A Survey of Superconducting Cuprates

Abstract

The cuprate superconductors are near to a metal-insulator transition. The physical properties of these materials are determined mainly by the charge carrier concentration. In the high-temperature superconductors with T c -values above 77 K, the charge carriers are defect electrons (holes), whose concentration is intimately connected to oxygen non-stoichiometry, cation disorder and cation doping effects. CuO2 planes are a common structural feature of all cuprate superconductors. The oxygen atoms occupy perovskite-like anion positions halfway between the copper atoms. In compounds with more than one CuO2 layer in the perovskite blocks, di- or tri-valent ions (Ca, Y or rare earth elements) are inserted between adjacent CuO2 sheets. Because of the common CuO2 planes, the a and b lattice parameters of cuprate superconductors are typically close to 0.39 nm. Double or single layers of BiO, TlO and HgO combined with BaO or SrO layers separate neighbouring CuO2blocks. Defects in these layers provide charge carriers for the doping of the CuO2 planes. Therefore, these layers are called charge carrier reservoirs. High pressure synthesis allows the introduction of additional elements such as C, Cu, Al, Ga and B into these charge carrier reservoirs. Typically, an inverse parabolic dependence of T c on the number of holes per CuO2 unit is observed. Perovskite-like blocks with 3 or 4 CuO2 sheets seem to favour a high critical temperature. The application of hydrostatic pressure typically leads to enhanced T c -values for hole-doped high-temperature superconductors. The considerably different maximum T c -values for Y-, Bi- and Hg-based cuprate superconductors may be connected to structural features of these compounds. The nearly plane CuO2 sheets in the Hg-based high-Tcsuperconductors seem to favour a high critical temperature. On the other hand, the buckling of the CuO2 planes in Y-123, and the incommensurate modulations present in Bi-based superconductors may be reasons for the moderate T c -values of these compounds.