Magnetic-field-induced increase of the critical fluctuation regime in the Fe-based superconductor Ba1-xKxFe2As2

We investigate the importance of critical order parameter fluctuations in Fe-based superconductors, using high-resolution specific heat and thermal expansion data of Ba1-xKxFe2As2 single crystals. By applying scaling relations of the 3d-XY and the 3d-Lowest-Landau-Level (3d-LLL) fluctuation models to data measured in different magnetic fields, we demonstrate that a strong increase of the critical fluctuations regime is responsible for the transition broadening in magnetic fields, which is a direct consequence of a magnetic-field-induced finite size effect due to a reduction of the effective dimensionality by a decreasing magnetic lengths scale related to the mean vortex separation and the confinement of quasiparticles in low Landau levels.