HIGH Tc SUPERCONDUCTORS AND RELATED OXIDES : HIGH PRESSURE PHASES. Mikio Takano, Institute for Chemical Research, Kyoto University.

High T_c cupric oxide superconductors and related quantum antiferromagnets prepared at high pressures of 6GPa typically will be reviewed.

The use of high pressure generally leads us to new phases through pressure-induced changes in structure and/or chemical composition of ambient pressure phases and also through stabilization of novel compositions and structures. All these cases can be seen for cupric oxides.

Superconductors prepared at high pressure are featured by the multiplicity of the 2D CuO₂ sheets (e.g. (Sr, Nd)CuO₂ and CuBa₂Ca₅Cu₆O₁₄), inclusion of 2D CuO₂ sheets capped with halogen atoms (e.g. (Ca,Na)CuO₂Cl₂ and Sr₂Ca₂Cu₃O₆F₂), etc. Many other new phases will be prepared in near future.

The speaker's group is now more involved in a research of spin-ladder compounds. SrCu₂O₃ and LaCuO_2.5 prepared at 3-6GPa contain two-leg ladders, the legs and the rungs of which are CuO chains and Cu-O-Cu bonds, respectively. The electronic ground state of SrCu₂O₃ has been found to be a singlet spin liquid, which is separated from the first excited magnetic state by a wide "spin gap" of 420 K. On the other hand, interladder interactions cause long range magnetic ordering in the latter oxide, though the ordering is easily destroyed by the substitution of Sr²⁺ for La³⁺. Novel physics of quantum spin ladders is progressing fast