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 0HLLL8 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.