Searching for Double Dirac Fermions*

The field of topological materials is developing rapidly, exhibiting new physical phenomena and inviting potential applications in the emerging fields of quantum computing and sensing. Normally, the maximal degeneracy of states in any electronic system is six. Topological considerations predict that in certain materials the double group cross product of two irreducible representations (irreps.) or the cross product of one irrep. on itself can yield eight states that are degenerate in energy at a high symmetry point in reciprocal space. Materials including CuBi₂O₄ and PdBi₂O₄ have been previously predicted to be such 8-fold fermion or double Dirac systems on the basis of their symmetries. However, their insulating behavior hinders photoemission and transport characterizations, which are crucial for unambiguous identification. Here we present a strategy for designing real materials with 8-fold fermions based on chemical principles and provide an update on experimental detection of this novel topological state.