The total spectral weight textit{S} of the emergent low-energy quasipaticles in high-temperature superconductors is explored by x-ray absorption spectroscopy. In order to examine the applicability of the Hubbard model, regimes that cover from zero doping to overdoping are investigated. In contrast to mean field theory, we found that textit{S} deviates from linear dependence on the doping level textit{p}. The slope of textit{S} versus textit{p} changes continuously throughout the whole doping range with no sign of saturation up to textit{p} = 0.23. Therefore, the picture of Zhang-Rice singlet remains intact within the most prominent doping regimes of HTSCs.
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