DIMENSIONAL CROSSOVER AND OXYGEN DEFICIENCY IN YBA2CU3OX SINGLE-CRYSTALS

The penetration depth anisotropy ratio gamma of equilibrated, oxygen-deficient YBa2Cu3Ox superconducting single crystals (7 > x > 6.35) has been measured by the equilibrium torque magnetometry method. The experiments were performed in a 1.8 T field at temperatures several K below the onset T(c) of superconductivity. The anisotropy gamma value of YBa2Cu3Ox increases sharply from gamma = 8.7 +/- 0.2 for x almost-equal-to 6.93 to gamma = 105 +/- 7 for x almost-equal-to 6.40. Oxygen deficiency in the CuO chain sites reduces the number of the 2-coordinated, monovalent Cu(1) ions, which in turn reduces the hole doping in the weakly coupled CuO2 planes and suppresses the superconductivity. Our experiment indicates that the interlayer coupling, or the anisotropy, is strongly correlated with the oxygen stoichiometry x or the in-plane hole concentration p(pl) per CuO2 unit. The YBa2Cu3Ox system changes from three-dimensional to two-dimensional when p(pl) is reduced from approximately 0.2 to approximately 0.05 with p(pl) almost-equal-to 0.1 as a crossover regime.