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

Nov. 30 15:50-16:15

*Chiheng Dong1,4, Zhe Cheng1, Huan Yang2, Qinghua Zhang3, Satoshi Awaji5, Lin Gu3, Hai-Hu Wen2, Yanwei Ma1,4
Key Laboratory of Applied Superconductivity, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China1
Center for Superconducting Physics and Materials, National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China2
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China3
University of Chinese Academy of Science, Beijing, 100049, China4
High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan5

Iron-based superconductors (IBS) have great potential for high-power applications due to their prominent high-field properties. IBS long wires made by the powder-in-tube (PIT) method have already been made into coils. However, there is still considerable scope for the increase of critical current density, Jc. One of the central issues is to reveal the roles and limitations of grain boundaries in supercurrent transport in IBS. 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 ε=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.

Keywords: Iron-based superconducting wires, Critical current density, Grain boundary engineering