Evolution of the density of states at EF of Bi2-yPbySr2-xLaxCuO6+d and   Bi2Sr2-xLaxCuO6+d cuprates with hole doping
  Bi2Sr2-xLaxCuO6+d and Bi2-yPbySr2-xLaxCuO6+d high-Tc superconductors in a wide doping range from overdoped to heavily underdoped were studied by X-ray absorption and photo-emission spectroscopy. The hole concentration p was determined by an analysis of the Cu L3-absorption edge. Besides the occupied density of states derived from photoemission, the un-occupied density of states was determined from the prepeak of the O K-absorption edge. Both, the occupied as well as the unoccupied density of states reveal the same dependence on hole doping, i.e. a continuous increase with increasing doping in the hole underdoped region and a constant density in the hole overdoped region. By comparing these results of single-layer BSLCO with previous results on single-layer LSCO it could be argued that besides the localized holes on Cu sites the CuO2-planes consist of two types of doped holes, from which the so-called mobile holes determine the intensity of the prepeak of the O 1s absorption edge. 
M. Schneider, R.-S. Unger, R. Mitdank, R. Mueller, A. Krapf, S.   Rogaschewski, H. Dwelk, C. Janowitz, R. Manzke