Search for Novel Superconductors with High-Tc in the Ru Oxide System and Possibility for Coexistence of Magnetism and Superconductivity

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It is quite important to find out new compounds which show new physical properties, leading to a possible breakthrough in the field of solid state sciences. In this context, the role of chemists is very important in this research field.We have investigated the syntheses and chemical and physical properties of various 4d and 5d transition metal oxide systems, and tried to find new and novel compounds which show new-type of quantum phenomena, especially, superconducting and metal-insulator transition compounds.As a result, recently, the Pyrochlore oxide Cd_2Re_2O_7 has been found to exhibit superconductivity at about 1 K, which is a firstly-found superconductor in the frustrated-spin system. We also found a pseudo-gap behavior in Cd_2Re_2O_7 below T^* of about 200K which resembles the spin-gap behavior in the cuprate high-T_c systems.We have also found new-type compounds with interesting physical and chemical properties in this research program as follows: EuSr_2RuCu_2O_8 (coexistence of ferromagnetism and superconductivity), Tl_2Ru_2O_7 (Metal-Insulator transition and spin-gap behavior below T_<M-1>), Bi_2Ru_2O_7 and Pb_2Ru_2O_7 (Pauli paramagnetism with antiferromagnetic spin fluctuations). Now, further study of physical properties of these compounds including spin dynamics is still in progress in detail, and will be reported elsewhere.