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

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.

K-doped Ba122 epitaxial thin film on MgO substrate by buffer engineering

“…Figure 3(a) shows the temperature dependence of resistivity of the film. The transition temperature is as high as 39.8 K with a sharp transition of ∆T c = 0.9 K. The observed T c is higher by ∼3 K than T c in K-doped Ba122 epitaxial thin films grown on fluoride substrates [14,23]. The T c enhancement is considered to be due to the epitaxial strain (also/or thermal expansion mismatch): a 0 = 3.917 Å for Ba 0.64 K 0.36 Fe 2 As 2 [24], 3.863 Å for CaF 2 , and 4.212 Å for MgO.…”

“…The transition temperature is as high as 39.8 K with a sharp transition of Tc = 0.9 K. The observed Tc is higher by ~3 K than Tc in K-doped Ba122 epitaxial thin films grown on fluoride substrates. [14,23] The Tc enhancement is considered to be due to the epitaxial strain (also/or thermal expansion mismatch): a0 = 3.917Å for Ba0.64K0.36Fe2As2, [24] 3.863Å for CaF2, and 4.212Å for MgO. The longer a0 of MgO should introduce inplane tensile and out-of-plane compressive strain.…”

Recent progress on epitaxial growth of Fe-based superconducting thin films

“…The key to success was to employ a lowtemperature growth and fluoride substrates. The film showed a surprisingly high J c of over 14 MA cm −2 at 5 K [23]. Additionally, the pinning force density of the film was higher than that of the pinning-enhanced K-doped single crystal.…”

Strengthened proximity effect at grain boundaries to enhance inter-grain supercurrent in Ba1-K Fe2As2 superconductors

“…However, the highest critical current density Jc ever reached in Ba1-xKxFe2As2 PIT tapes is still much smaller than the Ba1-xKxFe2As2 films. [9]. The key factors controlling the inter-grain transport of supercurrent continue to be controversial and subjects of intense interests.…”