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 H-c2 values, we suggest a thermodynamic estimation of the upper critical field slope dH(c2)/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 mu H-0(LLL)similar to 8 T. Finally, we estimate the coherence length values and we observe that they progressively increase with decreasing T-c. 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.