The Coulomb repulsion for two holes in the localized Cu 3d orbitals Udd= 6 eV is measured by X-ray photoelectron spectroscopy
"Early photoelectron core-level spectra (XPS and UPS) by Fujimori et al. (1987) and Bianconi et al. (1987) in (La1-xSrx)2CuO4-y and YBa2Cu3O7 did not reveal a 2p3d 8 final state owing to a Cu3+ 3d8 state (the underlining indicates a hole)."
Excerpt from the lecture delivered 8 December 1987, on the occasion of the presentation of the 1987 Nobel Prize in Physics by J. Georg Bednorz and K. Alex Müller published in Review of Modern Physics Vol. 60, No. 3, July 1988, 585-600; (from pag. 593)
see more by this:(1) "Localization of Cu 3d levels in the high Tc superconductor YBa2Cu3O7 by Cu 2p X-ray photoelectron spectroscopy" by A. Bianconi, A. Congiu Castellano, M. De Santis, P. Delogu, A. Gargano, and R. Giorgi published in Solid State Communications 63, 1135 (1987)
II. THE CARRIERS ARE HOLES ON THE OXYGEN ATOM
see more by this: (2) "L2,3 XANES of the high Tc superconductor YBa2Cu3O7 with variable oxygen content" by A. Bianconi, A. Congiu Castellano, M. De Santis, P. Rudolf, P. Lagarde, A. M. Flank, and A. Marcelli publishehd in Solid State Communications Vol. 63, pag. 1009 (1987)------------------------
see more by this: (3) "Experimental evidence of itinerant Cu 3d9-oxygen hole many body configuration in the high Tc superconductor YBa2Cu3O7 by A. Bianconi, A. Clozza, A. Congiu Castellano, S. Della Longa, M. De Santis, A. Di Cicco, K. Garg, P. Delogu, A. Gargano, R. Giorgi, P. Lagarde, A. M. Flank, and A. Marcelli publishewd in International Journal of Modern Physics B Vol. 1, No.3&4, 853-862 (1987)