Effect of impurity on anisotropy of high-T_C superconductivity in YBa₂(Cu_1-xZn_x)₃O_7-δ single crystals

The effect of Zn doping on anisotropy of high-T_c superconductivity is studied in single-crystal YBa₂(Cu_1-xZn_x)₃O_7-δ (YBCZO) within the range 0<x<0.03. The resistive transition curves under a magnetic field for these impurity-containing systems are successfully described for the first time by the renormalized fluctuation theory including ψ⁴ term. The derived coherence length ratio ξ_ab/ξ_c, which indicates anisotropy in superconductivity, decreases as the impurity concentration increases. However, both ρ{variant}_c and the normal state anisotropy ρ{variant}_c/ρ{variant}_ab increase. This situation is not consistent with the Lawrence-Doniach (LD) model. The anisotropic 3-D metal model seems to be appropriate for the YBCZO system. This result excludes certain types of high-T_C theories which assume strict two dimensionality of the electronic system.