Evidence for Dirac nodal-line fermions in a phosphorous square-net superconductor

The fastest Dirac fermions of 1200 km/s with a nodal loop are discovered here in the superconductor ZrP${}_{2\ensuremath{-}x}$Se${}_{x}$, surpassing the velocity record held by the point-node Dirac fermions in graphene. By using angle-resolved photoemission spectroscopy, the authors demonstrate that nonsymmorphic phosphorous-based square nets provide the hoped-for stage for high Dirac velocity, nodal line, and superconductivity to meet together. This finding should pave the way for future fast and low-loss electronics, and hopefully will lead to discovery of the elusive Majorana fermion.