Weakly increasing anisotropy in Zn doped YBa2Cu3O7-delta.

The electrical resistivity of single crystals of YBa2Cu3O7-delta with 1 and 3.5 % Zn replacing copper has been studied in magnetic field up to 12 T parallel to the c-axis. From the onset of c-axis resistance, ocurring at a higher temperature than for the in-plane resistivity, the phase diagram can be determined which separates the region of intact vortex tubes from that of decorrelated vortex segments moving independently in different ab-planes of the crystal. This phase separation occurs at a larger magnetic field for comparable temperatures and concentrations when compared to Fe-doped Y-123. This result and a quantitative description in terms of a published model for Josephson tunneling between parallel planes, show that Zn-doping increases anisotropy only weakly in contrast to Fe-doped Y-123.