  In cuprate high-temperature superconductors the small coherence lengths and
high transition termperatures result in strong thermal fluctuations, which
render the superconducting transition in applied magnetic fields into a wide
continuous crossover. A state with zero resistance is found only below the
vortex melting transition, which occurs well below the onset of superconducting
correlations. Here we investigate the vortex phase diagram of the novel
Fe-based superconductor in form of a high-quality single crystal of
Ba0.5K0.5Fe2As2, using three different experimental probes (specific heat,
thermal expansion and magnetization). We find clear thermodynamic signatures of
a vortex melting transition, which shows that the thermal fluctuations in
applied magnetic fields also have a considerable impact on the superconducting
properties of iron-based superconductors.
