Stiefel–Whitney classes and topological phases in band theory
Ahn Junyeong1, 2, 3, Park Sungjoon1, 2, 3, Kim Dongwook4, Kim Youngkuk4, Yang Bohm-Jung1, 2, 3, †
       

Stabilty of nodal lines in 3D PT-symmetric spinless fermion systems. (a) A Z2 trivial nodal line can be gapped via band re-inversion. (b) A nodal line carrying Z2 monopole charge cannot be gapped alone.