Multiple boundary states in bilayer and decorated Su-Schrieffer-Heeger-like models

Abstract: Topological boundary states have attracted widespread fascination due to their series of intriguing properties. In this paper, we investigate the multiple boundary states within the two kinds of extended Su-Schrieffer-Heeger (SSH) models. The coexistence of boundary states that exist both in the bulk and band gaps is realized based on the bilayer SSH-like model, which consists of two conventional square-root SSH models that are directly coupled. We further show the square-root topology within the decorated SSH-like model, which supports multiple boundary states that could be embedded into the bulk continuum by tuning the hopping parameters. In addition, the connection between the decorated SSH-like model and its effectively decomposed counterparts is revealed. Our results broaden insight into the multiple boundary states and open up an exciting avenue for the future exploration of square-root topology.