Upper and lower critical magnetic fields of superconducting NdFeAsO 1-xFx single crystals studied by Hall-probe magnetization and specific heat

Abstract

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 (Tc∼32-35-K). The anisotropy of the penetration depth (Γλ) is temperature independent and on the order of 4.0±1.5. Similarly specific-heat data lead to an anisotropy of the coherence length Γξ∼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 (∼5 K large at 2 T). © 2009 The American Physical Society.