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

We report the observation of an in-plane nodal loop accompanying highly dispersive Dirac fermions with a velocity of 1.2 x 10(6) m/s in a superconductor ZrP1.24Se0.57 by employing angle-resolved photoemission spectroscopy. This nodal loop arises in principle from the P square net as long as nonsymmorphic symmetry is preserved. We have also found that two quasisquare Fermi surfaces that branch from the nodal loop at -1.2 eV are largely parallel in favor of nesting, and that two additional electron pockets are present at the Brillouin-zone center unlike in the case of ZrSiS. Our findings demonstrate that a P square net embedded in a superconductor provides a new playground for emergent exotic phenomena coexisting with superconductivity.