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(a) Trigonal bipyramid coordination structure (in the middle is a cation); (b) Two-dimensional hexagonal lattice composed of triangular bipyramidal ligands by sharing the vertex angle and the superconducting party sequence parameter in real space release.
So far, scientists have discovered two types of well-known unconventional high-temperature superconductors: copper-based and iron-based superconductors. Both types of superconductors were accidentally discovered in experiments. The study of their superconducting mechanism is the most challenging frontier work in condensed matter physics.
The research group of Hu Jiangping, a researcher at the Institute of Physics of the Chinese Academy of Sciences/Beijing National Laboratory for Condensed Matter, summed up a series of research work in the past and proposed to explain the pairing symmetry in these two types of superconductors in a unified way. It must be determined that only superexchange causes Antiferromagnetic exchange coupling leads to superconductor pairing, and other magnetic exchange effects do not lead to superconductor pairing. Based on this conclusion, they further explained why unconventional high-temperature superconductivity is such a rare phenomenon that both types of materials have special electronic environments that other compounds do not possess. In this environment, d-electron orbitals that participate in the antiferromagnetic superexchange coupling appear separately from the Fermi level independently of other orbitals. In copper-based high-temperature superconductors, due to the octahedral coordination, this environment can only be achieved by the transition metal cation Cu2+, which is filled with nine electrons in the shell of the d atom. In iron-based high-temperature superconductors, due to the tetrahedral coordination, this environment can only be achieved by the transition metal cation Fe2+ in which six electrons are filled in the shell of the d atom. It can be said that this kind of special electronic environment is the electronic structural gene of unconventional high-temperature superconductors. Therefore, finding a new electronic structural gene that satisfies the same conditions can not only discover new possible high-temperature superconductors, but also establish the superconducting mechanism of unconventional high-temperature superconductors.
Based on the above conclusions, they found that an electronic structure environment that satisfies the conditions can be realized in a two-dimensional hexagonal lattice composed of trigonal bipyramid coordination by sharing apex angles. This lattice structure already exists in Mn-based and Fe-based compounds. However, in this structure, the electronic structure environment that satisfies the conditions must be realized by a cation Co2+/Ni3+ having 7 electrons in the d atom shell. They predict that if a new Co/Ni-based material with this structure can be synthesized, it will be a superconductor with d+id pair symmetry, and its highest superconducting transition temperature will exceed that of iron-based superconductors. The material prediction of the above research results is published in Phys. Rev. X 5, 041012 (2015).
The above work was funded by the Chinese Academy of Sciences.
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