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

Abstract

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 $d{H}_{c2}/dT$ based on the analysis of conductivity fluctuations in the critical regime. A high field slope as large as $\ensuremath{-}12\text{ }\text{T}/\text{K}$ 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 ${\ensuremath{\mu}}_{0}{H}_{\text{LLL}}\ensuremath{\sim}8\text{ }\text{T}$. 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.