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Significant enhancement of the intergrain coupling in lightly F-doped SmFeAsO superconductors

A series of polycrystalline SmFeAsO1−xFx bulks (x = 0–0.25) were synthesized by a one step solid state reaction method at a low heat temperature of 900 ° C, which was about 300 ° C lower than the conventional temperature (∼1200 ° C). The magnetic susceptibility measurements showed single step superconducting transitions for the lightly doped samples with 0.05 ≤ x ≤ 0.12, while the double-step-like transitions were observed in samples x ≥ 0.15, indicating weak-link behavior of the grains. The double step transition is due to the possible occurrence of intergranular (at low temperature) and intragranular (near Tc) superconductivity consisting of coupled superconducting grains, and becomes broad with increasing x. The lightly doped samples with x = 0.10–0.12 have the onset Tc up to 51 K and the intergranular critical current density of over 103 A cm−2 at 5 K in self-field. This intergrain Jc value is one order of magnitude higher than the optimally doped samples (x = 0.2, Tc = 57 K), as assessed by remanent magnetization measurements. These results suggest strong intergrain coupling, and are expected to motivate future research and development for lightly fluorine-doped regions of REFeAs(O, F) superconductors.