Dimensional crossover field as a function of oxygen stoichiometry in YBa2Cu3O7-δ thin films

The magnetic phase diagram of YBa2Cu3O7-δ thin films was studied as a function of anisotropy by systematically changing the oxygen stoichiometry (δ value). For small δ values the vortex-solid phase transition boundary Hg(T) is observed over the full accessible field range (50 kOe) to follow the (1-T/Tc)∼1.5 dependence that is expected for an anisotropic superconductor in the three-dimensional regime. As δ increases, a portion of the phase boundary no longer follows the (1-T/Tc)∼1.5 dependence making it possible to identify the anisotropy-dependent dimensional crossover field H0 at which the system begins to behave quasi-two-dimensionally. Plotting Hg(T) normalized by H0 as a function of (1-T/Tc) results in a collapse of the phase boundaries onto a single universal curve. Using published values of the anisotropy γ=8.5 and the upper critical field slope dHc2/dT=-20 kOe/K it is possible to determine the specific relation H0∼1.2φ0/(d2γ2), where d is the multilayer spacing, which can be used to determine the δ dependence of γ