Upper and Lower Critical Magnetic Fields of Superconductingndfeaso1−xfxsingle Crystals Studied by Hall-probe Magnetization and Specific Heat

The upper and lower critical fields have been deduced from specific heat and Hall-probe magnetization measurements in nonoptimally doped Nd(O,F)FeAs single crystals (T-c similar to 32-35 K). The anisotropy of the penetration depth (Gamma(lambda)) is temperature independent and on the order of 4.0 +/- 1.5. Similarly specific-heat data lead to an anisotropy of the coherence length Gamma(xi)similar to 5.5 +/- 1.5 close to T-c. Our results suggest the presence of rather large thermal fluctuations and the existence of a vortex liquid phase over a broad temperature range (similar to 5 K large at 2 T).