With ARXPS, we investigated the surface layer on c-axis oriented high Tc superconducting YBa2Cu3O7-δ thin films. In the Y 3d feature, no chemical shift has been observed, indicating that Y is present in one chemical state only. In the Ba 3d and Ba 4d features, contributions of Ba components in two different chemical states have been found. We assign these two Ba lines to Ba in the superconducting YBa2Cu3O7-δ material and to Ba in a non superconducting top layer. Also in the O 1s feature, two O lines have been discovered, one belonging to O in the YBa2Cu3O7-δ and one belonging to O in the top layer. From the ARXPS measurements, indications have been found that in the surface layer 30 at. % O is missing. The ARXPS experiments show that the stable surface of c-axis oriented high Tc superconducting YBa2Cu3O7-δ thin films is the Ba-O plain, above the Y and that the superconducting film is covered with a non superconducting top layer with a typical thickness of 1.1 nm, containing Ba- and Cu-oxides and, due to transport through ambient, graphite, some carbonates and hydroxides.
Aarnink, W. A. M., Gao, J., Rogalla, H., & van Silfhout, A. (1990). Composition and thickness of the surface layer on high Tc superconducting YBa2Cu3O7-δ thin films. Journal of the less-common metals, 164–165(Part 1), 321-328. https://doi.org/10.1016/0022-5088(90)90229-D