Topological magnon bands in the flux state of Shastry-Sutherland lattice model

We investigate low-energy magnon excitations above the noncollinear flux state and noncoplanar canted flux state in a Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interaction (DMI) on a Sashtry-Sutherland lattice. While previous studies have shown the presence of topological magnetic excitation in the dimer and ferromagnetic phases on the Shastry-Sutherland lattice, our results establish the nontrivial topology of magnons in the antiferromagnetic flux and canted flux states. Our results uncover the existence of a multitude of band topology transitions in the magnon sector---evidenced by the changing Chern numbers of the single magnon bands---as the Hamiltonian parameters are varied, even when the ground state remains unchanged. The thermal Hall conductivity is calculated and its derivative is shown to exhibit a logarithmic divergence at the boundaries of band topology transitions, independent of the type of band touching involved. A simple analytical form of the temperature dependence of a logarithmic peak is derived for a generic spin model. This may provide a useful means to identify the energy at which the band topological transition of magnon bands occurs. We also propose the way to realize the studied model in a practical material.