On August 7, the reporter learned that researcher Jin Changqing from the Key Laboratory of Extreme Condition Physics, Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics, and his collaborators have been carrying out the design and high-pressure synthesis of new structures of copper-based superconducting materials for a long time.
Focus on copper and alkaline earth oxide systems, which are the simplest chemical components that can form the basic structure of copper-based superconductors. The uniqueness of choosing this simple composition is that it can focus on the core elements of the copper-based superconductor, while avoiding the expensive and toxic elements such as rare earth, bismuth, mercury, etc., which are required for the preparation of copper-based superconducting materials at atmospheric pressure. Further application and expansion of new materials.
Using high-pressure and high-temperature preparation technology, they have successively discovered the “copper series” (Physica C 223, 238 (1994); Phys. Rev. B 61, 778 (2000)); Chinese Science 48, 87405 (2018)), “Top Angle Oxygen” doped system (Nature 375, 301 (1995); Phys. Rev. B 74, 100506(R) (2006); Phys. Rev. B 80, 94523 (2009) (Editor’s Suggestion)) and other new structures Copper-based superconducting material system. Among them, the Tc of “copper-based” superconducting materials can be as high as 118K, and was selected as the cover of the “Science Bulletin” album to commemorate the 30th anniversary of the discovery of superconducting materials in the liquid nitrogen temperature zone (Science Bulletin 62, 3947 (2017)). The team has been around for more than 20 years The systematic research of new copper-based superconducting materials has formed its own characteristics internationally.
Copper oxide high-temperature superconductor (copper-based superconductor for short) is a superconducting material system with the highest transition temperature so far under normal pressure conditions. Its microscopic mechanism has been selected as one of 125 major scientific problems in “Science”, and it is still a condensed matter science. One of the biggest mysteries and challenges. Due to the strong Jahn Teller effect and interlayer Coulomb effect of the copper-based superconductor, the copper-oxygen bond length along the c-direction is greater than the bond length in the copper-oxygen plane, resulting in the basic electronic configuration of the copper-oxygen hexacoordination octahedron showing a tensile state . For the stretched coordination structure, the 3d x2–y2 orbital of copper is located above the 3z2-r2 orbital and strongly hybridizes with the 2P orbital of in-plane oxygen. This image constitutes the starting point for understanding copper-based superconducting materials
Through the innovation of 100,000 bar ultra-high oxygen pressure synthesis technology (MRS Advances 2, 2587 (2017)), Jin Changqing guided graduate student Li Wenmin to prepare and discover a new class of superconducting material Ba2CuO4-y. This is currently the only copper-based superconducting material that exhibits compressed copper-oxygen local coordination. For the compression type coordination configuration, the 3d 3z2-r2 orbital of copper will be located above the x2-y2 orbital, which is significantly different from the orbital order of the “traditional” stretch type coordination. X-ray absorption spectrum experiments show that the Ba2CuO4-y superconductor is in the over-doped region, which corresponds to the non-superconducting phase region of the “traditional” copper-based superconductor. The current mainstream theory believes that the compressed coordination configuration, the excess of the doped carrier concentration, and the possible special in-plane structure are not conducive to superconductivity. Ba2CuO4-y still exhibits a superconducting transition temperature as high as 73K. . The same as the basic crystal structure, the Tc of Ba2CuO4-y is increased by more than 80% compared to the La2CuO4 system with normal orbital order. These experimental phenomena show that Ba2CuO4-y belongs to a new class of copper-based superconducting materials, which is different from the traditional types in the past.
The above work was recently published in the Proceedings of the National 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, winner of the Bartin Award, and a well-known superconductivity theory expert, wrote a special review on the topic of “A different branch of the high Tc family” (this material has short The apex angle of oxygen distance and over-doping characteristics mean that it belongs to different branches of copper-based high-temperature superconducting materials, which poses a challenge to many existing high-temperature superconducting mechanisms.
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