Effects of disorder in iron-based superconductors

Abstract : The pinning of vortices is used as a probe for the identification of disorder and its effect on superconductivity in 122-type iron-based superconductors. Using a new analysis method taking into account the interaction of individual vortices with their neighbors, pinning energies and pinning forces in Ba(Fe1-xCox)2As2 are extracted from the vortex distributions in the regime of small fields. The correlation of measurements of the critical current density Jc with the spatial distribution of vortices shows that pinning in this particular regime is due to the heterogeneity of superconducting properties, on the scale of 20-100 nm. Application of the same analysis procedure on the vortex structure in BaFe2(As1-xPx)2 with less density fluctuations, shows that the pinning forces and energies depend on the doping level x. Both Jc measurements and pinning force distributions independently yield a mean distance between pinning centers of about 90 nm, increasing with increasing P-content x. Combination of the above results and critical current density measurements lead to the conclusion that the low field plateau observed in Jc curves, followed by a power-law decrease, emerges from strong pinning due to nm scale heterogeneity of superconducting properties. Attention is also paid to the weak collective pinning contribution that manifests itself at higher fields > 1 T. Notably, this contribution is consistently analyzed in terms of quasiparticle scattering and mean free path fluctuations. In order to test this premise irradiation of Co, Ni and P-doped 122-type iron-based compounds with high-energy 2.5 MeV electrons is performed for several doping levels of the materials and to different doses. Such irradiation introduces atomic sized point-like defects. Following irradiation it appears that the critical temperature Tc shows a similar depression for all studied materials. The weak collective contribution to Jc in Co-doped is found to clearly increase. Moreover this contribution appears after irradiation of the P-doped compound in which it was previously absent. This allows one to confirm the role of atomic point-like pins as scatterers in Ni and Co-doped compounds, as well as the hypothesis that these defects are at the origin of the weak collective pinning contribution to Jc at larger fields.
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Sultan Demirdis. Effects of disorder in iron-based superconductors. Superconductivity [cond-mat.supr-con]. Ecole Polytechnique X, 2012. English. ⟨pastel-00778943⟩

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