Reversible magnetization, critical fields, and vortex structure in grain-aligned YBa2Cu4O8

Reversible-magnetization data have been studied for YBa2Cu4O8 in order to determine the thermodynamic properties and the magnetic-field dependence of the superfluid density. By fitting the Hao-Clem variational model to the reversible magnetization data below Tc, superconducting parameters such as the Ginzburg-Landau parameter , the thermodynamic critical field Hc, and the superconducting order parameter are determined. The upper and lower critical fields Hc2,Hc1, the penetration depth , and the coherence length are derived. Within the model there is a direct connection between the magnetization and the superfluid density, so the vortex structure, the behavior of the average order parameter in (H,T) plane, and the magnetic-field dependence of the superfluid density are obtained. In the high-field region, the evidence of strong vortex fluctuation effects is observed and explained by vortex fluctuation for a three-dimensional superconductor. The mean-field transition temperature Tc(H) and dHc2/dT near Tc derived from the analysis of vortex fluctuation effects are comparable with fitting results.