Surface stoichiometry and valence electronic structure of YBa2Cu3O7-x

We report x-ray photoemission data from YBa2Cu3O7-x showing that the measured composition and the electronic structure changes significantly upon the photoelectron 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. These results are interpreted assuming that the surface is mainly composed by Ba-O planes. This interpretation is shown to be consistent with electronic structure calculations for the bulk and for a YBa2Cu3O7-x surface terminated with a Ba plane and produced by cutting the Cu2 O4 bond normal to the c axis. The local density of states calculated on this basis and taking into account electron correlation effects is shown to be consistent with many significant behaviors experimentally observed in the YBa2Cu3O7-x valence band