Impurity effects on the superconducting coherence length in Zn- or Ni-doped YBa2Cu3O6.9 single crystals

The superconducting coherence length xi of Zn- or Ni-doped YBa2Cu3O6.9 Sing le crystals was measured through the diamagnetic susceptibility in the reve rsible region, and through the resistivity in the: mixed state. Upon impuri ty doping, xi increases along both the in- and out-of-plane directions, whi ch suggests that the doped impurities act as pair breakers. The in-plane xi is well explained by the pair-breaking theory of d-wave superconductivity. On the other hand, the increase of the out-of-plane xi is larger than the theoretical prediction, which might indicate that the interplane coupling o f the order parameter is modified by the impurities. [S0163-1829(99)06625-4 ].