Weyl semimetals are materials with topologically protected point-like degeneracies (Weyl nodes) in their band structures. When tuning external parameters, such as magnetic field or strain, isolated nodes move in momentum space, but small perturbations cannot gap them out. Changes in the number of Weyl nodes occur through merging processes, usually involving a pair of oppositely charged nodes. More complicated processes involving multiple Weyl nodes are also possible, but they typically require fine tuning and are thus less stable. In this work, we use singularity-theoretical tools to study how symmetries affect the allowed merging processes and their stability, focusing on the combination of a two-fold rotation and time-reversal (C2T) symmetry. We find in this case that, counter-intuitively, processes involving a merging of three nodes are more generic than processes involving only two nodes. Our work suggests that multi-Weyl-merging may be observed in a large variety of quantum materials, and we discuss MoTe2, SrSi2, and bilayer graphene as potential candidates.

Reference: arXiv:2403.08518 (PRL in press)