ROLE OF OUT-OF-PLANE COPPER AND THALLIUM ORBITALS IN THALLIUM CUPRATE

Simultaneous total and fluorescence yield XAS measurements on Cu L-3-e dge and, for the first time, total yield measurements at Tl L-3-edge o f well oriented Tl(2212) thin films are reported in this work. Also, C u L-3 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 cupra tes al least for overdoped samples. Our results suggest that use of a bulk sensitive technique is necessary in order to get a reliable estim ation of slates having Cu 3d(z2-r2) character and thus a precise knowl edge of sample stoichiometry. No direct correlation is found between T c and out-of-plane covalent and doping hole densities such that theori es based on the existence of large fraction of there out-of-plane unoc cupied states are ruled out. To understand the role of out-of-plane co pper orbitals, polarization dependent measurements of Cu L-3-absorptio n edge on well characterised Tl(2212) thin films have been performed. The density of unoccupied states having 3d(z -r)(2) 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 f ound to be doping dependent. The probable reason for the observed shif t is discussed.