Resonant photoemission from the surface of a high-temperature Y---Ba---Cu---O superconductor

Photoemission measurements using synchrotron radiation are reported on YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-x</SUB> having a superconducting transition temperature of 90 K. The experiments were performed over a photon energy range of 40-161 eV as well as using Mg X-rays. An unusual enhancement of photoemission intensity associated with Ba(5p) and Ba(5s) levels was observed for a photon energy of ~104 eV. Our results suggest a different chemical composition of the surface and volume, two types of oxygen sites in the material, and a predominantly monovalent state of copper after Ar<SUP>+</SUP> etching