The iron-based superconductors are characterized by strong fluctuations due to high transition temperatures and small coherence lengths. We investigate fluctuation behavior in the magnetic iron-pnictide superconductor $\mathrm{Rb}\mathrm{Eu}{\mathrm{Fe}}_4{\mathrm{As}}_4$ by calorimetry and transport. We find that the broadening of the specific-heat transition in magnetic fields is very well described by the lowest-Landau-level scaling. We report calorimetric and transport observations for vortex-lattice melting, which is seen as a sharp drop of the resistivity and a step of the specific heat at the magnetic-field-dependent temperature. The melting line in the temperature–magnetic field plane lies noticeably below the upper-critical-field line and its location is in quantitative agreement with theoretical predictions without fitting parameters. Finally, we compare the melting behavior of $\mathrm{Rb}\mathrm{Eu}{\mathrm{Fe}}_4{\mathrm{As}}_4$ with other superconducting materials showing that thermal fluctuations of vortices are not as prevalent as in the high-temperature superconducting cuprates, yet they still noticeably influence the properties of the vortex matter.

• Received 24 June 2019
• Revised 23 August 2019

DOI:https://doi.org/10.1103/PhysRevB.100.094518