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

TL;DR

This study investigates fluctuation conductivity and pseudogap behavior in EuFeAsO(0.85)F(0.15), revealing magnetic interactions and fluctuation regimes similar to other Fe-based superconductors, using the Local Pair model.

Contribution

First application of the Local Pair model to analyze pseudogap and fluctuation conductivity in EuFeAsO(0.85)F(0.15), highlighting magnetic interactions in Fe-based superconductors.

Findings

Identification of 3D and 2D fluctuation regimes near Tc

Determination of the pseudogap parameter Δ*(T) for the first time

Evidence of enhanced magnetic interactions affecting superconductivity

Abstract

The study of excess conductivity $\sigma'(T)$ 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 $\sigma'(T)$ 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 $T_c$ $\sigma'(T)$ 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 $T_0 \approx 11.7\,K$ $\sigma'(T)$ 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, $\Delta^*(T)$, was determined for the first time. $\Delta^*(T)$ 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 $\Delta T_{3D}$ and enhanced $\sigma'(T)$, including linear $\Delta^*(T)$ 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 $\Delta^*(T)$ 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 $\sigma'(T)$ analysis even in magnetic superconductors.