Conductivity of layered superconducting crystals in the normal state: Case of Bi(2+x)Sr2Ca(1-y)Cu2O(8+delta) - Importance of the oxygen and cationic concentrations, role of superconductive fluctuations above Tc

Resistivity anisotropy is well known, in the normal state, for all the superconductive strongly layered crystals. The first measurements made on Bi-2212 have shown that the out-of-plane resistivity rho-c (resistivity along c) was, at room temperature, about 10 exp 3 to 10 exp 4 times higher than the in-plane resistivity rho-ab (resistivity in the ab plane), revealing a very high degree of anisotropy of the electrical properties for these crystals. Moreover, rho-c was given with a semiconducting behavior and rho-ab with a metallic one. More recent results have shown that rho-c could be metallic and that the rho-c/rho-ab ratio could be greatly reduced, going along with an increase of the density of charge carriers in the valence band, itself related to an increase of the oxidation degree of the crystal (increase of delta). It was also shown that Tc is directly related to the oxygen concentration, a maximum value Tc(max) of Tc being associated with an optimal delta-i value of delta close to 0.25; Tc(sub max) is also probably obtained for an optimal cationic composition, including an excess of Bi and probably a lack of Ca. From our own results Tc(max) can reach 99 K going along with a metallic behavior along c. (Author)