Approaching the ultimate superconducting properties of (Ba,K)Fe2As2 by naturally formed low-angle grain boundary networks

Kazumasa Iida,Satoshi Hata,Takafumi Hatano,Chao Wang,Hongye Gao,Akiyasu Yamamoto,Dongyi Qin,Zimeng Guo,Hikaru Saito,Chiara Tarantini,Michio Naito

doi:10.1038/s41427-021-00337-5

Abstract

The most effective way to enhance the dissipation-free supercurrent in the presence of a magnetic field for type II superconductors is to introduce defects that act as artificial pinning centers (APCs) for vortices. For instance, the in-field critical current density of doped BaFe2As2 (Ba122), one of the most technologically important Fe-based superconductors, has been improved over the last decade by APCs created by ion irradiation. The technique of ion irradiation has been commonly implemented to determine the ultimate superconducting properties. However, this method is rather complicated and expensive. Here, we report a surprisingly high critical current density and strong pinning efficiency close to the crystallographic c-axis for a K-doped Ba122 epitaxial thin film without APCs, achieving performance comparable to ion-irradiated K-doped Ba122 single crystals. Microstructural analysis reveals that the film is composed of columnar grains with widths of approximately 30–60 nm. The grains are rotated around the b- (or a-) axis by 1.5° and around the c-axis by −1°, resulting in the formation of low-angle grain boundary networks. This study demonstrates that the upper limit of in-field properties reached in ion-irradiated K-doped Ba122 is achievable by grain boundary engineering, which is a simple and industrially scalable manner.

Highlights

Significant progress in the growth of Fe-based superconductor (FBS) thin films has been achieved over the past decade
Our preliminary study shows that grain boundaries (GBs) are present in K-doped Ba122 despite no sign of weak-link behaviors
Several studies have shown a proof of principle of this concept by growing P- and Co-doped Ba122 thin films on technical substrates with oxide buffer layers having a different in-plane spread prepared by ion

Summary

Introduction

Significant progress in the growth of Fe-based superconductor (FBS) thin films has been achieved over the past decade. High-quality, epitaxial thin films of technologically important FBS [e.g., Fe(Se, Te), doped AeFe2As2 (Ae: alkaline earth elements) and doped LnFeAsO (Ln: lanthanoid elements)] are realized on different kinds of single-crystalline substrates and technical substrates[1,2,3,4,5] except for (Ba,K)Fe2As2 (Kdoped Ba122). The realization of epitaxial K-doped Ba122 has been challenging due to the difficulty in controlling volatile potassium. GBs with a small misorientation angle less than θc do not impede the supercurrent flow. Dislocation arrays in low-angle GBs (LAGBs) contribute to flux pinning[3,7,8], leading to improvements in the critical current properties of FBS thin films. Several studies have shown a proof of principle of this concept by growing P- and Co-doped Ba122 thin films on technical substrates with oxide buffer layers having a different in-plane spread prepared by ion

Methods

Results

Conclusion