The magnetization around the superconducting transition was recently measured in a high-quality Ba(1-x)KxFe2As2 single crystal with magnetic fields applied along and transverse to the crystal Fe-layers [J. Mosqueira et al., Phys. Rev. B 83, 094519 (2
011)]. Here we extend this study to the finite field (or Prange) regime, in which the magnetic susceptibility is expected to be strongly dependent on the applied magnetic field. These measurements are analyzed in the framework of the three-dimensional anisotropic Ginzburg Landau (3D-aGL) approach generalized to the short wavelength regime through the introduction of a total-energy cutoff in the fluctuation spectrum. The results further confirm the adequacy of GL approaches to describe the fluctuation effects close to the superconducting transition of these materials.
We report on isofield curves of $sqrt{-M}$ vs. T, where M is the reversible magnetization, of YBa_2Cu_3O_{6.95}, YBa_2Cu_3O_{6.65}, and Bi_2Sr_2CaCu_2O_{8+x} with the magnetic field, H, applied parallel to the c-axis of the samples (and also parallel
to the ab- planes for YBaCuO). For temperatures close to the critical temperature, T_c, the quantity sqrt{-M} is proportional to the order parameter amplitude |psi|. Curves of sqrt{-M} vs. T allowed to study the asymptotic behavior of the form (T_a-T)^m of |psi| near T_c, as a function of field. Results for the studied samples produced values of T_a(H) lying above T_c, suggesting that the magnetic field gradually allows to probe a region of temperatures where phase correlations persist above T_c. The study performed here in YBaCuO samples allowed to study how phase correlations evolve with doping in the pseudo-gap region of YBaCuO. sqrt{-M} vs. T curves for all samples show a rather large amplitude fluctuation with no phase correlation extending well above T_a(H) which is interpreted in terms of a Gaussian Ginzburg-Landau approach with a {total-energy} cutoff in the fluctuation spectrum. Resulting values for the exponent m found for all samples, 0.5 < m < 0.7, are interpreted as due to phase fluctuations of the d-wave pairing symmetry of the order parameter in the ab- planes.
