Theoretical investigations on electronic structure and properties in the charged model clusters relating to high-Tc S-substituted YBCO superconductors

Using the extended Huckel molecular orbital method in connection with the tight binding model, we have studied electronic structure and related properties of superconducting YBa2Cu3O7-x crystals in which O-atoms in regular sites were selectively replaced with S atoms. The calculations are based on the crystal structure of Y-Ba-Cu-O obtained by Beno et al.. We use atomic coordinates that refer to the unrelaxed Y-Ba-Cu-O system. In analogy to the isomerism problem with molecules, we discuss all possible combinations of S-substitutions in O-sites with one, two and four S atoms. The calculations are carried out within charged clusters model for the analogous of YBa-copperoxide and of the YBa-free copperoxide. Our results suggest that the electronic structure of the symmetrically S-substituted or S-added compound is closer to that of the oxygen-deficient superconducting compound than that obtained from unsymmetrical substitution. This applies in particular if 0 is replaced within an 0(1) site. Symmetrical substitutions in the CuO2 layers give rise to large variations in the electronic structure of YBa2Cu3O7. This suggests that superconductivity is very sensitive to the oxygen content of the CuO2 layers. (orig.)