The electronic conductivity mechanism in Bi-Sr-Ca-Cu-O glass-ceramics

The results of electrical conductivity measurements in Bi2O3-SrO-CaO-CuO gl ass-ceramics are reported. It is shown that a sharp increase in the conduct ivity observed at 460 degrees C is attributed to the crystallization of the Bi2Sr2CuO6 (Bi-2201) superconducting phase in the glass matrix. The mechan ism of electrical conductivity in this glass-ceramic can be explained as a result of discontinuous metallic structures of the Bi-2201 phase which appe ar in the glass matrix during annealing. The low-temperature conductivity o beys the law sigma(T) = sigma(o) exp[(-T-o/T)(n)] with n changing from 1/4 to 1/2 with decrease in the temperature. This cross-over is related to a gr anular structure in which a parabolic Coulomb gap exists in the density of states N(E) near E-F. We also show that the superconductivity effect should be taken into account for T-1/2 conductivity behaviour. It is shown that t he Bi-Sr-Ca-Cu-O glass after a very short heat treatment at 820 degrees C e xhibits the superconducting transition. In the normal state the conductivit y is typical for granular metals and the activation energy continuously dec reases with decreasing temperature.