Definitive physical evidence for superconductivity requires both a zero-resistance state and magnetic flux expulsion, known as the Meissner effect. However, establishing unambiguous Meissner signatures in emerging classes of pressure-induced superconductors, such as layered nickelates and hydrides, remains challenging under high pressure, raising questions about the bulk nature of superconductivity and the factors that obscure its magnetic signatures.
To investigate these issues, researchers publishing in Matter and Radiation at Extremes, affiliated with Jilin University, the Center for High Pressure Science and Technology Advanced Research, Shanghai Advanced Research in Physical Sciences (SHARPS), Brookhaven National Laboratory, and the China Academy of Engineering Physics systematically investigated how hydrostaticity affects DC magnetization, electrical transport, and crystal structure in optimally doped Bi₂Sr₂CaCu₂O₈₊δ (Bi-2212) single crystals, using them as a model superconducting system.
The researchers compared these properties under different pressure-transmitting media (PTMs): solid sodium chloride (NaCl), liquid silicone oil, helium, and a control condition without a PTM. The Meissner diamagnetic signal was strongly suppressed under the nonhydrostatic or quasi-hydrostatic conditions provided by NaCl and silicone oil. With NaCl, the magnetic signal was suppressed at lower pressures than with silicone oil, even while electrical signatures of superconductivity remained detectable. This difference shows that electrical transport and magnetic susceptibility can provide different indications of superconductivity under pressure. Better hydrostatic conditions also mitigated the pressure-induced increase in normal-state resistance, consistent with improved preservation of the superconducting state.
Single-crystal X-ray diffraction analyses showed that the suppression of the magnetic response was associated with pressure-induced lattice distortion. The diffraction results revealed lattice strain and incipient polycrystallization under less hydrostatic conditions. These structural changes may generate flux-pinning centers that impede flux expulsion and weaken the Meissner response in DC magnetic susceptibility measurements.
By contrast, helium provided more hydrostatic conditions, with diffraction spots remaining nearly sharp and round up to 20.5 GPa. The Meissner diamagnetic transitions remained observable up to 20 GPa, indicating that the magnetic response persisted at higher pressures under these conditions. Comparative electrical transport measurements further demonstrated that the zero-resistance state persisted to 26.0 GPa with silicone oil, whereas the Meissner diamagnetic signal with this medium remained observable up to 17.1 GPa, possibly because electrical conduction requires only a minimal percolative pathway, whereas magnetic susceptibility depends upon the total superconducting volume fraction exceeding instrument detection thresholds. These results show that the pressure ranges over which electrical and magnetic signatures remain detectable can differ.
In conclusion, these comprehensive findings establish that an absent or weak diamagnetic signal under pressure cannot automatically be taken as proof that bulk superconductivity has vanished. Furthermore, the study emphasizes that the pressure-transmitting medium and its ability to limit deviatoric stress are critical factors in high-pressure measurements. Careful control of hydrostaticity is therefore essential for interpreting magnetic susceptibility data and assessing superconductivity under extreme pressure conditions, including in emerging superconducting systems, such as nickelates and hydrides, where clear Meissner signals remain difficult to establish.
Reference:
Zhenfang Xing, Di Peng, Feiyang Zhang, Genda Gu, Yuxin Liu, Fujun Lan, Tao Luo, Guangrun Zhong, Chuangqi Xu, Shuai Liu, Yu Wang, Hongbo Lou, Yonghao Han, Zhidan Zeng, Lin Zhao, Qiaoshi Zeng; Effect of hydrostaticity on DC magnetic susceptibility of Bi2Sr2CaCu2O8+δ single crystals under high pressure. Matter Radiat. Extremes 1 October 2026; 11 (6): 067801. https://doi.org/10.1063/5.0340333
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