Topological indicators for systems with open boundaries: Application to the Kitaev wire

TL;DR

This paper extends topological indicators for open-boundary systems, applying them to the Kitaev wire to better understand edge states and phase transitions, introducing new measures compatible with open boundaries.

Contribution

It introduces an open-boundary analog of the Zak phase and classifies edge states by symmetry, enhancing topological characterization of open systems.

Findings

Both D and γ_Z indicators effectively identify topological phases.

γ_Z remains π in the topological phase, diverging from D which diverges.

Eigenvalues of the shift operator reveal additional phase transition signatures.

Abstract

To clarify the relationship between edge electronic states in open-boundary crystalline systems and their corresponding bulk electronic structure, Alase et al. [Phys. Rev. Lett. 117, 076804 (2016)] have recently generalized Bloch's theorem to lattice models with broken translational symmetry. Their formalism provides a bulk-boundary correspondence indicator, D, sensitive directly to the localization of edge states. We extend this formalism in two significant ways. First, we explicitly classify the edge-state basis provided by the theory according to their underlying protecting symmetries. Second, acknowledging that the true topological invariant is the Zak phase, inherently defined only for periodic boundary conditions, we introduce an analogous quantity $\gamma_Z$, suitable for open-boundary systems. We illustrate these developments on the example of the Kitaev wire model. We demonstrate that both indicators, $D$ and our open-boundary analog, $\gamma_Z$, effectively capture topological localization: while D diverges within the topological phase, $\gamma_Z = \pi$ . We further examine eigenvalues ($z$) of the lattice shift operator, showing that variations in these eigenvalues as a function of chemical potential provide additional signatures of topological phase transitions. Additionally, we study a periodic Kitaev wire containing a bond impurity, revealing that while the topological phase remains characterized by $\gamma_Z = \pi$, significant state localization near the impurity emerges only at critical values of the chemical potential.