Unveiling three types of fermions in a nodal ring topological semimetal through magneto-optical transitions

Abstract

We investigate the quasiparticles of a single nodal ring semimetal SrAs${}_3$ through axis-resolved magneto-optical measurements. We observe three types of Landau levels scaling as $\epsilon \sim \sqrt{B}$, $\epsilon \sim B^{2/3}$, and $\epsilon \sim B$ 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 endpoints 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.

35 pages, 23 figures

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