Hidden chiral symmetry for the kagome lattice and its analogs

Chiral symmetry plays an essential role in condensed matter physics. In tight-binding models, it is often attributed to bipartite lattice structures, and its typical consequence is the ``particle-hole symmetric'' band structures, where positive and negative eigenenergies appear in a pairwise manner. In this paper, we address chiral symmetry for nonbipartite lattice models with flat bands. Our argument relies on what we call the molecular-orbital representation, by which we can guarantee the existence of the flat band. We show that chiral symmetry is preserved for the nonflat bands when the molecular orbitals are normalized and divided into two sets, within which they do not overlap. The chiral operator is constructed by the same molecular orbitals as the Hamiltonian. This results in the characteristic relation between the chiral operator and the Hamiltonian, which we call the Pythagoras relation. We present examples of such models, including the kagome lattice and its one-dimensional analog, and address the characteristics originating from chiral symmetry, such as the emergence of the topological edge modes. We also show an example where the numbers of molecular orbitals in each set are different, resulting in multiple flat bands with different energies.