CRYSTALLINE PHASES AND ELECTRONIC-STRUCTURES IN SUPERCONDUCTING BI-SR-CA-CU OXIDES

Two classes of samples, designated A and B, of layered Bi-Sr-Ca-Cu oxi des having the same nominal composition 4:3:3:4, but different thermal histories, were investigated by using field modulated microwave absor ption (ESR), powder x-ray diffraction (XRD), x-ray photoelectron spect roscopy (XPS), and x-ray absorption near the edge structure (XANES). P revious electrical resistivity measurements showed that the B samples presented only two superconducting phases with midpoints of the transi tion temperatures at similar to 80 K and similar to 105 K. The microwa ve absorption technique indicated instead the presence of islands whic h became superconducting at the above-mentioned temperatures also in t he A samples. The crystalline and electronic structures of the two typ es of samples are illustrated and discussed. A plausible theoretical i nterpretation of the experimental results, based on a quantum percolat ion model with Coulomb interaction, is also given.