Multifold nodal points in magnetic materials

Abstract

We describe the symmetry protected nodal points that can exist in magnetic space groups and show that only three-, six-, and eightfold degeneracies are possible (in addition to the two- and fourfold degeneracies that have already been studied). The three- and sixfold degeneracies are derived from “spin-1” Weyl fermions. The eightfold degeneracies come in different flavors. In particular, we distinguish between eightfold fermions that realize nonchiral “Rarita-Schwinger fermions” and those that can be described as four degenerate Weyl fermions. We list the (magnetic and nonmagnetic) space groups where these exotic fermions can be found. We further show that in several cases, a magnetic translation symmetry pins the Hamiltonian of the multifold fermion to an idealized exactly solvable point that is not achievable in nonmagnetic crystals without fine-tuning. Finally, we present known compounds that may host these fermions and methods for systematically finding more candidate materials.