Dimensional cross-over in the superconducting state of oxygen-deficient YBa2Cu3O6.7

We have investigated the dimensionality of the superconducting state in oxygen reduced YBa2Cu3O6.7 (T-c = 63 K) via measurements of the low-field magnetization M(H) with the field applied parallel to the CuO2-planes. At low temperatures, the c-axis surface screening currents are found to be strongly suppressed by vortices penetrating between the planes, indicating that the individual CuO2-planes are weakly coupled via the Josephson interaction, Since in optimally doped YBa2Cu3O7, the M(H) behaviour is consistent with anisotropic three-dimensional superconductivity, these observations clearly demonstrate that the removal of oxygen from the chains results in a dimensional cross-over of the superconducting state in YBa2Cu3O7-delta. On raising the temperature towards T-c, three-dimensional behaviour is again observed in YBa2Cu3O6.7 when the c-axis coherence length becomes comparable to the interlayer spacing. (C) 1997 Elsevier Science B.V.