Effect of hydrostaticity on DC magnetic susceptibility of Bi2Sr2CaCu2O8+ δ single crystals under high pressure

Definitive evidence for superconductivity requires not only a zero-resistance state, but also the observation of magnetic flux expulsion, i.e., the Meissner effect. However, in many emerging pressure-induced superconductors, including hydrides and nickelates, unambiguous Meissner effect signals in DC magnetic susceptibility measurements remain difficult to establish under high pressure. This has raised persistent questions about the bulk nature of superconductivity in these systems and about the experimental factors that may obscure the Meissner response under pressure. Here, we systematically investigate the hydrostaticity effect on DC magnetization, electrical transport, and single-crystal structure of optimally doped Bi2Sr2CaCu2O8+δ under high pressure, using this well-established cuprate superconductor as a model system. We demonstrate that the Meissner diamagnetic signal is highly sensitive to the degree of hydrostaticity. Under non- or quasi-hydrostatic pressure conditions generated using NaCl or silicone oil as pressure-transmitting media, the diamagnetic response is strongly suppressed, while electrical signatures of superconducting transition remain observable. This suppression is directly correlated with pressure-induced lattice distortion revealed by single-crystal structural analysis. By contrast, when helium is used as a nearly ideal hydrostatic pressure-transmitting medium, the Meissner diamagnetic signal persists to substantially higher pressures with nearly intact single-crystal quality. These results demonstrate that reduced nonhydrostaticity can severely obscure the Meissner response in high-pressure DC susceptibility measurements, even in a canonical bulk superconductor. Our findings provide an important experimental benchmark for interpreting magnetic susceptibility data and for definitively validating emerging high-temperature superconductors under extreme pressure conditions.