Upper critical field and fluctuation conductivity in the critical regime of doped SmFeAsO

We measure magnetotransport of F-doped SmFeAsO samples up to 28 T and we extract the upper critical fields, using different criteria. In order to circumvent the problem of criterion-dependent Hc2 values, we suggest a thermodynamic estimation of the upper critical field slope dHc2 / dT based on the analysis of conductivity fluctuations in the critical regime. A high field slope as large as -12 T / K is thus extracted for the optimally doped sample. We find evidence of a two-dimensional lowest Landau level LLL scaling for applied fields larger than 0HLLL 8 T. Finally, we estimate the coherence length values and we observe that they progressively increase with decreasing Tc . In all cases, the coherence length values along the c axis are smaller than the interplanar distance, confirming the two-dimensional nature of superconductivity in this compound.