Pairing symmetry revealed by single particle tunneling measurements in compressively strained bilayer nickelate thin films

The discovery of high-temperature superconductivity in nickelate system has stimulated tremendous interest in the community [1,2]. Among the nickelate superconductors, two typical systems have attracted the most attention, namely the infinite layer RE1-xSrxNiO2 with Tc reaching about 40 K, and the bilayer system La3Ni2O7 with Tc above 90 K. In this talk we will report the progress of research on the symmetry of the superconducting order parameter. We have carried out single particle tunneling measurement on the nickelate superconducting thin film Nd1-xSrxNiO2, a dominant V-shape spectrum (with a d-wave gap around 4 meV) was discovered [3], which is mixed with a passive s-wave component (~2meV). Recently, we have also succeeded in growing the compressively strained thin films of La2PrNi2O7 with Tc^onset = 41.5 K. Tunneling spectra are successfully measured on the terraces after we remove the surface layer by using the tip-excavation technique. The spectrum shows a two-gap structure with Delta1~ 20 meV, Delta2 ~ 6-8 meV, and fittings based on the Dynes model indicate that the dominant gap should have an anisotropic s-wave structure, this allows us to pin down the s+- among the two arguable pairing models: s+- and d-wave in this type of thin films [4]. Our results indicate the undoubted role played by the spin fluctuations in establishing superconductivity in the two major nickelate superconducting systems.