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, †
       

(a) Band structure near a nodal line with zero Z2 monopole charge. (b) Band structure near a nodal line carrying a unit Z2 monopole charge (monopole nodal line) linked with another nodal line below the Fermi level (EF). (c) Atomic structure of ABC-stacked graphdiyne. (d) Band structure of ABC-stacked graphdiyne, where thick orange lines indicate degenerate nodal lines above and below EF. (e) The shape of two monopole nodal lines (red loops) at EF (E = 0) linked with a nodal line below EF (yellow line) in ABC-stacked graphdiyne. Figures are adopted from Ref. [7].