Fluctuation conductivity and possible pseudogap state in FeAs-based superconductor EuFeAsO0.85F0.15

The study of excess conductivity σ ^\prime (T) in the textured polycrystalline FeAs-based superconductor EuFeAsO_0.85F_0.15 ({T}_{{c}}=11 {{K}}) prepared by the solid state synthesis is reported for the first time. The σ ^\prime (T) analysis has been performed within the local pair (LP) model based on the assumption of the LPs 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 ^\prime (T) is adequately described by the 3D term of the Aslamasov-Larkin (AL) theory but the range of the 3D-AL fluctuations, {{Δ }}{T}_3{{D}}, is relatively short. Above the crossover temperature {T}₀≈ 11.7 {{K}} σ ^\prime (T) is described by the 2D Maki-Thompson (MT) fluctuation term of the Hikami-Larkin theory. But enhanced 2D-MT fluctuation contribution being typical for the magnetic superconductors is observed. Within the LP model the PG parameter, {{{Δ }}}^* (T), was determined for the first time. It is shown that {{{Δ }}}^* (T) demonstrates the narrow maximum at {T}_s≈ 160 {{K}} followed by the descending linear length down to {T}_{SDW}={T}_{NFe}≈ 133 {{K}}. Observed small {{Δ }}{T}_3{{D}}, enlarged 2D σ ^\prime (T) and linear {{{Δ }}}^* (T) are considered to be the evidence of the enhanced magnetic interaction in EuFeAsO_0.85F_0.15. Importantly, the slop of the linear {{{Δ }}}^* (T) and its length are found to be the same as it is revealed for SmFeAsO_0.85. The results suggest both the similarity of the magnetic interaction processes in different Fe-pnictides and applicability of the LP model to the σ ^\prime (T) analysis even in magnetic superconductors.