Locally equivalent quasifree states and index theory

TL;DR

This paper explores the classification of quasifree ground states of the self-dual CAR algebra using index theory, linking topological invariants to symmetry protected topological phases and $K$-homology.

Contribution

It generalizes Clifford module index constructions to $KO$-theory invariants for deformations of $C^{*,\mathfrak{r}}$-algebras, connecting topological obstructions to $K$-homology classes.

Findings

Clifford module indices characterize topological obstructions.

Invariants in $KO_\ast(A^\mathfrak{r})$ are constructed for deformations.

The coarse assembly map relates $K$-homology classes to SPT phases.

Abstract

We consider quasifree ground states of Araki's self-dual CAR algebra from the viewpoint of index theory and symmetry protected topological (SPT) phases. We first review how Clifford module indices characterise a topological obstruction to connect pairs of symmetric gapped ground states. This construction is then generalised to give invariants in $KO_\ast(A^\mathfrak{r})$ with $A$ a $C^{*,\mathfrak{r}}$-algebra of allowed deformations. When $A=C^*(X)$, the Roe algebra of a coarse space $X$, and we restrict to gapped ground states that are locally equivalent with respect $X$, a $K$-homology class is also constructed. The coarse assembly map relates these two classes and clarifies the relevance of $K$-homology to free-fermionic SPT phases.