Possibility of local pair existence in optimally doped SmFeAsO1−x in pseudogap regime

Abstract

We report the analysis of pseudogap ∆ * derived from resistivity experiments in FeAs-based superconductor SmFeAsO0.85, having a critical temperature Tc = 55 K. Rather specific dependence ∆ * (T ) with two representative temperatures followed by a minimum at about 120 K was observed. Below Ts ≈ 147 K, corresponding to the structural transition in SmFeAsO, ∆ * (T ) decreases linearly down to the temperature TAF M ≈ 133 K. This last peculiarity can likely be attributed to the antiferromagnetic (AFM ) ordering of Fe spins. It is believed that the found behavior can be explained in terms of Machida, Nokura, and Matsubara (MNM) theory developed for the AFM superconductors.

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

“…Strictly speaking, apart from our investigation of the FLC and PG in SmF eAsO 0.85 [14] we have no information about the similar experiments performed by another research groups. As a result, the possibility of a PG state in the FeAs-based HTS's still remains controversial [37].…”

“…However, over the temperature range T * ≈ 171 K to T ≈ 210 K ρ(T ) varies linearly with T at rates dρ/dT = 2.0 µ Ω cm K −1 . Above 210 K ρ(T ) exhibits a downwards deviation from the linear dependence which is typical for the FePn's [1,2,14] (refer to Fig. 1).…”

“…Found ξ c (0) = (2.84 ± 0.02)Å is about 1.7 and 2.0 times of that obtained for the YBCO film (T c = 87.4 K) [43] and correspondingly for SmF eAsO 0.85 polycrystal (T c = 55 K) [14], which are considered to be the reference samples (Table I). It is not surprising seeing we assume [63].…”

“…Besides, up to now there is no rigorous theory to describe the excess conductivity in the whole temperature range from T * down to T c in the HTS's. That is why we have tried to analyze found σ ′ (T ) within our LP model approach paying more attention at the possible difference in revealed results and parameters in comparison with those obtained for YBCO films [43] and SmF eAsO 0.85 polycrystals [14,43]. Here we focus on the analysis of the fluctuation conductivity (FLC) and possible pseudogap (PG) derived from measured excess conductivity within our LP model [43,44].…”

“…The common feature for all families is a structural transition from a tetragonal to an orthorhombic phase at T s = (150−190) K which is closely related to the formation of a spin-density-wave (SDW) type magnetic instability at T = T SDW due to antiferromagnetic (AF) ordering of the Fe spins [2]. For "1111" systems T SDW < T S [2,14] whereas for "122" compound, e.g for EuF e 2 As 2 [15], T SDW ≈ T s . Apparently, the superconductivity emerges from the FeAs or FeSe layers which are the building blocks of the corresponding quasi two-dimensional crystal structures suggesting the analogy to the cuprate-based high-T c superconductors (HTS's).…”