On-line SPICE-SPIN+X Seminars
On-line Seminar: 14.07.2021 - 15:00 German Time
Topological protectorates of Fermi surfaces
Christian Pfleiderer, TU Munich
Following over a decade of intense research to enable major technological progress by means of materials in which the electronic structure exhibits non-trivial topological properties, three key challenges are still unresolved. First, the identification of topological band degeneracies that are generically rather than accidentally located at the Fermi level. Second, the ability to easily control such topological degeneracies. And third, to identify generic topological degeneracies in large, multi-sheeted Fermi surfaces.
Combining quantum oscillatory studies with density functional theory and comprehensive band-topology calculations, we report the identification of symmetry-enforced nodal planes in the B20 compounds CoSi and MnSi. The nodal planes enforce topological protectorates with substantial Berry curvatures at their intersection with the Fermi surface regardless of the complexity of the FS. In CoSi we show that the nodal planes provide the missing topological charges of an entire network of band-crossings comprising, in addition, multifold degeneracies and Weyl points, such that the fermion doubling theorem is satisfied. Moreover, in the ferromagnetic state of MnSi, the existence of the nodal planes may be controlled with the direction of the applied magnetic field.
The identification of symmetry-enforced topological protectorates of the Fermi surfaces of CoSi and MnSi suggests the existence of similar properties in a large number of materials. In particular, deriving the symmetry conditions underlying topological nodal planes, we show that the 1651 magnetic space groups comprise 7 grey groups and 26 black-and-white groups with topological nodal planes, including the space group of ferromagnetic MnSi.
 M. Wilde et al. Nature 594, 374 (2021)
 N. Huber et al., arXiv/2107.02820