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

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 BaFe 2 As 2 (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. Superconductivity: Engineering crytal structure A way to optimize superconductivity by carefully controlling the atomic-level crystal structure has been developed by scientists in Japan and the USA. Superconductivity is a quantum effect, usually only seen at very low temperatures, in which an electrical current can pass through a material without facing any resistance. One approach to increasing the operating temperature is to intentionally introduce atomic-level defects using a technique called ion irradiation but this is technically complicated and expensive. Kazumasa Iida from Nagoya University, Japan, and co-workers have managed to achieve the levels of superconductivity associated with ion irradiation by using so-called grain boundary engineering. Regions of different crystalline orientation are known as grains. The team showed that by controlling the alignment of these grains during synthesis of the superconductor (Ba,K)Fe 2 As 2 , they could maximize the material’s superconductivity. High critical current density and strong pinning efficiency for Fe-based superconductor, K-doped BaFe 2 As 2 , were achieved by natural