The remarkable discovery of high-Tc superconductivity and the following enthusiastic research in the last decade have clearly exemplified how the finding of new materials would give a great impact on the progress of solid state physics. Now the related topics are spreading over not only superconductivity but also unusual metallic behavior which is generally seen near the metal-insulator transition in the strongly correlated electron systems. We believe that for the next few decades it will become more important to explore novel physics through searching for new materials. Transition-metal oxides are one of the most typical systems where the effect of Coulomb interaction plays a critical role on their magnetic and electronic properties. Especially interesting is what is expected when electrons localized due to the strong Coulomb repulsion start moving by changing the bandwidth or the number of carriers. We anticipate there an unknown, dramatic phenomenon governed by quantum fluctuations. One of the topics we study now is a quantum spin lattice based on triangle geometry where a magnetic frustration tends to suppress ordinary long-range order and may lead to an unusual spin liquid ground state. Recently, we found superconductivity for the first time in the pyrochlore oxides Cd2Re2O7 and AOs2O6 (A = K, Rb, Cs) which crystallize in such a three-dimensionally frustrated lattice.
Superconducting transitions observed in resistivity for the β-pyrochlore oxide superconductors AOs2O6. The Tc is 3.3 K, 6.3 K and 9.6 K for A = Cs, Rb and K, respectively.
*High-Field Phase Diagram and Spin Structure of Volborthite Cu3V2O7(OH)2⋅2H2O: M. Yoshida, M. Takigawa, S. Krämer, S. Mukhopadhyay, M. Horvati´c, C. Berthier, H. Yoshida, Y. Okamoto and Z. Hiroi, J. Phys. Soc. Jpn.81 (2012) 024703 (1-9).