Dimensionless Hierarchical Topological Phononic States

Abstract: Topological insulators exhibit unique boundary states that are protected by the topology of the bulk bands, a phenomenon that has now been extended to classical systems such as phononics and mechanics. Typically, nontrivial topology in an $n$-dimensional bulk leads to the emergence of $(n-1)$-dimensional topologically protected boundary states. However, these states can often be gapped out by breaking the symmetry that protects them, resulting in the possible creation of new in-gap higher-order topological modes. A notable example of this is the higher-order topological insulator (HOTI), where gapping out surface states leads to the formation of lower-dimensional topological modes, such as hinge or corner states. This process reduces the spatial dimensionality of the protected modes from $(n-1)$ to $(n-2)$ or even lower. In this work, we propose an alternative method to achieve higher-order topological modes using a one-dimensional Su-Schrieffer-Heeger model. Instead of relying on dimensional reduction, we manipulate the positions of domain walls to gap out the originally topologically protected domain-wall states, thereby inducing new higher-order topological states. These new higher-order topological states can be characterized using a generalized winding number calculation. This approach allows for the realization of multiple (and even infinite) topological orders within simple 1D lattices while maintaining the principle of bulk-boundary correspondence. Our study reveals a new mechanism that enriches topological hierarchies beyond conventional classifications. Such a mechanism could also be extended to higher dimensions, potentially creating intricate networks of topological states and advancing our control over wave phenomena.