Topological quadrupolar semimetals

Conventional topological semimetals are characterized by the energy momentum separation of the bulk nodes in the energy spectrum. Here, the authors demonstrate a few types of topological semimetals that are gapped in the bulk, but host one or a half of a Dirac semimetal on multiple surfaces. They show that these semimetals can be characterized by the bulk nodes in the hybrid Wannier spectrum, rather than the energy spectrum. This works points to a new direction in studying the higher order topology in semimetallic systems.