Photoemission on highT _c superconductors

Abstract

XPS and UPS photoemission results on the new high-T _c oxide superconductors La_2−xMe_xCuO₄ (Me=Sr, Ba) with a maximumT _c≈40 K show the following: 1) the electronic density of states atE _F is small (smaller than in pure Cu); the work function is metallic like with a value around 4.2 eV. 2) The Cu-2p spectra show hardly a possible small tendency to a mixed valent Cu³⁺/Cu²⁺state with increasing Me content, but smaller than expected from the Me²⁺ concentration suggesting that oxygen defects are responsible for the charge compensation. 3) The core level spectra of La, Me, and O show satellites which are at least partially related to O²⁻ defects and which change in intensity with increasing surface sensitivity of the experiments. First core-level spectra on y_1.2Ba_0.8CuO₄ with aT _c ≈ K indicate a complicated composition of these new compounds.