Upper and lower critical fields in NdFeAs(O,F) single crystals : a study 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 non-optimally doped NdFeAs(O,F) single crystals ($T_c\sim 32-35$K). The anisoptropy of the penetration depth (${\mathrm{\Gamma }}_{\lambda }$) is temperature independent and on the order of $4.0 \pm 1.5$\mathrm{.}Similarlyspecificheatdataleadananisotropyofthecoherencelenght$\Gamma_\xi \sim 5.5 \pm 1.5$closeto$T_c$. Our results suggest the presence of rather large thermal fluctuations and to the existence of a vortex liquid phase over a broad temperature range ($\sim 5$K large at 2T).