Manifestation of Hubbard and covalent correlations in the absorption spectra of YBa 2 Cu 3 O 6+x films

The absorption spectra of single-crystal YBa 2 Cu 3 O 6+x films with various doping levels in the range from x≈0.35 to x≈0.9 are measured in the energy region 0.3–3 eV. An analysis of the spectral composition of the absorption is made with allowance for intraband and interband transitions and the local dd transitions in the Cu 2+ ion. It is concluded that the dd band (the transition d xy →d x 2 −y 2 at 1.5 eV) reflects the enhancement of the covalent bonding (pd hybridization) upon metallization and that the spectral feature at ≈1.8 eV carries information about the contribution of electronic correlations, since it is sensitive to the opening of a spin gap in the insulator and to antiferromagnetic fluctuations in the metal. Although the covalent (≈1.5 eV) and correlation (≈1.8 eV) absorption peaks compete with each other, the coexistence of these bands in the metal supports the validity of a model based on the correlation polaron — a charge carrier which creates a region of covalent bonding in a Hubbard matrix of antiferromagnetic fluctuations.