Temperature-dependent spectral weight transfer in YBa$_2$Cu$_3$O$_x$ probed by x-ray absorption spectroscopy

The x-ray absorption spectroscopy was utilized to critically examine the temperature dependency of the spectral weight in YBa$_2$Cu$_3$O$_x$. Large excess spectral weight for the Zhang- Rice singlet due to dynamics of holes is found with its doping dependence showing similar doom-like shape as that for $Tc$. Furthermore, appreciable spectral weight transfer from the upper Hubbard band to Zhang-Rice singlet was observed as the temperature acrosses the onset temperature for the pseudogap. The observed spectral weight transfer follows the change of the pseudogap, indicating a strong link between pseudogap and the upper Hubbard band.