${\mathrm{Bi}}_2{\mathrm{Sr}}_{2-x}{\mathrm{La}}_x\mathrm{Cu}{\mathrm{O}}_{6+\delta }$ and ${\mathrm{Bi}}_{2-y}{\mathrm{Pb}}_y{\mathrm{Sr}}_{2-x}{\mathrm{La}}_x\mathrm{Cu}{\mathrm{O}}_{6+\delta }$ 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 $L_3$-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 $\mathrm{Cu}{\mathrm{O}}_2$-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.

• Received 16 March 2005

DOI:https://doi.org/10.1103/PhysRevB.72.014504