Conductivity of layered superconducting crystals in the normal state:: Case of Bi2+xSr2Ca1-yCu2O8+δ -: 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. It has been studied by many authors for Bi-2212 crystals (of real composition: Bi2+xSr2Ca1-yCu2O8+delta) which can be obtained easily. The first measurements made on Bi-2212 have shown that the out-of-plane resistivity pc (resistivity along c) was, at room temperature, about 10(3) to 10(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 pc was given with a semiconducting behaviour and rho(ab) with a metallic one. More recent results have shown that pc 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 shown also that Te is directly related to the oxygen concentration, a maximum value Tc-max of Tc being associated to an optimal delta(i) value of delta close to 0.25; Tcmax is also probably obtained for an optimal cationic composition including an excess of Bi and probably a lack of Ca. From our own results Tcmax can reach 99K going along with a metallic behaviour along c. Out of the interesting case of highly oxygen doped crystals, the out-of-plane resistivity of Bi-2212 crystals is semiconducting like. But the increase of resistivity with reducing temperature, above Tc, is quicker than expected from the Arrhenius law. This point is at the origin of many studies. Some authors relate this excess of out-of-plane resistivity, above Tc, to superconductive fluctuations, while it is unanimously accepted that superconductive fluctuations reduce the in-plane resistivity of crystals. This point will be discussed in details and another possible origin of this phenomenon will be given.