Manifestation of Hubbard and covalent correlations in the absorption spectra of YBa2Cu3O6+x films

The absorption spectra of single-crystal YBa2Cu3O6+x films with various doping levels in the range from x approximate to 0.35 to x approximate to 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 Cu2+ 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 approximate to 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 (approximate to 1.5 eV) and correlation (approximate to 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. (C) 2000 American Institute of Physics. [S1063-777X(00)00108-0].