Corner states are not topologically protected

TL;DR

This paper challenges the notion that corner states in higher-order topological insulators are topologically protected, demonstrating their dependence on boundary choices and unit cell definitions rather than intrinsic topological invariants.

Contribution

It shows that Wannier centers are not true topological invariants and that corner states can exist in both trivial and topological phases depending on boundary conditions.

Findings

Corner states depend on boundary choices, not topological invariants.

Wannier centers vary with unit cell choices, undermining their role as topological markers.

Corner states can be present in trivial phases with appropriate boundaries.

Abstract

Recently there is a surge of interests in the so-called topologically protected corner states in 2D and 3D systems. Such systems are considered as high order topological insulators. Wannier centers are used as topological invariants to characterize bulk systems. The existence of corner states is considered as a reflection of the topological non-triviality of bulk energy bands. We demonstrate that the Wannier centers are not topological invariants by showing they depend on the choices of unit cells. The same bulk system can be considered as both topological and trivial with two equally possible types of unit cells. We show the existence of corner states only reflects different choices of boundaries of the same bulk system. The corner states disappears in the so-called topological state if we choose a different boundary with the same symmetry as the original one. On the other hand, equally robust corner states can be realized in the so-called trivial state.