Abstract

Superconductivity in cuprates cannot be fully understood using free-electron theory and the BCS model. Experiments during the last thirty years have shown that structural dynamics and localization cannot be ignored. The Marcus model is applied here for a theoretical assignment of the optical spectra. Hubbard-U absorption at »2 eV is assigned as metal/metal (MM) charge transfer transition of type 2Cu(II) + hn ® Cu(I) + Cu(III). The mid-infrared (MIR) sharp line at 0.35 eV in La₂CuO₄ is interpreted as a vibrational transition with intensity borrowing from the crossing Cu(I) + Cu(III) energy curve. Hole-doping decreases U, since holes become available as acceptors. The relationship between the pseudogap and U in the doped cuprate is discussed. The absorption at 0.13 eV and the activation energy at 0.035 eV, existing only after doping, are very likely due to vertical and adiabatic electron exchange of type Cu(III)+Cu(II)® Cu(II)+Cu(III). The pseudogap involves two-electron states.

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