Topology of nonsymmorphic crystalline insulators and superconductors

TL;DR

This paper extends topological classification to nonsymmorphic crystalline insulators and superconductors, revealing new $$ and $$ topological phases enabled by nonsymmorphic symmetries, with implications for boundary states and surface modes.

Contribution

It provides a complete classification of topological phases with nonsymmorphic symmetries using twisted equivariant K-theory, introducing novel $$ and $$ phases and their invariants.

Findings

Nonsymmorphic symmetries allow $$ topological phases without time-reversal symmetry.

Coexistence of nonsymmorphic symmetries with TRS or PHS yields $$ topological phases.

Boundary modes are protected by nonsymmorphic symmetries, leading to gapless surface states.

Abstract

Topological classification in our previous paper [K. Shiozaki and M. Sato, Phys. Rev. B ${\bf 90}$, 165114 (2014)] is extended to nonsymmorphic crystalline insulators and superconductors. Using the twisted equivariant $K$-theory, we complete the classification of topological crystalline insulators and superconductors in the presence of additional order-two nonsymmorphic space group symmetries. The order-two nonsymmorphic space groups include half lattice translation with $Z_2$ flip, glide, two-fold screw, and their magnetic space groups. We find that the topological periodic table shows modulo-2 periodicity in the number of flipped coordinates under the order-two nonsymmorphic space group. It is pointed out that the nonsymmorphic space groups allow $\mathbb{Z}_2$ topological phases even in the absence of time-reversal and/or particle-hole symmetries. Furthermore, the coexistence of the nonsymmorphic space group with the time-reversal and/or particle-hole symmetries provides novel $\mathbb{Z}_4$ topological phases, which have not been realized in ordinary topological insulators and superconductors. We present model Hamiltonians of these new topological phases and the analytic expression of the $\mathbb{Z}_2$ and $\mathbb{Z}_4$ topological invariants. The half lattice translation with $Z_2$ spin flip and glide symmetry are compatible with the existence of the boundary, leading to topological surface gapless modes protected by such order-two nonsymmorphic symmetries. We also discuss unique features of these gapless surface modes.