Role of out-of-plane copper orbitals in thallium cuprate

To understand the role of out-of-plane copper orbitals, polarization dependent measurements of Cu L₃-absorption edge on well characterized 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. {} 1996 The American Physical Society.