Topological states from topological crystals

TL;DR

This paper introduces a classification scheme for topological states protected by both onsite and spatial symmetries, demonstrating that all such states can be represented as topological crystals, which are real-space assemblies of lower-dimensional topological states.

Contribution

It provides a comprehensive classification of topological crystalline insulators by explicitly constructing and enumerating topological crystals for all 230 space groups in three dimensions.

Findings

All symmetry-protected topological states can be deformed into topological crystals.

Explicit enumeration of topological crystals for non-interacting time-reversal symmetric insulators.

Unified classification including strong and crystalline topological insulators.

Abstract

We present a scheme to explicitly construct and classify general topological states jointly protected by an onsite symmetry group and a spatial symmetry group. We show that all these symmetry protected topological states can be adiabatically deformed (allowing for stacking of trivial states) into a special class of states we call topological crystals. A topological crystal in, for example, three dimensions is a real-space assembly of finite-sized pieces of topological states in one and two dimensions protected by the local symmetry group alone, arranged in a configuration invariant under the spatial group and glued together such there is no open edge or end. As a demonstration of principle, we explicitly enumerate all inequivalent topological crystals for non-interacting time-reversal symmetric electronic insulators with significant spin-orbit coupling and any one of the 230 space groups in three dimensions. Because every topological crystalline insulator can be deformed into a topological crystal, the enumeration of the latter gives topological crystalline insulators a full classification and for each class an explicit real-space construction. We also extend these results to give a unified classification including both strong topological insulators and topological crystalline insulators.