Nontrivial topology in one- and two-dimensional asymmetric systems with chiral boundary states

Abstract: Symmetry plays an important role in the topological band theory. In contrary, study on the topological properties of the asymmetric systems is rather limited, especially in higher-dimensional systems. In this work, we explore a new theory to study the topology in various one-dimensional (1D) and two-dimensional (2D) asymmetric systems with chiral boundary states. Starting from the simple SSHm model, we show the chiral topology of its edge states by redefining sublattices. Meanwhile, based on its Rice-Mele-like effective Hamiltonian, a new topological invariant $\bar{Z}$ can be defined and the bulk-edge correspondence is established. With this clear physical picture, our theory can be extended to the more complex asymmetric ladder models, or even the 2D asymmetric systems. In the 2D BBH3 model, new chiral corner states with redefined lattices are found based on our method. These corner states are independent of any spatial symmetry and exhibit the characteristics of topological bound states in the continuum (TBICs). Moreover, the topological invariant can be calculated by introducing $\bar{Z}$ into 2D. At last, we propose an acoustic experiment of the BBH3 model where chiral corner states are numerically observed. Our work exhibits a new approach to study the topological properties of asymmetric systems. By redefining sublattices, we find that the models with entirely different structures might share the same topological origins.