Photoemission valence band spectra and electronic density of states in copper oxides and copper based ceramic super-conductors

Photoemission valence band spectra with three different photon energies (21.2 eV, 40.8 eV, 1253.6 eV) for Cu₂O and CuO and a number of copper oxide based superconducting ceramics are investigated, namely La₂CuO₄ (LCO), Y₁Ba₂Cu₃O₇ (YBCO), Bi₂Sr₂CaCu₂O₈ (BSCCO) and Tl₂Ba₂Ca₂Cu₃O₁₀ (TBCCO). From the Cu₂O data it can be infered, that the He-II (40.8 eV) spectra give a fair representation of the density of states (DOS). In addition the agreement between the calculated density of states (DOS) and the He-II photoemission spectrum is almost perfect for Cu₂O. This agreement is worse for CuO and the other materials which all have a Cu groundstate close to a 3d ⁹ configuration indicating a large contribution of thed-d correlation energy to the excitation spectra which is absent in Cu₂O because of the filled 3d-shell. In all cases the experimental DOS atE _F is very small and only the spectra of BSCCO show a well defined Fermi edge. The relevance of these findings with respect to the theoretical local functional density DOS calculations is discussed.