Description
Whether infinite-layer (IL) nickelates host intrinsic superconductivity in their undoped phase remains an outstanding question. Although superconducting signatures have been reported near zero doping, definitive hallmarks—zero resistance and the diamagnetic response—have not been established in bare undoped compounds. Here we realize bulk superconductivity with zero resistance and a diamagnetic response in uncapped PrNiO2 by optimizing film synthesis and minimizing non-stoichiometry. Angle-resolved photoemission spectroscopy shows that the superconducting films retain the characteristic undoped electronic structure without evidence of additional hole doping. Superconductivity remains robust over a wide range of isovalent La substitution, yet is rapidly suppressed by slight hole doping. Considering that a superconducting dome re-emerges at higher hole concentrations, the undoped superconducting phase is well separated from the doped dome in the phase diagram. The undoped compound further exhibits an unusually large upper critical field anisotropy than optimally doped samples. These results establish undoped IL-nickelates as hosting a distinct superconducting state potentially different from that of their optimally cation-substituted counterparts.
