Revealing superconducting gap in La3Ni2O7-δ by Andreev reflection spectroscopy under high pressure.

The recent discovery of compressed superconductivity at 80 K in La3Ni2O7-δ has brought nickelates into the family of unconventional high-temperature superconductors. However, due to the challenges of directly probing the superconducting pairing mechanism under high-pressure, the pairing symmetry and gap structures of nickelate superconductors remain under intense debate. In this work, we successfully determine the microscopic information on the superconducting gap structure of La3Ni2O7-δ samples subjected to pressures exceeding 20 GPa, by constructing different conductance junctions within diamond anvil cells. By analyzing the temperature-dependent differential conductance spectra within the Blonder-Tinkham-Klapwijk (BTK) model, we have determined the superconducting energy gap at high pressure. The differential conductance curves reveal a two-gap structure with Δs1 = 23 meV and Δs2 = 6 meV, while the BTK fitting consistent with an s-like, two-gap spectrum. The gap ratio 2Δs1(0)/kBTc is found to be 7.41, belonging to a family of strongly coupled superconductors. Our findings provide valuable insights into the superconducting gap structures of the pressure-induced superconducting nickelates.