Lin Er Chow1,Sujith Kunniniyil Sudheesh1,Proloy Nandi1,Shengwei Zeng1,Zhaoting Zhang1,Xiaomeng Du1,Zhi Shiuh Lim1,Ee Min Elbert Chia2,Ariando Ariando1
National University of Singapore1,Nanyang Technological University2
Lin Er Chow1,Sujith Kunniniyil Sudheesh1,Proloy Nandi1,Shengwei Zeng1,Zhaoting Zhang1,Xiaomeng Du1,Zhi Shiuh Lim1,Ee Min Elbert Chia2,Ariando Ariando1
National University of Singapore1,Nanyang Technological University2
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 of important physics questions such as the superconducting gap symmetry. Here we perform in-plane London penetration depth measurement on optimally doped (Nd,Sr)NiO<sub>2</sub> and (La,Ca)NiO<sub>2</sub> 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<sub>x<sup>2</sup>-y<sup>2</sup></sub>-wave pairing symmetry which was predicted in theoretical calculations while suggesting a dominant s-wave pairing with two gaps of different T<sub>c</sub> 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<sub>x<sup>2</sup>-y<sup>2</sup></sub> electronic band in the origin of unconventional superconductivity in cuprates.