Impurity effects on the superconducting coherence length in Zn- or Ni-doped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.9}$ single crystals

The superconducting coherence length $\xi$ of Zn- or Ni-doped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.9}$ single crystals was measured through the diamagnetic susceptibility in the reversible region, and through the resistivity in the mixed state. Upon impurity doping, $\xi$ increases along both the in- and out-of-plane directions, which 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 of the order parameter is modified by the impurities.