Evidence for three-dimensional XY critical properties in underdoped YBa2Cu3O7-δ

We perform a detailed analysis of the reversible magnetization data of Salem-Sugui and Babic of underdoped and optimally doped YBa2Cu3O7-delta single crystals. Near the zero field transition temperature we observe extended consistency with the properties of the three-dimensional XY universality class, even though the attained critical regime is limited by an inhomogeneity induced finite size effect. Nevertheless, as T-c falls from 93.5 to 41.5 K, the critical amplitude of the in-plane correlation length xi(ab0), the anisotropy gamma=xi(ab0)/xi(c0) and the critical amplitude of the in-plane penetration depth lambda(ab0) increase substantially, while the critical amplitude of the c-axis correlation length xi(c0) does not change much. As a consequence, the correlation volume V (-)(corr) increases and the critical amplitude of the specific heat singularity A(-) decreases dramatically, while the rise of lambda(ab0) reflects the behavior of the zero temperature counterpart. Conversely, although xi(ab0) and lambda(ab0) increase with reduced T-c, the ratio lambda(ab0)/xi (-)(ab0), corresponding to the Ginzburg-Landau parameter kappa(ab), decreases substantially and YBa2Cu3O7-delta crosses over from an extreme to a weak type-II superconductor.