Figure 1: Dielectric function of optimally doped Bi₂Sr₂Ca_0.92Y_0.08Cu₂O_8+δ.

(a) The high-energy region of the imaginary part of the ab-plane dielectric function, measured at T=20 K for the optimally doped sample (estimated hole concentration p=0.16, ref. 46), is shown. The thin black line is the fit to the data described in the text. The values of the parameters resulting from the fit are reported in the Supplementary Table S1. The contributions of the individual interband oscillators at ω_0i∼1.46, 2, 2.72 and 3.85 eV are indicated as colour-patterned areas. The real part of the dielectric function, with the respective fit, is shown in the inset. Supplementary Figure S1 shows the fits to the reflectivity for T=300 and 100 K and the extracted I²χ(Ω). (b) Schematics of the ZRS and of the in-plane CT process between Cu and O atoms. (c) The atomic orbitals involved in the charge transfer and Zhang-Rice singlet (ZRS) are schematically indicated. (d) Cartoon of the electron density of states (DOES) (ref. 27). The red and blue arrows represent transitions between mixed Cu–O states and involving a 3d¹⁰ configuration in the final state, respectively.