Temperature and doping dependence of x-ray absorption spectral weight in YBa$_2$Cu$_3$O$_y$*

The comprehensive study of the temperature dependent x-ray absorption spectroscopy (XAS) could be attributed to a dynamical spectral weight $\alpha$ in YBa$_2$Cu$_3$O$_y$ (YBCO). Large spectral weight changes with the temperature for both the Upper Hubbard band and the Zhang-Rice band due to dynamics of holes are experimentally found in the underdoped regime. These spectral weight changes become larger when the doping level $p$ goes deeper into the underdoped regime, but quickly vanishes as $p$ goes to the undoped limit. Our results clearly indicate that the pseudogap is related to the double occupancy and originates from bands in higher energies.