Calculation of the exciton spectrum of YBa₂Cu₃O₇ using the intermediate exciton theory

Starting from the correlation corrected (quasiparticle) band structures of the CuO₂ plane and the CuO chain (always including the apical oxygen atom) of YBa₂Cu₃O₇ we have calculated the exciton spectra of both systems. For this the intermediate exciton theory was applied in its first-principles form. This leads to a Green’s-matrix equation for the determination of the E_q exciton energies. We have applied in the denominator of the Green’s-matrix quasiparticle one-electron energies instead of the Hartree-Fock ones. In this way we have found very good agreement with experiment for the calculated exciton energies of the CuO₂ plane (theoretical values 4.4–5.7 eV, experimental values 4.5–5.5 eV). Further there is a reflexion spectrum peak of YBa₂Cu₃O_6.9 at 8.0 eV which agrees well with our results obtained for the exciton energies of the chain.