Sub-symmetry protected topological states

TL;DR

This paper reveals that boundary states in symmetry-protected topological phases can be protected by a weaker sub-symmetry, challenging the traditional understanding that full symmetry is necessary for protection.

Contribution

It introduces the concept of sub-symmetry protection in SPT phases, supported by theoretical models and experimental photonic lattice demonstrations.

Findings

Boundary states are protected by sub-symmetry even when the topological phase is destroyed.

Experimental demonstration of sub-symmetry protection in photonic lattices.

Long-range hopping symmetry clarifies the topological nature of corner states in BKLs.

Abstract

A hallmark of symmetry-protected topological phases (SPTs) are topologically protected boundary states, which are immune to perturbations that respect the protecting symmetry. It is commonly believed that any perturbation that destroys an SPT phase simultaneously destroys the boundary states. However, by introducing and exploring a weaker sub-symmetry (SubSy) requirement on perturbations, we find that the nature of boundary state protection is in fact more complex. We demonstrate that the boundary states are protected by only the SubSy using prototypical Su-Schrieffer-Heeger (SSH) and breathing Kagome lattice (BKL) models, even though the overall topological invariant and the SPT phase are destroyed by SubSy preserving perturbations. By employing judiciously controlled symmetry breaking in photonic lattices, we experimentally demonstrate such SubSy protection of topological states. Furthermore, we introduce a long-range hopping symmetry in BKLs, which resolves a debate on the topological nature of their corner states. Our results apply to other systems beyond photonics, heralding the possibility of exploring the intriguing properties of SPT phases in the absence of full symmetry in different physical contexts.