Research on Spinel Trpe Cu sulfide Superconductors

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Superconductivity of Cu oxide high temperaturu superconductors is based on the electronic state of Cu(3d^9)-O(3p^6).To extend the understanding about the origin of the superconductivity, it is quite interesting to find basic properties of Cu(3d^9)-S(3p^6) orbits of Cu sulfide superconductors with low anisotropy. We performed exploratory work about various spinel type Cu sulfide superconductors, and found that the Cu_<1+X>Co_<2-X>S_4(x=0-0.5) system with a low spin state of Co^<3+>(3d^6) shows antiferromagnetic behavior at 20K and superconductor transition at 5K.Research activity was focused on this system and related Cu salfide materials of (Cu, M)Co_2S_4(M=Ni and Co), Cu[Cu_XRh_<2-X>]S_4 and CuV_2S_4. Hard work was needed to develop reproducible process of sample synthesis, and was a measure reason for the research schedule postponed a year. In fact, even today, the sample is produced in our laboratory only in the world in spite the unique material was found 5 years ago. The high quality sample are now available, and the magnetism and superconductivity data are being renewed well.Inthis program, material synthesis, crystallographic data, electronic conductivity, superconductivity and microscopic NMR data were studied with the collaborate teams and obtained very effective yields. Material synthesis and the basic properties were done in Ehime University, some low temperture measurements in Solid State Technical Institute for Tokyo University, standard NMR study in Tokushima University and advanced NMR study in Kobe University. As the result, various unique phenomena and physics were obtained to explain the superconductivity, spin correlation, and electron correlation in the Cu(3d)-S(3p) system, and we have published 12 major technical papers.Low temperature specific heat, neutron diffraction, Seebeck effect and higt-pressure effect are also being studied and stimulating results are being given. These are targeted to explore in near future.