THERMAL-CONDUCTIVITY OF YBA2(CU1-XZNX)(3)O7-DELTA - RELATION BETWEEN X AND DELTA

The effect of oxygen stoichiometry content delta and zinc doping x on the thermal conductivity kappa of YBa2Cu3O7-delta compounds is investi gated. Various experimental results for 0 less than or equal to x less than or equal to 0.017 and 0 less than or equal to delta less than or equal to 0.5 are analyzed by using a theoretical model for the electr onic contribution to the thermal conductivity in the case of a two-dim ensional d-wave superconductor with Van Hove singularities (VHS) in th e density of states. It is shown that the data can be quite well repro duced by considering the effect of the position of these VHS from the Fermi level on the thermal conductivity peak observed below T-c. From this analysis, a simple relation between x and delta can be extracted. We argue that this relation is correlated to the variation of the Cu( 2) valence with oxygen content and Zn concentration, and that the Cu(2 ) valence controls the critical temperature of such cuprates, further strengthening the Van Hove scenario of high-T-c superconductivity.