Low-Field Phenomenology and Characteristic Lengths of Hydrogen- and Halogen-Doped YBa₂Cu₃O_7-δ Compounds

We measure the diamagnetic response of YBa₂Cu₃O_yA_x (A = Cl, F, H) ceramics. We obtain low-field effective penetration depths λ of a few 10 μm, first flux-penetration field H_I distinguishable from the volume penetration field H_CI and two characteristic temperatures T_s and T_c, T_c < T_s, defined by the onset of the real and imaginary parts of the susceptibility, respectively. H_I(T) goes to zero as a power law H_I(T, x) = H_I(0, x) (T - T/T_c)^2.7 near T_c. From the values of λ and H_I we infer the correlation length ξ at T = 4.2 K. A large range of values for ξ(4.2) suggests that chemical disorder plays an essential role in the phenomenology of cuprates. We find λ(4.2) ∼ H_I(4.2)^-1/3. This, and the power law ξ(4.2) ∼ H_I(4.2)^-2/3 result from the scaling properties of a phase transition governed by disorder.