Asymmetric real topology of conduction and valence bands

Abstract: Previously, it was believed that conduction and valence bands exhibit a symmetry: they possess opposite topological invariants (e.g., the Chern numbers of conduction and valence bands for the Chern insulator are $\pm C$). However, we present a counterexample: The second Stiefel-Whitney numbers for conduction and valence bands over the Klein bottle may be asymmetric, with one being nontrivial while the other trivial. Here, the Stiefel-Whitney classes are the characteristic classes for real Bloch functions under $PT$ symmetry with $(PT)^2=1$, and the Klein bottle is the momentum-space unit under the projective anti-commutation relation of the mirror reflection reversing $x$ and the translation along the $y$-direction. The asymmetry originates from the algebraic difference of real cohomology classes over Klein bottle and torus. This discovery is rooted in the fundation of topological band theory, and has the potential to fundamentally refresh our current understanding of topological phases.