Non-local order parameters for fermion chains via the partial transpose

TL;DR

This paper introduces non-local order parameters based on anti-unitary symmetries for classifying fermionic topological phases in one dimension, providing a homotopy invariant approach that enhances understanding of these phases.

Contribution

It proposes a new method using non-local order parameters defined via anti-unitary symmetries, offering a homotopy invariant classification for fermionic chains.

Findings

Non-local order parameters are homotopy invariants.

For matrix product states, invariants determine the real division super algebra.

The approach simplifies classifying topological phases with anti-unitary symmetries.

Abstract

In the last two decades, a vast variety of topological phases have been described, predicted, classified, proposed, and measured. While there is a certain unity in method and philosophy, the phenomenology differs wildly. This work deals with the simplest such case: fermions in one spatial dimension, in the presence of a symmetry group $G$ which contains anti-unitary symmetries. A complete classification of topological phases, in this case, is available. Nevertheless, these methods are to some extent lacking as they generally do not allow to determine the class of a given system easily. This paper will take up proposals for non-local order parameters defined through anti-unitary symmetries. They are shown to be homotopy invariants on a suitable set of ground states. For matrix product states, an interpretation of these invariants is provided: in particular, for a particle-hole symmetry, the invariant determines a real division super algebra $\mathbb{D}$ such that the bond algebra is a matrix algebra over $\mathbb{D}$.