Electronic structure ofTlBa2CaCu2O7−δ

Abstract

The core levels of ${\mathrm{TlBa}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (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 (${\mathrm{E}}_{\mathrm{F}}$) for the stoichiometric compound (\ensuremath{\delta}=0), while for 50% oxygen vacancies in the Tl-O layer (\ensuremath{\delta}=0.5) ${\mathrm{E}}_{\mathrm{F}}$ 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 ${\mathrm{T}}_{\mathrm{c}}$, consistent with a shift of ${\mathrm{E}}_{\mathrm{F}}$ closer to the VHS. Comparisons are made to the core levels and valence bands of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Tl-2212) and ${\mathrm{HgBa}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ (Hg-1212). The similarity of the Cu ${2\mathrm{p}}_{3\mathrm{/}2}$ 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 ${2\mathrm{p}}_{3\mathrm{/}2}$ signals exhibit differences which suggest that the replacement of ${\mathrm{Tl}}^{3+}$ with ${\mathrm{Hg}}^{2+}$ results in a decrease in the O 2p\ensuremath{\rightarrow}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.