DIMENSIONAL CROSSOVER AND OXYGEN DEFICIENCY IN YBA2CU3OX SINGLE-CRYSTALS

The penetration depth anisotropy ratio gamma of equilibrated, oxygen-d eficient YBa2Cu3Ox superconducting single crystals (7 > x > 6.35) has been measured by the equilibrium torque magnetometry method. The exper iments were performed in a 1.8 T field at temperatures several K below the onset T(c) of superconductivity. The anisotropy gamma value of YB a2Cu3Ox 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 defici ency in the CuO chain sites reduces the number of the 2-coordinated, m onovalent Cu(1) ions, which in turn reduces the hole doping in the wea kly coupled CuO2 planes and suppresses the superconductivity. Our expe riment indicates that the interlayer coupling, or the anisotropy, is s trongly correlated with the oxygen stoichiometry x or the in-plane hol e concentration p(pl) per CuO2 unit. The YBa2Cu3Ox system changes from three-dimensional to two-dimensional when p(pl) is reduced from appro ximately 0.2 to approximately 0.05 with p(pl) almost-equal-to 0.1 as a crossover regime.