Ion displacement and electron-hole localization in perovskite-like layeredcuprates

The electron-hole localization can be realized on a boundary ab plane of la yered crystals if the ion charge e(i) and the c-direction force constant k( i) satisfy the relation of (1/S)Sigma(i is an element of V)(e(i)(2)/k(i))gr eater than or equal to 0.3 Angstrom, where S and V represent the boundary s urface and the layer volume, respectively. In many perovskite-like layered cuprates, the relation is satisfied mainly due to the smallness of k(Cu) co mpared to other k(i)'s in typical perovskite crystals. This indicates the p ossibility of the electron-hole localization on the CuO2 plane, where high- T-c superconductivity can be realized.