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

TL;DR

This study demonstrates that overdoping in Ba1-xKxFe2As2 superconductors enhances the proximity effect at grain boundaries, significantly improving inter-grain supercurrent transport through engineered grain boundary properties.

Contribution

The paper introduces a detailed microscopic analysis and a sandwich model based on the proximity effect to explain how overdoping strengthens inter-grain supercurrents in iron-based superconductors.

Findings

Inter-grain critical current density peaks at x~0.458 doping.

Grain boundary transparency improves monotonically with doping.

Overdoping enhances the proximity effect, boosting intergrain supercurrent.

Abstract

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 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-superconductor (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 parameters of iron-based superconductors by grain boundary engineering.