Electronic structure of the Cu3O4 plane of Ba2Cu3O4Cl2:: Experiment and theory

We present a detailed joint experimental and theoretical investigation of the valence band electronic structure of single crystals of the model cuprate Ba2Cu3O4Cl2. This oxychloride system possesses a Cu3O4 plane which can be regarded as a superposition of two subsystems: a CuAO2 cuprate plane and an extra Cu-B site, and thus represents an ideal trial system for assessing the impact of deviations from the CuO2 plane stoichiometry upon the electronic structure in cuprate materials. From polarization-dependent, k-resolved photoemission measurements the dispersion relations and symmetry of a number of the observed valence band features are determined and compared with the results of band structure calculations carried out within the LSDA+U formalism, which include a detailed analysis of the character and symmetry of the individual bands. Upon electron removal, the extra copper site makes its presence felt via the formation of a second Zhang-Rice singlet located on the Cu-B subsystem, in addition to that originating from the CuAO2 subsystem. The main valence band edge at similar to 2 eV binding energy is shown to be mainly due to bands involving combinations of O 2p(x,y) orbitals, some of which exhibit essentially pure O 2p character. These low-lying oxygen bands are different in origin from the nonmixing O 2p states observed, for example, at (pi,pi) in Sr2CuO2Cl2, as the orbital combination responsible for the latter is involved in Ba2Cu3O4Cl2 in the formation of the Zhang-Rice singlet state on the Cu, sublattice. [S0163-1829(98)00717-6].