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

Abstract

The study of excess conductivity ${\sigma }^{\prime }(T)$ in the textured polycrystalline FeAs-based superconductor $EuFeAs{O}_{0.85}{F}_{0.15}$ ($T_c=11K$) prepared by the solid state synthesis is reported. The ${\sigma }^{\prime }(T)$ 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 $T^{\ast }\gg T_c$. Similarly to the cuprates, near $T_c$ ${\sigma }^{\prime }(T)$ is adequately described by the 3D term of the Aslamasov-Larkin (AL) theory but the range of the 3D-AL fluctuations, $\Delta T_{3D}$, is relatively short. Above the crossover temperature $T_0\approx 11.7K$ ${\sigma }^{\prime }(T)$ 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, ${\Delta }^{\ast }(T)$, was determined for the first time. ${\Delta }^{\ast }(T)$ shows the narrow maximum at $T_s\approx 160K$ followed by the linear drop down to $T_{SDW}=T_{NFe}\approx 133K$. Both small $\Delta T_{3D}$ and enhanced ${\sigma }^{\prime }(T)$, including linear ${\Delta }^{\ast }(T)$ drop, are considered to be the evidence of the enhanced magnetic interaction in $EuFeAs{O}_{0.85}{F}_{0.15}$. Importantly, the slop of the linear ${\Delta }^{\ast }(T)$ and its length are found to be the same as observed for $SmFeAs{O}_{0.85}$. The results suggest both the similarity of magnetic interaction processes in different Fe-pnictides and applicability of the LP model approach to the ${\sigma }^{\prime }(T)$ analysis even in magnetic superconductors.

11 pages, 7 figures, 1 table

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