Bosonic Crystalline Symmetry Protected Topological Phases Beyond the Group Cohomology Proposal

TL;DR

This paper classifies three-dimensional bosonic crystalline SPT phases using a new mathematical framework involving twisted group cohomology, extending beyond previous models by including configurations of $E_8$ states on 2-cells.

Contribution

It introduces a new summand in the classification scheme that accounts for $E_8$ state configurations, expanding the understanding of crystalline SPT phases beyond the group cohomology proposal.

Findings

Classification for all 230 space groups established

New summand $H_{ ext{phi}}^{1}(G; ext{Z})$ identified and interpreted

Classification aligns with the generalized cohomology hypothesis

Abstract

It is demonstrated by explicit construction that three-dimensional bosonic crystalline symmetry protected topological (cSPT) phases are classified by $H_{\phi}^{5}(G;\mathbb{Z})\oplus H_{\phi}^{1}(G;\mathbb{Z})$ for all 230 space groups $G$, where $H^n_\phi(G;\mathbb{Z})$ denotes the $n$th twisted group cohomology of $G$ with $\mathbb{Z}$ coefficients, and $\phi$ indicates that $g\in G$ acts non-trivially on coefficients by sending them to their inverses if $g$ reverses spacetime orientation and acts trivially otherwise. The previously known summand $H_{\phi}^{5}(G;\mathbb{Z})$ corresponds only to crystalline phases built without the $E_8$ state or its multiples on 2-cells of space. It is the crystalline analogue of the "group cohomology proposal" for classifying bosonic symmetry protected topological (SPT) phases, which takes the form $H_{\phi}^{d+2}(G;\mathbb{Z})\cong H_{\phi}^{d+1}(G;U(1))$ for finite internal symmetry groups in $d$ spatial dimensions. The new summand $H_{\phi}^{1}(G;\mathbb{Z})$ classifies possible configurations of $E_8$ states on 2-cells that can be used to build crystalline phases beyond the group cohomology proposal. The completeness of our classification and the physical meaning of $H_{\phi}^{1}(G;\mathbb{Z})$ are established through a combination of dimensional reduction, surface topological order, and explicit cellular construction. The value of $H_{\phi}^{1}(G;\mathbb{Z})$ can be easily read off from the international symbol for $G$. Our classification agrees with the prediction of the "generalized cohomology hypothesis," which concerns the general structure of the classification of SPT phases, and therefore provides strong evidence for the validity of the said hypothesis in the realm of crystalline symmetries.