Thermodynamic estimation of the upper critical field slope of doped SmFeAsO from fluctuation conductivity in the critical regime

We measure magnetotransport in SmFeAs(O$_{1-x}$F$_{x})$ polycrystalline samples up to 28T and we extract the upper critical fields, using different criteria. Due to fluctuation effects, not negligible magnetoresistance and resistivity not saturating to a residual value at Tc, H$_{c2}$ values turn out to be strongly criterion-dependent. In order to circumvent this problem, we propose a thermodynamic estimation of the upper critical field slope dH$_{c2}$/dT based on the analysis of conductivity fluctuations in the critical regime at high fields. Indeed, in this regime we find evidence of a two-dimensional lowest Landau level (LLL) scaling for applied fields larger than $\mu _{0}$H$_{LLL}\sim $8T, which allows to extract a high field slope as large as -12T/K for the optimally doped sample SmFeAs(O$_{0.85}$F$_{0.15})$. A comparison of the fluctuation behavior with that of high-T$_{c}$ cuprates indicates that this H$_{LLL}$ value may be related to the H$_{c2}$ and $\kappa $ values higher than those of cuprates..