The temperature dependences of the fluctuation conductivity and pseudogap in SmFeAsO1−x are determined from resistivity measurements. It is shown that the temperature dependence of the pseudogap contains a number of features that substantially distinguish it from that of HTSC-cuprates.
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.
In contacts normal metal–superconductor Bi1.6Pb0.4Sr1.8Ca2.2Cu3O10 (Bi2223), the energy gap Δ(T) of the superconductor and the excess current Iexc(T) were measured by means of the Andreev spectroscopy. It is shown that the parameter Δ(T) turns to zero at the superconducting transition temperature of the cuprate, Tc ≈ 110 K, and the dependence Δ(T) follows the BCS theory; at the same time, the excess current Iexc(T) persists up to the temperature Tpair ≈ 180 K, significantly higher than Tc. The result obtained indicates a possible existence in Bi2223 of a wide temperature range Tc < T < Tpair where a condensate of Cooper pairs starts to form.
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