Imaging the Meissner effect in pressurized bilayer nickelate with integrated multi-parameter quantum sensor.

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. Although zero resistance has been successfully achieved under improved hydrostatic pressure conditions, the Meissner effect of La3Ni2O7-δ under high pressure remains controversial. Here, using shallow nitrogen-vacancy centers implanted on the culet of diamond anvils as in situ quantum sensors, we observe compelling evidence for the Meissner effect in polycrystalline bilayer nickelate samples: magnetic field expulsion during both field-cooling and field-warming processes. In particular, we explore the multi-parameter measurement capacity of the diamond quantum sensors to extract the weak demagnetization signal of La3Ni2O7-δ. The correlated measurements of Raman spectra and magnetic imaging indicate an incomplete structural transformation related to the displacement of oxygen ions emerging in the non-superconducting region. Our work clarifies the controversy about the Meissner effect of La3Ni2O7-δ and contributes to the development of quantum sensing of weak signals under high-pressure conditions.