Dimensional crossover and oxygen deficiency in YBa 2Cu 3O χ single crystals

Abstract

The penetration depth anisotropy ratio γ of equilibrated, oxygen-deficient YBa 2Cu 3O χ superconducting single crystals (7> χ > 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 γ value of YBa 2Cu 3O χ increases sharply from γ = 8.7 ± 0.2 for χ ≈ 6.93 to γ = 105 ± 7 for x ≈ 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 CuO 2 planes and suppresses the superconductivity. Our experiment indicates that the interlayer coupling, or the anisotropy, is strongly correlated with the oxygen stoichiometry χ or the in-plane hole concentration p pl per CuO 2 unit. The YBa 2Cu 3O χ system changes from three-dimensional to two-dimensional when p pl is reduced from ∼ 0.2 to ∼ 0.05 with p pl ≈ 0.1 as a crossover regime.