Protected Gapless Edge States In Trivial Topology

TL;DR

This paper demonstrates that protected gapless edge states can exist in trivial topological insulators without bulk topological features, protected instead by mirror antisymmetry and characterized by entanglement spectrum spectral flow.

Contribution

It introduces a new class of trivial topological insulators with protected edge states arising from mirror antisymmetry, challenging the traditional bulk-boundary correspondence paradigm.

Findings

Protected edge states can exist without Wannier obstruction or Wilson-loop winding.

Edge state protection stems from mirror antisymmetry, not crystalline symmetry.

Spectral flow in entanglement spectrum characterizes these edge states.

Abstract

Bulk-boundary correspondence serves as an important feature of the strong topological insulators, including Chern insulators and $Z_2$ topological insulators. Under nontrivial band topology, the protected gapless edge states correspond to the Wannier obstruction or Wilson-loop winding in the bulk. Recent studies show that the bulk topological features may not imply the existence of protected gapless edge states. Here we address the opposite question: Does the existence of protected gapless edge states necessarily imply the Wannier obstruction or Wilson-loop winding? We provide an example where the protected gapless edge states arise without the aforementioned bulk topological features. This trivialized topological insulator belongs to a new class of systems with non-delta-like Wannier functions. Interestingly, the gapless edge states are not protected by the crystalline symmetry; instead the protection originates from the mirror antisymmetry, a combination of chiral and mirror symmetries. Although the protected gapless edge states cannot be captured by the bulk topological features, they can be characterized by the spectral flow in the entanglement spectrum.