Origin of Robust $\mathbb{Z}_2$ Topological Phases in Stacked Hermitian Systems: Non-Hermitian Level Repulsion

Zhiyu Jiang, Masatoshi Sato, Hideaki Obuse

Published 2024-07-30Version 1

Quantum spin Hall insulators, which possess a non-trivial $\mathbb{Z}_2$ topological phase, have attracted great attention for two decades. It is generally believed that when an even number of layers of the quantum spin Hall insulators are stacked, the $\mathbb{Z}_2$ topological phase becomes unstable due to $\mathbb{Z}_2$ nature. While several researchers report counterexamples of the instability, there is no systematic understanding. In this work, we show that the $\mathbb{Z}_2$ topological phase is robust against the stacking if it has an additional chiral symmetry, in terms of the level repulsion in the corresponding non-Hermitian system. We demonstrate this by mapping a one-dimensional class DIII superconductor with $\mathbb{Z}_2$ topology to the corresponding non-Hermitian system in AII$^\dagger$ with $\mathbb{Z}_2$ point-gap topology.