The Madelung Energy in Copper-Oxide-Based Ceramics

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The Madelung energy in the (Y, Ca)(Ba, La).Cu,0.., system has been calculated inorder to find the distribution of hole carriers among sites. We regard the compoundas a superconductor, when the lowest-energy distribution is such that holes enter sitesin Cub. sheets, whereas we regard it as a semiconductor, when no holes enter sites inthe Cub. sheets. Based on this criterion, we find a phase diagram, where the supercon-ducting region is separated from the semi-conducting region by a boundary. A goodagreement with the experimental finding by Tokura et al. is obtained, when account istaken of the ordering of exygen vacancies in the basal plane of Fig. 1 in the text. It isfound that for some compounds with small l>( - 0.25), holes are spontaneously pro-duced in Cub. sheets, so that the compounds may be a superconductor although theirnominal hole content is zero.
社団法人日本物理学会の論文
1988-12-15