Hourglass Fermion Surface States in Stacked Topological Insulators with Nonsymmorphic Symmetry

TL;DR

This paper introduces a simple stacked model that demonstrates the emergence of hourglass fermion surface states protected by nonsymmorphic symmetry, confirming their universality in such topological systems.

Contribution

The paper presents a minimal stacking model that reproduces hourglass fermion surface states and analyzes their robustness under symmetry-preserving perturbations.

Findings

The model successfully reproduces hourglass fermion surface states.

The derived Dirac theory matches the numerical results.

Hourglass states are shown to be robust under certain perturbations.

Abstract

Recently a nonsymmorphic topological insulator was predicted, where the characteristic feature is the emergence of a "hourglass fermion" surface state protected by the nonsymmorphic symmetry. Such a state has already been observed experimentally. We propose a simple model possessing the hourglass fermion surface state. The model is constructing by stacking the quantum-spin-Hall insulators with the interlayer coupling introduced so as to preserve the nonsymmorphic symmetry and the time reversal symmetry. The Dirac theory is also derived, whose analytical results reproduce the hourglass fermion surface state remarkably well. Furthermore, we discuss how the hourglass state is destroyed by introducing perturbations based on the symmetry analysis. Our results show that the hourglass fermion surface state is universal in the helical edge system with the nonsymmorphic symmetry.