Evolution of the density of states at the Fermi level of Bi_2−yPb_ySr_2−xLa_xCuO_6+δ and Bi₂Sr_2−xLa_xCuO_6+δ cuprates with hole doping

Bi₂Sr_2−xLa_xCuO_6+δ and Bi_2−yPb_ySr_2−xLa_xCuO_6+δ high-T_c superconductors in a wide doping range from overdoped to heavily underdoped were studied by x-ray absorption and photoemission 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 unoccupied 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.