Topological Majorana flat bands in the Kitaev model on a Bishamon-kikko lattice

Abstract: We unveil an interesting example of topological flat bands of Majorana fermions in quantum spin liquids. We study the Kitaev model on a periodically depleted honeycomb lattice, under a magnetic field within the perturbation theory. The model can be straightforwardly extended while maintaining the exact solvability, and its ground state is a quantum spin liquid as on the honeycomb lattice. As fractionalized excitations, there are unpaired localized Majorana fermions in addition to the itinerant Majorana fermions and $\mathbb{Z}_2$ fluxes. We show that in the absence of the magnetic field the Majorana fermions have completely flat bands at zero energy, and by applying the magnetic field, they turn into topological flat bands with nonzero Chern number. By varying the anisotropy of the interactions and the magnitude of the magnetic field, we clarify that the system exhibits a variety of topological phases that do not appear in the original model. We emphasize that the topological flat bands that give this rich topology come from the hybridization of the Majorana flat bands and unpaired Majorana fermions, which is unique to the flat bands of fractionalized excitations in quantum spin liquids. Our findings would stimulate the exploration of a new type of Kitaev materials exhibiting rich topology from topological Majorana flat bands.