X-Ray Photoelectron Study of Surface of Superconductive Y-Ba-Cu-O as Dependent on Cleaning, Temperature, Composition and Doping

The core level X-ray photoelectron spectra of the high-temperature superconductor system YBa 2 Cu 3 O x are studied in dependence on the oxygen composition x, temperature of the sample, doping by Ag and quality of cleaning the sample surface. Nowadays there are many X-ray photoelectron investigations, but comparative investigations of electron structure in superconductive and unsuperconductive states are absent in the majority of them. The X-ray electron magnetic spectrometer used in our work allows one to obtain spectra in the temperature range from room temperature to the temperature of liquid nitrogen with simultaneous mechanical cleaning of the sample surface from the contaminations and degraded layers. Thus the obtained results are unique and allow one to obtain for the first time the following conclusions. Three charge states of copper atoms are shown to exist in this system — Cu 1+, Cu 2+ and Cu 3+ — but only the Cu 2+ state plays the main role in the transition of the system to the superconductive state. New peaks arise in O1s, Ba3d and Ba4d spectra after the transition of the system to the superconductive state, which implies the changes in the surroundings of O and Ba atoms in this state. In Ag-doped samples similar changes in X-ray photoelectron spectra are observed and they are even more pronounced than in the undoped ones.