states. In the ferromagnetic phase in which all the spins point along the $+\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ or $\ensuremath{-}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ direction, the bulk bands are separated from each other by finite energy gaps. This makes it possible to calculate the Berry curvature at all momenta, and hence the Chern numbers for every band; the number of edge states is related to the Chern numbers. Interestingly, we find that there are four different regions in this phase where the Chern numbers are different. Hence there are four distinct topological phases even though the ground-state spin configuration is identical in all these phases. We calculate the thermal Hall conductivity of the magnons as a function of the temperature in the above ferromagnetic phase; we find that this can distinguish between the various topological phases. These results are valid for all values of $S$. In the other phases, there are no gaps between the different bands; hence the edge states are not topologically protected.