Valence-band states in ${\mathrm{Bi}}_2$(Ca,Sr,La${)}_3$${\mathrm{Cu}}_2$${\mathrm{O}}_8$

We have used photoemission spectroscopy to examine the symmetry of the occupied states of the valence band for the La-doped superconductor ${\mathrm{Bi}}_2$(Ca,Sr,La${)}_3$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_8$. While the oxygen states near the bottom of the 7-eV wide valence band exhibit predominantly O 2$p_z$ symmetry, the states at the top of the valence band extending to the Fermi level are found to have primarily O 2$p_x$ and O 2$p_y$ character. We have also examined anomalous intensity enhancements in the valence-band features for photon energies near 18 eV. These enhancements, which occur at photon energies ranging from 15.8 to 18.0 eV for the different valence-band features, are not consistent with either simple final-state effects or direct O 2s transitions to unoccupied O 2p states.