Pressure effect on the 2D-electronic state and the superconductivity in oxide superconductors

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Unconventional superconductivity is frequently found at adjacencies of the magnetic ordering related to strong electron-electron correlation. It is known that the normal state is characterized by strong anisotropy and strong electron-electron correlation, as is representatively seen in high-TィイD2cィエD2 cuprates. To understand the effect of these characters in normal state on the superconductivity, we perform pressure experiments in oxide superconductors.(1) Possibility of pressure induced ferromagnetism in SrィイD22ィエD2RuOィイD24ィエD2As is known pressure, which can control the electron-electron correlation is suitable technique to discover the magnetic grand state in SrィイD22ィエD2RuOィイD24ィエD2 system. We measure temperature and pressure dependence of resistivity in SrィイD22ィエD2RuOィイD24ィエD2. A pressure induce hump at around 40K is found in the temperature dependence of in-plane resistivity under 1.5-5GPa. Moreover, the resistivity shows TィイD14/3ィエD1 -dependence after suppressing the hump under pr … More essure over than 5GPa. These behaviors strongly indicate a pressure-induced ferromagnetism.(2) Role of two-dimensional electronic state in superconductivity in LaィイD22-xィエD2SrィイD2xィエD2CuOィイD24ィエD2Strong two-dimensionality, which is after expressed in exotic behavior of the c-axis resistivity, is a key character of oxide superconductors, though low dimensionality is generally known as a destructive factor for conventional superconductivity. To explore how the two-dimensionality in the electronic state is reflected in TィイD2cィエD2, we have measured the c-axis resistivity for LaィイD22-xィエD2SrィイD2xィエD2CuOィイD24ィエD2 under anisotropic pressure. The c-axis compression, which decreases the c-axis resistivity, reduces TィイD2cィエD2 drastically, whereas c-axis extension, which increases the c-axis resistivity, enhances TィイD2cィエD2 from 38K at ambient pressure to 51 .6K at 8GPa. We find that the variation of TィイD2cィエD2 scales as a function of the c-axis resistivity, and that the c-axis pressure coefficient is much stronger than the ab-axis one, These imply that TィイD2cィエD2 depends primarily on the interlayer, rather than the in-plane, lattice parameter. Less