  X-ray absorption spectra on the overdoped high-temperature superconductors
Tl_2Ba_2CuO_{6+delta} (Tl-2201) and La_{2-x}Sr_xCuO_{4+delta} (LSCO) reveal a
striking departure in the electronic structure from that of the underdoped
regime. The upper Hubbard band, identified with strong correlation effects, is
not observed on the oxygen K edge, while the lowest-energy prepeak gains less
intensity than expected above p ~ 0.21. This suggests a breakdown of the
Zhang-Rice singlet approximation and a loss of correlation effects or a
significant shift in the most fundamental parameters of the system, rendering
single-band Hubbard models inapplicable. Such fundamental changes suggest that
the overdoped regime may offer a distinct route to understanding in the
cuprates.
