Abstract
Topological semimetals with massless Dirac and Weyl fermions1,2 represent the forefront of
quantum materials research. In two dimensions (2D), a peculiar class of fermions that are
massless in one direction and massive in the perpendicular direction was predicted fifteen years
ago3–5
. These highly exotic quasiparticles – the semi-Dirac fermions – ignited intense theoretical
interest6–14 but remain undetected. Using magneto-optical spectroscopy, we demonstrate the
defining feature of semi-Dirac fermions – 𝐵
2/3
scaling of Landau levels – in a prototypical
nodal-line metal ZrSiS15,16
. In topological metals, including ZrSiS, nodal-lines extend the band
degeneracies from isolated points to lines, loops17,18 or even chains19–22 in the momentum space.
With ab initio calculations and theoretical modeling, we pinpoint the observed semi-Dirac
spectrum to the crossing points of nodal-lines in ZrSiS. Crossing nodal-lines exhibit a continuum
absorption spectrum but with singularities that scale as 𝐵
2/3
at the crossing. Our work sheds light
on the hidden quasiparticles emerging from the intricate topology of crossing nodal-lines
see also: https://arxiv.org/pdf/2311.03735v1
