Probing the Meissner effect in pressurized bilayer nickelate superconductors using diamond quantum sensors

Recent reports on the signatures of high-temperature superconductivity with a critical temperature T_c close to 80 K have triggered great research interest and extensive follow-up studies^1-8. Although zero-resistance state has been successfully achieved under improved hydrostatic pressure conditions^3,9, there is no clear evidence of superconducting diamagnetism in pressurized La₃Ni₂O_7-δ due to the low superconducting volume fraction and limited magnetic measurement techniques under high pressure conditions¹⁰. Here, using shallow nitrogen-vacancy centers implanted on the culet of diamond anvils as in-situ quantum sensors, we observe convincing evidence for the Meissner effect in polycrystalline samples La₃Ni₂O_7-δ and La₂PrNi₂O₇: the magnetic field expulsion during both field cooling and field warming processes. The correlated measurements of Raman spectra and NV-based magnetic imaging indicate an incomplete structural transformation related to the displacement of oxygen ions emerging in the non-superconducting region. Furthermore, comparative experiments on different pressure transmitting media (silicone oil and KBr) and nickelates (La₃Ni₂O_7-δ and La₂PrNi₂O₇) reveal that an improved hydrostatic pressure conditions and the substitution of La by Pr in La₃Ni₂O_7-δ can dramatically increase the superconductivity. Our work clarifies the controversy about the Meissner effect of bilayer nickelate and contributes to a deeper understanding of the mechanism of nickelate high-temperature superconductors.