Theoretical investigation on electronic structure and properties in the charged model clusters relating to highT_e 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 YBA₂Cu₃O_7-m crystals in which O-atoms in regular sites were selectively replaced with S atoms. The calculations are based on the crystal structure of YBaCuO obtained by Beno et al.. We use atomic coordinates that refer to the unrelaxed YBaCuO system. In analogy ot 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 analogues 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 subctitution. This applies in particular if O is replaced within an O(1) site. Symmetrical substitution 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 conetent of the CuO₂ layers.