Unveiling Three Types of Fermions in a Nodal Ring Topological Semimetal through Magneto-Optical Transitions

We investigate the quasiparticles of a single nodal ring semimetal SrAs3 through axis-resolved magneto-optical measurements. We observe three types of Landau levels scaling as µ∼B, µ∼B2/3, and µ∼B that correspond to Dirac, semi-Dirac, and classical fermions, respectively. Through theoretical analysis, we identify the distinct origins of these three types of fermions present within the nodal ring. In particular, semi-Dirac fermions - a novel type of fermion that can give rise to a range of unique quantum phenomena - emerge from the end points of the nodal ring where the energy band disperses linearly along one direction and quadratically along the perpendicular direction, a feature not achievable in nodal point or line structures. The capacity of the nodal ring to simultaneously host multiple fermion types, including semi-Dirac fermions, establishes it as a valuable platform to expand the understanding of topological semimetals.