Efficient local classification of parity-based material topology

TL;DR

This paper introduces a real-space, scalable method for classifying parity-based topological phases in aperiodic materials, overcoming limitations of traditional momentum-space approaches and enabling analysis of complex quasicrystals.

Contribution

The authors develop a novel, efficient spectral localizer-based framework with scalable sparse algorithms for topological classification in aperiodic systems, applicable to various physical materials.

Findings

Identified quantum spin Hall effect in quasicrystals.

Diagnosed fragile topology in photonic quasicrystals.

Provided a robust, local topological invariant for aperiodic systems.

Abstract

Although the classification of crystalline materials can be generally handled by momentum-space-based approaches, topological classification of aperiodic materials remains an outstanding challenge, as the absence of translational symmetry renders such conventional approaches inapplicable. Here, we present a numerically efficient real-space framework for classifying parity-based $\mathbb{Z}_2$ topology in aperiodic systems based on the spectral localizer framework and the direct computation of the sign of a Pfaffian associated with a large sparse skew-symmetric matrix. Unlike projector-based or momentum-space-based approaches, our method does not rely on translational symmetry, spectral gaps in the Hamiltonian's bulk, or gapped auxiliary operators such as spin projections, and instead provides a local, energy-resolved topological invariant accompanied by an intrinsic measure of topological protection. A central contribution of this work is the development of a scalable sparse factorization algorithm that enables the reliable determination of the Pfaffian's sign for large sparse matrices, making the approach practical to realistic physical materials. We apply this framework to identify the quantum spin Hall effect in quasicrystalline class AII systems, including gapless heterostructures, and to diagnose fragile topology in a large $C_2 \mathcal{T}$-symmetric photonic quasicrystal. Overall, our results demonstrate that the spectral localizer, combined with efficient sparse numerical methods, provides a unified and robust tool for diagnosing parity-based topological phases in aperiodic electronic, photonic, and acoustic materials where conventional band-theoretic indexes are inapplicable.