Chemical bonding in Tl cuprates studied by x-ray photoemission

Epitaxial thin films of the Tl cuprate superconductors Tl₂Ba₂CaCu₂O₈, Tl₂Ba₂Ca₂Cu₃O₁₀, and Tl0.78Bi0.22Ba0.4Sr1.6Ca₂Cu₃O_9-δ are studied with x-ray photoemission spectroscopy. These data, together with previous measurements in this lab of Tl₂Ba₂CuO_6+δ and TlBa₂CaCu₂O_7-δ, comprise a comprehensive data set for a comparative study of Tl cuprates with a range of chemical and electronic properties. In the Cu $2p$ spectra, a larger energy separation between the satellite and main peaks (Eₛ-Eₘ) and a lower intensity ratio (Iₛ/Iₘ) are found to correlate with higher values of Tc. Analysis of these spectra within a simple configuration interaction model suggests that higher values of Tc are related to low values of the O2→pCu3d charge transfer energy. In the O1s region, a smaller bond length between Ba and Cu-O planar oxygen is found to correlate with a lower binding energy for the signal associated with Cu-O bonding, most likely resulting from the increased polarization screening by Ba²⁺ ions. For samples near optimum doping, maximum Tc is observed to occur when the Tl 4f7/2 binding energy is near 117.9 eV, which is near the middle of the range of values observed for Tl cuprates. Higher Tl4f7/2 binding energies, corresponding to formal oxidation states nearer Tl¹⁺, are also found to correlate with longer bond lengths between Ba and Tl-O planar oxygen, and with higher binding energies of the O1s signal associated with Tl-O bonding.