Superconductivity and cu valence of Bi-Sr-Ca-Cu-O thin films

Abstract

Bi-Sr-Ca-Cu-O superconducting films were prepared by rf magnetron sputtering using a target with modulated composition and successive annealing in oxygen. The resistivity and Cu valence of the films varied before and after annealing. The zero-resistance temperature of the films was 10 K before the annealing, and rose up to 96 K (midpoint 105 K) after annealing. Essentially no change in the characteristic layer spacing ($d$ value) of \ensuremath{\cong} 18 \AA{}, corresponding to $\frac{c}{2}$, was observed before and after annealing as seen by x-ray diffraction. The Cu valence of the films was investigated by x-ray photoemission before and after annealing. The photoemission core-level spectra and $\mathrm{LMM}$ Auger emission spectra revealed that the Cu valence after annealing became larger than that obtained before annealing. The superconductivity obviously is correlated with the Cu valence of the films.