Experimental hole densities in HgBa2Can-1CunO2n+2+delta compounds from near-edge x-ray-absorption spectroscopy

Near-edge x-ray-absorption spectroscopy measurements have been performed on the O K'- and Cu L-absorption edges on a series of underdoped and optimally doped HgBa2Can-1CunO2n+2+delta samples in order to get quantitative information on the changes in the electronic structure under doping with excess O. The density of doped holes with O 2p character per CuO2 plane per unit cell in the optimized samples is found to be close to that of other p-type doped superconducting cuprates with optimized T-c. There is also some evidence for a decreasing density of theses doping-induced O 2p hole states per CuO2 plane per unit cell with increasing number n of CuO2 sheets for the samples with optimized T-c. Finally, a quantitative analysis of the experimental data leads to doping levels in the Hg systems that are half of what is expected in a simple ionic picture.