Control of the charge inhomogeneity and high-Tc superconducting properties in homologous series of multi-layered copper oxides

A systematic view stemmed for the most important parts from the authors’ own research results is presented for tailoring high-Tc superconductive copper oxides from the standpoint of materials chemistry. The present article first elucidates that, in the multi-layered superconductive copper-oxide structures presently known to exist, the distribution of charge/charge carriers is inhomogeneous over several different spatial dimensions throughout the crystal. At the same time useful guiding rules are derived based on the concepts of tolerance parameter and bond-valence for controlling the charge distribution. Then experimental and semi-experimental evidences are presented to establish the relationships between the inhomogeneous distribution of holes and superconducting properties. To reach this goal, phenomenological aspects of the high-Tc superconductive copper oxides are described systematically, and selected important techniques for estimating the local carrier concentration and distribution are discussed. That is, the following points are included in the present article: (i) general crystallographic categorization and naming scheme of multi-layered copper oxides based on the concept of homologous series, (ii) crystallographic and chemical factors to control the oxygen non-stoichiometry and charge distribution, (iii) techniques for probing the charge/carrier distribution in the layered copper-oxide crystal, and (iv) empirical relationships between the carrier distribution and superconducting properties including the superconductivity transition temperature, Tc, the magnetic irreversibility field, Hirr, and the so-called peak effect/fishtail phenomenon. Finally, the unified view is summarized and some indespensable questions to be still answered in future work are mentioned.