Strengthened proximity effect at grain boundaries to enhance inter-grain supercurrent in Ba1-xKxFe2As2 superconductors

Iron-based superconductors have great potential for high-power applications due to their prominent high-field properties. One of the central issues in enhancing the critical current density of iron-based superconducting wires is to reveal the roles and limitations of grain boundaries in supercurrent transport. Here, we finely tuned the electronic properties of grain boundaries by doping Ba1-xKxFe2As2 superconductors in a wide range (0.25 <= x <= 0.598). It is found that the intra-grain Jcintra peaks near x-0.287, while the inter-grain Jcinter has a maximum at about x-0.458. Remarkably, the grain boundary transparency parameter defined as epsilon = Jcinter/Jcintra rises monotonically with doping. Through detailed microscopic analysis, we suggest that the FeAs segregation phase commonly existing at grain boundaries and the adjacent grains constitute superconductor-normal metal -super-conductor (SNS) Josephson junctions which play a key role in transporting supercurrent. A sandwich model based on the proximity effect and the SNS junction is proposed to well interpret our data. It is found that overdoping in superconducting grains largely strengthens the proximity effect and consequently enhances the intergrain supercurrent. Our results will shed new insights and inspirations for improving the application pa-rameters of iron-based superconductors by grain boundary engineering.

Automated summary

Researchers have finely tuned the electronic properties of grain boundaries in iron-based superconductors by doping them with different concentrations of a certain element. The study reveals that the contribution of grain boundaries to supercurrent transport is closely related to the doping concentration. The presence of a special structure at the grain boundaries and adjacent grains plays a key role in the transport of supercurrent. This study provides important insights for improving the application performance of iron-based superconductors through grain boundary engineering.