Upper and lower critical magnetic fields of superconducting NdFeAsO 1 − x F x single 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 $({T}_{c}\ensuremath{\sim}32\char21{}35\text{ }\text{K})$. The anisotropy of the penetration depth $({\ensuremath{\Gamma}}_{\ensuremath{\lambda}})$ is temperature independent and on the order of $4.0\ifmmode\pm\else\textpm\fi{}1.5$. Similarly specific-heat data lead to an anisotropy of the coherence length ${\ensuremath{\Gamma}}_{\ensuremath{\xi}}\ensuremath{\sim}5.5\ifmmode\pm\else\textpm\fi{}1.5$ 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 ($\ensuremath{\sim}5\text{ }\text{K}$ large at 2 T).