Exactly soluble local bosonic cocycle models, statistical transmutation, and simplest time-reversal symmetric topological orders in 3+1D

TL;DR

This paper constructs exactly soluble local bosonic models in 3+1D and 2+1D that realize various topological orders, including those with time-reversal symmetry and emergent fermions, revealing new phenomena like statistical transmutation and fractionalized symmetries.

Contribution

It introduces a versatile algebraic topology-based framework for constructing exactly soluble models of topological orders with novel properties in 3+1D and 2+1D.

Findings

Realization of 3+1D $Z_2$ gauge theory with emergent fermionic Kramer doublet

Identification of point-like excitations with fractionalized time-reversal symmetry

Discovery of string-like excitations carrying anomalous $Z_2$ symmetry

Abstract

We propose a generic construction of exactly soluble \emph{local bosonic models} that realize various topological orders with gappable boundaries. In particular, we construct an exactly soluble bosonic model that realizes a 3+1D $Z_2$ gauge theory with emergent fermionic Kramer doublet. We show that the emergence of such a fermion will cause the nucleation of certain topological excitations in space-time without pin$^+$ structure. The exactly soluble model also leads to a statistical transmutation in 3+1D. In addition, we construct exactly soluble bosonic models that realize 2 types of time-reversal symmetry enriched $Z_2$-topological orders in 2+1D, and 20 types of simplest time-reversal symmetry enriched topological (SET) orders which have only one non-trivial point-like and string-like topological excitations. Many physical properties of those topological states are calculated using the exactly soluble models. We find that some time-reversal SET orders have point-like excitations that carry Kramer doublet -- a fractionalized time-reversal symmetry. We also find that some $Z_2$ SET orders have string-like excitations that carry anomalous (non-on-site) $Z_2$ symmetry, which can be viewed as a fractionalization of $Z_2$ symmetry on strings. Our construction is based on cochains and cocycles in algebraic topology, which is very versatile. In principle, it can also realize emergent topological field theory beyond the twisted gauge theory.