Anomalies and symmetry fractionalization in reflection-symmetric topological order

TL;DR

This paper explores how certain reflection-symmetric topological phases in 2+1 dimensions can have their anomalies canceled by a 2D SPT phase, revealing new insights into symmetry fractionalization and anomaly classification.

Contribution

It demonstrates that anomalies in reflection-symmetric topological orders can be canceled by a 2D SPT, and classifies fractionalization patterns related to these anomalies.

Findings

Anomalies occur when both electric and magnetic quasiparticles have nontrivial fractional reflection quantum numbers.

Reflection symmetry fractionalization patterns are classified for $ ext{Z}_N$ topological order.

Some anomalies are canceled by a 2D SPT embedded in a trivial 3D bulk.

Abstract

One of the central ideas regarding anomalies in topological phases of matter is that they imply the existence of higher-dimensional physics, with an anomaly in a D-dimensional theory typically being cancelled by a bulk (D+1)-dimensional symmetry-protected topological phase (SPT). We demonstrate that for some topological phases with reflection symmetry, anomalies may actually be cancelled by a D-dimensional SPT, provided that it comes embedded in an otherwise trivial (D+1)-dimensional bulk. We illustrate this for the example of $\mathbb{Z}_N$ topological order enriched with reflection symmetry in (2+1)D, and along the way establish a classification of anomalous reflection symmetry fractionalization patterns. In particular, we show that anomalies occur if and only if both electric and magnetic quasiparticle excitations possess nontrivial fractional reflection quantum numbers.