Chemical bonding in Tl cuprates studied by x-ray photoemission

Abstract

Epitaxial thin films of the Tl cuprate superconductors ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8},$ ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10},$ and ${\mathrm{Tl}}_{0.78}{\mathrm{Bi}}_{0.22}{\mathrm{Ba}}_{0.4}{\mathrm{Sr}}_{1.6}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{9\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ are studied with x-ray photoemission spectroscopy. These data, together with previous measurements in this lab of ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CuO}}_{6+\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{TlBa}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}},$ 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 ${(E}_{s}\ensuremath{-}{E}_{m})$ and a lower intensity ratio ${(I}_{s}{/I}_{m})$ are found to correlate with higher values of ${T}_{c}.$ Analysis of these spectra within a simple configuration interaction model suggests that higher values of ${T}_{c}$ are related to low values of the $\mathrm{O}2\stackrel{\ensuremath{\rightarrow}}{p}\mathrm{Cu}3d$ charge transfer energy. In the $\mathrm{O}1s$ 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 ${\mathrm{Ba}}^{2+}$ ions. For samples near optimum doping, maximum ${T}_{c}$ is observed to occur when the Tl ${4f}_{7/2}$ binding energy is near 117.9 eV, which is near the middle of the range of values observed for Tl cuprates. Higher $\mathrm{Tl}{4f}_{7/2}$ binding energies, corresponding to formal oxidation states nearer ${\mathrm{Tl}}^{1+},$ are also found to correlate with longer bond lengths between Ba and Tl-O planar oxygen, and with higher binding energies of the $\mathrm{O}1s$ signal associated with Tl-O bonding.