Thermal conductivity ofYBa2(Cu1−xZnx)3O7−δ:Relation betweenxand δ

The effect of oxygen stoichiometry content δ and zinc doping $x$ on the thermal conductivity κ of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ compounds is investigated. Various experimental results for $0<~x<~0.017$ and 0⩽δ⩽0.5 are analyzed by using a theoretical model for the electronic contribution to the thermal conductivity in the case of a two-dimensional $d$-wave superconductor with Van Hove singularities (VHS) in the density of states. It is shown that the data can be quite well reproduced 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 δ 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.