$K$-theory classification of Wannier localizability and detachable topological boundary states

TL;DR

This paper uses $K$-theory to classify the relationship between Wannier localizability and the detachability of topological boundary states, unifying bulk and boundary perspectives in topological insulators.

Contribution

It provides a $K$-theoretic framework linking Wannier localizability with boundary state detachability, unifying bulk-boundary correspondence classifications.

Findings

Classifies intrinsic and extrinsic non-Hermitian topology.

Shows boundary and bulk classifications agree.

Develops a unified $K$-theoretic understanding of topological phases.

Abstract

A hallmark of certain topology, including the Chern number, is the obstruction to constructing exponentially localized Wannier functions in the bulk bands. Conversely, other types of topology do not necessarily impose Wannier obstructions. Remarkably, such Wannier-localizable topological insulators can host boundary states that are detachable from the bulk bands. In our accompanying Letter [D. Nakamura {\it et al.}, Phys. Rev. Lett. 135, 096601 (2025), arXiv:2407.09458], we demonstrate that non-Hermitian topology underlies detachable boundary states in Hermitian topological insulators and superconductors, thereby establishing their tenfold classification based on internal symmetry. Here, using $K$-theory, we elucidate the relationship between Wannier localizability and detachability of topological boundary states. From the boundary perspective, we classify intrinsic and extrinsic non-Hermitian topology, corresponding to nondetachable and detachable topological boundary states, respectively. From the bulk perspective, on the other hand, we classify Wannier localizability through the homomorphisms of topological phases from the tenfold Altland-Zirnbauer symmetry classes to the threefold Wigner-Dyson symmetry classes. Notably, these two approaches from the boundary and bulk perspectives lead to the same classification. We clarify this agreement and develop a unified understanding of the bulk-boundary correspondence on the basis of $K$-theory.