Cu valence and the formation of high T c superconductor oxides studied by x-ray photoemission spectroscopy on 200 Å Bi-Sr-Ca-Cu oxide thin films

Abstract

We have used x‐ray photoemission spectroscopy to study the core level spectra of 200 Å Bi‐Sr‐Ca‐Cu oxide thin films, in the hope that the importance of the surface sensitivity of photoemission is minimized for these samples, whose thickness is only 6–7 unit cells. The samples were annealed in oxygen at temperatures ranging from 600 to 870 °C, over which temperature range the thin films are converted from an amorphous insulating phase to an ordered superconducting phase with T_c (R=0)=80 K. Regardless of the annealing temperature, the Cu 2p spectra of all films show satellite structure indicative of Cu²⁺ states. Even the spectrum of an as‐deposited film has a satellite similar to those observed for the annealed films, including the superconducting film. The similarity of the satellite structure in the core level spectra for all films suggests that the Cu‐O interactions, such as the Cu d‐O p orbital charge transfer energies and the hopping integrals, remain similar when the oxides undergo the insulator‐superconductor transition through high‐temperature anneal. That is, the same local Cu‐O interaction exists before and after the superconducting phase sets in. This implies that the Cu valence alone does not determine the properties of high T_c superconductors.