Fluctuation conductivity and possible pseudogap state in FeAs-based superconductor EuFeAsO(0.85)F(0.15)

The study of excess conductivity $σ'(T)$ in the textured polycrystalline FeAs-based superconductor $EuFeAsO_{0.85}F_{0.15}$ ($T_c = 11\, K$) prepared by the solid state synthesis is reported. The $σ'(T)$ analysis has been performed within the Local Pair (LP) model based on the assumption of the LP formation in cuprate high-$T_c$ superconductors (cuprates) below the pseudogap (PG) temperature $T^*\gg T_c$. Similarly to the cuprates, near $T_c$ $σ'(T)$ is adequately described by the 3D term of the Aslamasov-Larkin (AL) theory but the range of the 3D-AL fluctuations, $ΔT_{3D}$, is relatively short. Above the crossover temperature $T_0 \approx 11.7\,K$ $σ'(T)$ is described by the 2D Maki-Thompson (MT) fluctuation term of the Hikami-Larkin (HL) theory. But enhanced 2D-MT fluctuation contribution, being typical for the magnetic superconductors, is observed. Within the LP model approach the PG parameter, $Δ^*(T)$, was determined for the first time. $Δ^*(T)$ shows the narrow maximum at $T_s \approx 160\,K$ followed by the linear drop down to $T_{SDW}=T_{NFe}\approx 133\,K$. Both small $ΔT_{3D}$ and enhanced $σ'(T)$, including linear $Δ^*(T)$ drop, are considered to be the evidence of the enhanced magnetic interaction in $EuFeAsO_{0.85}F_{0.15}$. Importantly, the slop of the linear $Δ^*(T)$ and its length are found to be the same as observed for $SmFeAsO_{0.85}$. The results suggest both the similarity of magnetic interaction processes in different Fe-pnictides and applicability of the LP model approach to the $σ'(T)$ analysis even in magnetic superconductors.

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