To decipher the mechanism of high temperature superconductivity, it is important to know how the superconducting pairing emerges from the unusual normal states of cuprate superconductors, including pseudogap, anomalous Fermi liquid and strange metal (SM). A long-standing issue under debate is how the superconducting pairing is formed and condensed in the SM phase because the superconducting transition temperature is the highest in this phase. Although a lot of theoretical progress on the superconducting mechanism of these high-T_C superconductors has been made, a unified understanding on how the SC state connects with these unusual normal states is still lacking. Because the SM state of the optimally-doped superconductor not only can develop the SC state with the highest T_C but also links the PG and the anomalous FL states, it is of great interest to take the SM state as a breakthrough point to further reveal the underlying physics of cuprate superconductors. Recently, a team led by Prof. SUN Liling, from Institute of Physics (IOP), Chinese Academy of Sciences, chose the optimally-doped Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) single crystal, a typical nearly-two-dimensional high-Tc superconductor with the SM normal state and widely studied in recent years, as the investigated material. They performed high pressure studies on these samples through state-of-the-art in-situ high-pressure measurements of resistance, magnetoresistance and magnetic susceptibility and discovered a pressure-induced crossover from two- to three-dimensional superconducting states in the optimally-doped Bi₂Sr₂CaCu₂O_8+δ bulk superconductor at a pressure above 2.8 GPa. They find that the two-dimensional (2D) superconductivity bridges the SM state and 3D superconducting state and that the two-dimensional (2D) superconducting transition exhibits a Berezinski-Kosterlitz-Thouless-like behavior. These results provide direct and strong evidence that the SM state is predominantly 2D-like.

In this study, they are the first to perform the combined in-situ high-pressure measurements of ac susceptibility and resistance for the same sample in the same diamond anvil cell. This kind of measurements is of technical challenges because the integration of the standard four-probes for the resistance measurement and the coils for the ac susceptibility measurements into the same pressure cell is very difficult, indicating that their high pressure technique adopted in this study is among the best levels of the world.

The high quality single crystals investigated in this study are provide by Genda Gu from Brookhaven National Laboratory, USA, whose samples of Bi-2212 have been widely studied by using other methods. High pressure X-ray diffraction measurements were performed at 15U beamline at the Shanghai Synchrotron Facilities.

These results have been published on Nature Physics. GUO Jing, ZHOU Yazhou and HUANG Cheng are the first coauthors (https://www.nature.com/articles/s41567-019-0740-0.pdf).

The study was supported by the National Science Foundation, the Ministry of Science and Technology of China, the Chinese Academy of Sciences and grants from US.