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Bismuth Valence Studies of As-Grown Superconducting Bi-Sr-Ca-Cu-O Thin Films with Tc(zero) from 98 K to 66 K

Bismuth valence of Bi-Sr-Ca-Cu-O superconducting film has been studied by X-ray photoemission spectroscopy. As-grown highly oriented Bi-Sr-Ca-Cu-O 2223 single-phase films with various critical temperatures from 98 K to 66 K were prepared by changing the Bi content. The Bi 4f core level electron spectra are decomposed into two components. These components are assigned to BiO and to Bi2O3 which is located at an energy higher than the peak of BiO by about 1 eV. Then it was empirically determined that the critical temperature can be controlled by the Bi mean valence, and changes irrespective of the number of Cu-O planes. The critical temperature of the film increases with a decrease in the Bi mean valence which was calculated from the ratio of BiO/Bi2O3.