Higher-order topology in honeycomb lattice with Y-Kekulé distortions

Abstract: We investigate higher-order topological states in honeycomb lattice with Y-Kekul\'e distortions that preserve $C_{6v}$ crystalline symmetry. The gapped states in expanded and shrunken distortions are adiabatically connected to isolated hexamers and Y-shaped tetramer states, respectively, where the former possesses nontrivial higher-order topology characterized by a $\mathbb{Z}_6$ invariant. Topological corner states exist in a flake structure with expanded distortion where the hexamers are broken at the corners. Our work reveals that honeycomb lattice with Y-Kekul\'e distortions serves as a promising platform to study higher-order topological states.