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

The study of excess conductivity sigma'(T) in the textured polycrystalline FeAs-based superconductor EuFeAsO0.85F0.15 (T-c = 11 K) prepared by the solid state synthesis is reported for the first time. The sigma'(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* >> 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 approximate to 11.7 K sigma'(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, Delta*(T), was determined for the first time. It is shown that Delta*(T) demonstrates the narrow maximum at T-s approximate to 160 K followed by the descending linear length down to T-SDW = T-NFe approximate to 133 K. Observed small Delta T-3D, enlarged 2D sigma'(T) and linear Delta*(T) are considered to be the evidence of the enhanced magnetic interaction in EuFeAsO0.85F0.15. Importantly, the slop of the linear Delta*(T) and its length are found to be the same as it is revealed for SmFeAsO0.85. The results suggest both the similarity of the magnetic interaction processes in different Fe-pnictides and applicability of the LP model to the sigma'(T) analysis even in magnetic superconductors.