Non-Bloch Dirac Points and Phase Diagram in the Stacked Non-Hermitian SSH Model

TL;DR

This paper uncovers non-Bloch Dirac points in a non-Hermitian stacked SSH model, revealing a boundary-dependent bulk-boundary correspondence and mapping real spectra to Hermitian semimetals.

Contribution

It introduces the concept of non-Bloch Dirac points with real spectra in non-Hermitian systems and demonstrates their topological characterization and boundary sensitivity.

Findings

Discovery of non-Bloch Dirac points with real spectra

Boundary-dependent locations of Dirac points

Mapping of non-Hermitian spectra to Hermitian semimetals

Abstract

Topological semimetals exhibit protected band crossings in momentum space, accompanied by corresponding surface states. Non-Hermitian Hamiltonians introduce geometry-sensitive features that dissolve this bulk-boundary correspondence principle. In this paper, we exemplify this phenomenon by investigating a non-Hermitian 2D stacked SSH chain model with non-reciprocal hopping and on-site gain/loss. We derive an analytical phase diagram in terms of the complex energy gaps in the open-boundary spectrum. The phase diagram reveals the existence of non-Bloch Dirac points, which feature a real spectrum and only appear under open boundary conditions but disappear in Bloch bands under periodic boundary conditions. Due to the reality of the spectrum in the vicinity of non-Bloch Dirac points, we can locally map it to Hermitian semimetals within the Altland-Zirnabuer symmetry classes. Based on this mapping, we demonstrate that non-Bloch Dirac points are characterized by an integer topological charge. Unlike the band crossings in Hermitian semimetals, the locations of the non-Bloch Dirac points under different boundary geometries do not match each other, indicating a geometry-dependent bulk-boundary correspondence in non-Hermitian semimetals. Our findings provide new pathways into establishing unconventional bulk-boundary correspondence for non-Bloch Dirac metals in non-Hermitian systems.