Role of Out-of-Plane Copper and Thallium Orbitals in Thallium Cuprate

Abstract
Simultaneous total and fluorescence yield XAS measurements on Cu L₃-edge and, for the first time, total yield measurements at Tl L₃-edge of well oriented Tl(2212) thin films are reported in this work. Also, Cu L₃ and O K-edge measurements on overdoped Tl(2212) thin films using bulk sensitive fluorescence yield detection mode are reported showing that large out-of-plane hole densities do exist in the thallium cuprates at least for overdoped samples. Our results suggest that use of a bulk sensitive technique is necessary in order to get a reliable estimation of states having Cu 3d_z²_r² character and thus a precise knowledge of sample stoichiometry. No direct correlation is found between Tc and out-of-plane covalent and doping hole densities such that theories based on the existence of large fraction of these out-of-plane unoccupied states are ruled out. To understand the role of out-of-plane copper orbitals, polarization dependent measurements of Cu L₃-absorption edge on well characterised Tl(2212) thin films have been performed. The density of unoccupied states having 3d_z²_-r² character is found to depend on the amount of doping suggesting that out-of-plane copper orbitals essentially play a role of hole reservoir. The energy shift in the positions of white lines of E//(a,b) and E//c spectra is also found to be doping dependent. The probable reason for the observed shift is discussed.