Phy$icaC 162-164 (1989) 1385-1386 North-Holland SURFACE AND MIXED VALENCE EFFECTS IN XP8 8PIICTRA FROM YBCO F. Parmigiani; G. Samoggia; C. Calandra CISE SpA, Segrate, Italy; Universitb di Pavia, Italy. Italy; Universitb di Modena, We report X-ray photoemission data from YBa2Cu307 showing that the measured composition and the electronic structure depend significantly upon the photo- electron escape depth. For large take-off angles the valence band spectrum near the Fermi edge is well structured and shows a number of features,arising from Cu- O hybrid states. Their intensities are drastically reduced in the energy distribution curves measured at grazing angles. The results are interpreted assuming that the surface is mainly composed by Ba-O planes. This interpretation is shown to be consistent with etectronic structure calculations for the bulk and the Ba-O surface. After the discovery of high Tc super- conductivity in copper oxide materials (HTSCs) a large number of papers have appeared dealing with their electronic structure. In spite of the great effort done on this subject, some disagreements still exist (1-2). The aim of this paper is to demonstrate that the disagreement between the experimental data is mainly due to the difference between the surfa- ce and bulk stoichiometry. In particular the surface appears to be richer in Ba than the bulk, while no significant cha- nge in the oxygen content is observed. To understand these observations we have performed a theoretical calculation of the surface electronic structure of YBa2CusO~ assuming a Ba-O4 termination of the crystal. To make the results com- parable with the experimental data we evaluate self-energy effects using an approximate approach to treat the electron correlation. Details of the YBCO sample preparation are given in Ref.3. The XPS apparatus and the experimental procedures have be- en discussed in a previous work (4}.The sample stoichiometry, calculated from Cu2p, 01s, Ba3d and Y4d core level lines taken at different take off angles show- ed near the surface a lower copper con- tent. The observed gradient of the Ba/Cu ratio, reported also by other authors,is here attributed to the fact that the Ba- O plane is the most stable. In the light of these observations the valence band measurements were also performed at a take-off angle of 90 ° and 15 ° using the monochromatized A1 Ks line and a final resolution of the analyzer of z0.35 eV. Fig.la reports the valence band spectrum between 0 and 18 eV of a sintered YBCO sample measured at 90". Beside the structures P4-Pa, already observed and widely discussed in the literature( j-2~, the salient features are a first band (Pt) centered at about 0.7 eV followed by two other structures at about 1.6 (P2) and 2.3 (Ps) respectively. The in- tensity of these lines decreases signi- ficantly when the valence band measure- ments are performed at 15", i.e. in a Cu deficient region, as shown in Fig.lb. Therefore, it is possible to infer that the Pl, Pz and P$ structures arise from states having a significant Cu contribu- 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)