Electronic structure

The core levels of (Formula presented)(Formula presented)(Formula presented) (Tl-1212) epitaxial films have been measured with x-ray photoelectron spectroscopy (XPS). The valence electronic structure has been determined using the full-potential linear muffin-tin-orbital band-structure method and measured with XPS. The calculations show that a van Hove singularity (VHS) lies above the Fermi level ((Formula presented)) for the stoichiometric compound (δ=0), while for 50% oxygen vacancies in the Tl-O layer (δ=0.5) (Formula presented) is in close proximity to the VHS. Samples annealed in nitrogen (to reduce the hole overdoping by the removal of oxygen) exhibit higher core-level binding energies and a higher (Formula presented), consistent with a shift of (Formula presented) closer to the VHS. Comparisons are made to the core levels and valence bands of (Formula presented)(Formula presented)(Formula presented)(Formula presented) (Tl-2212) and (Formula presented)(Formula presented)(Formula presented) (Hg-1212). The similarity of the Cu (Formula presented) spectra for Tl-1212 and Tl-2212 indicates that the number of Tl-O layers has little effect on the Cu-O bonding. However, the Tl-1212 and Hg-1212 Cu (Formula presented) signals exhibit differences which suggest that the replacement of (Formula presented) with (Formula presented) results in a decrease in the O 2p→Cu 3d charge-transfer energy and differences in the probabilities of planar vs apical oxygen charge transfer and/or Zhang-Rice singlet-state formation. Differences between the Tl-1212 and the Tl-2212 and Hg-1212 measured valence bands are consistent with the calculated Cu 3d and (Tl, Hg) 6s/5d partial densities of states.