Revealing superconducting gap in La$_3$Ni$_2$O$_7$-$δ$ by Andreev reflection spectroscopy under high pressure

Abstract: The recent discovery of compressed superconductivity at 80~K in La$3$Ni$_2$O$_7$-$\delta$ 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 La$_3$Ni$_2$O$_7$-$\delta$ 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 $\Delta{1} = 23~\mathrm{meV}$ and $\Delta_{2} = 6~\mathrm{meV}$, while the BTK fitting is consistent with an $s$-like, two-gap spectrum. The gap ratio $2\Delta_{s1}(0) / k_{\mathrm{B}}T_{c}$ is found to be 7.61, belonging to a family of strongly coupled superconductors. Our findings provide valuable insights into the superconducting gap structures of the pressure-induced superconducting nickelates.