Pairing symmetry in infinite-layer nickelate superconductors

The recent success in the fabrication of superconducting nickelate thin film has opened a path to understand the origin of unconventional superconductivity in high-temperature cuprates. However, the challenging material growth, complicated strain and interface effects from the substrate, and a complex multiorbital picture of nickelates have obstructed our understanding on the important physics questions such as the superconducting gap symmetry. Here we perform in-plane London penetration depth measurement on optimally doped (Nd,Sr)NiO₂ and (La,Ca)NiO₂ to determine the pairing symmetry of the infinite-layer nickelate superconductor family. In contradiction to the cuprate analog picture, our result concretely disproves a dominant d_x²-y²-wave pairing symmetry which was predicted in theoretical calculations while suggesting a dominant s-wave pairing with two gaps of different T_c in nickelates. Such observation cannot be convincingly described by the existing theoretical viewpoints and calculations. Our findings could revolutionize understanding of the role of the copper oxide plane and 3d_x²-y² electronic band in the origin of unconventional superconductivity in cuprates.