Copper oxide high-temperature superconductor (copper-based superconductor for short) is the superconducting material system with the highest transformation temperature to date under normal pressure conditions. Its micromechanism is cracked and selected into 125 major scientific problems in Science. At present, it is still condensed matter science. One of the biggest mysteries and challenges. Due to the strong Jahn Teller effect and interlayer Coulomb interaction of copper-based superconductors, the copper-oxygen bond length along the c-direction is greater than the bond length in the copper-oxygen plane, which results in the copper-oxygen hexacoordinate octahedron in the basic electronic configuration exhibiting a tensile state. . For the stretch-type coordination structure, the 3d x2–y2 orbital of copper is above the 3z2-r2 orbital, and is strongly hybridized with the 2P orbital of in-plane oxygen. This image constitutes the starting point for the understanding of copper-based superconducting materials (see: Keimer et al Nature 518, 179 ~ 186 (2015)).
Researcher Jin Changqing and the collaborators of the Institute of Physics, Chinese Academy of Sciences / Beijing National Research Center for Condensed Matter Physics have been conducting long-term design and high-pressure synthesis of new structures for copper-based superconducting materials. The research objects are focused on copper and alkaline earth oxide systems. This is the simplest chemical component capable of forming the basic structure of a copper-based superconductor. The uniqueness of choosing this simple component is that it can focus on the core elements of copper-based superconductivity and avoid the expensive and toxic elements such as rare earth, bismuth, mercury, etc. that copper-based superconducting materials require at atmospheric pressure, which helps Further application and expansion of new materials. Using high pressure and high temperature preparation technology, they have successively discovered the "copper system" (Physica C 223, 238 (1994); Phys. Rev. B 61, 778 (2000)); Chinese Science 48, 87405 (2018)), "apex angle Oxygen-doped systems (Nature 375, 301 (1995); Phys. Rev. B 74, 100506 (R) (2006); Phys. Rev. B 80, 94523 (2009) (Editor's Suggestion))) Copper-based superconducting material system. The Tc of the "copper" superconducting material can be as high as 118K, and was selected into the cover of the 30th anniversary album of the "Science Bulletin" commemorating the discovery of superconducting materials in the liquid nitrogen temperature zone (Science Bulletin 62, 3947 (2017)). The systematic research of new copper-based superconducting materials has formed its own characteristics internationally.
Through the innovation of 100,000 bar super-high oxygen pressure synthesis technology (MRS Advances 2, 2587 (2017)), Jin Changqing instructed graduate student Li Wenmin to prepare a new class of superconducting material Ba2CuO4-y. This is currently the only copper-based superconducting material that exhibits compressive copper-oxygen local coordination. For the compression-type coordination configuration, copper's 3d 3z2-r2 orbit will be above the x2-y2 orbit, which is significantly different from the "traditional" stretch-type coordination orbital sequence. X-ray absorption spectroscopy experiments show that the Ba2CuO4-y superconductor is in a region beyond the doping region, corresponding to the non-superconducting phase region of a "traditional" copper-based superconductor. Current mainstream theories suggest that compressive coordination configurations, excess doped carrier concentrations, and possibly special in-plane structures are not conducive to superconductivity, and Ba2CuO4-y still exhibits a superconducting transition temperature of up to 73K. . Compared with the basic crystal structure, La2CuO4 system with normal orbital order, the Tc of Ba2CuO4-y is increased by more than 80%. These experimental phenomena show that, unlike the traditional types in the past, Ba2CuO4-y belongs to a new type of copper-based superconducting material. The above work was recently published in the journal of the American Academy of Sciences (W. M. Li et al. Proceedings of the National Academy of Sciences 116, 12156 (2019)). Professor Scalapino, a member of the American Academy of Sciences and the Badin Prize winner, and a well-known superconducting theory expert, wrote a special review on the topic "A different branch of the high Tc family" (this material has short The apical angle oxygen distance and over-doping characteristics mean that it belongs to different branches of copper-based high-temperature superconducting materials, which challenges many existing high-temperature superconducting mechanisms: Nevertheless, the remarkably high Tc of this highly overdoped cuprate with its short Cu apical O separation and its O vacancies in the CuO2 plane suggest that it is a member of a different branch of high Tc cuprate materials, which challenges the basic tenants of many high Tc theories). Professor QZHuang of the United States National Bureau of Standards, Professor ZWHu of the Max Planck Institute for Physical Chemistry, and Professor Uemura of Columbia University have cooperated closely in experimental characterizations such as neutron diffraction, photoelectron absorption spectroscopy, and uSR spectroscopy. Professor Uchida of the University of Tokyo participated in the discussion of the experimental results; the research work was funded by the major national research and development plans and major international cooperation projects of the Funding Committee.