Topologie excitovaných elektronových stavů

Weyl semimetals are three-dimensional phases of solids, where gapless crossing of two electronic states (so called Weyl nodes) is protected by symmetry and topology. Their special electronic properties provide topologically protected surface states (Fermi arcs), where electron spin is connected to electron direction of propagation. Nowadays, investigation of Weyl nodes focuses mostly on materials where Weyl node is situated in the vicinity of Fermi level, making design and fabrication of those materials much more challenging, and in case of ferromagnets such materials do not exist so far. Therefore, we suggest to prove existence of Weyl nodes and corresponding surface states in excited electronic states using magnetooptic spectroscopy, where position of Weyl node on Fermi surface is not required. We further suggest to employ surface states as a source of spin-current. Finally, we describe quadrupole and octupole sources of Berry curvature, a new topological feature not mentioned yet.