Realization of epitaxial thin films of the superconductor K-doped BaFe2As2

Abstract

The iron-based superconductor Ba1−xKxFe2As2 is emerging as a key material for high magnetic field applications owing to the recent developments in superconducting wires and bulk permanent magnets. Epitaxial thin films play important roles in investigating and artificially tuning physical properties; nevertheless, the synthesis of Ba1−xKxFe2As2 epitaxial thin films remained challenging because of the high volatility of K. Herein, we report the successful growth of epitaxial Ba1−xKxFe2As2 thin films by molecular-beam epitaxy with employing a combination of fluoride substrates (CaF2, SrF2, and BaF2) and a low growth temperature (350-420ºC). Our epitaxial thin film grown on CaF2 showed sharp superconducting transition at an onset critical temperature (Tc) of 36 K, slightly lower than bulk crystals by ~2 K due presumably to the strain effect arising from the lattice and thermal expansion mismatch. Critical current density (Jc) determined by the magnetization hysteresis loop is as high as 2.2 MA/cm 2 at 4 K under self-field. In-field Jc characteristics of the film are superior to the bulk crystals. The realization of epitaxial thin films opens opportunities for tuning superconducting properties by epitaxial strain and revealing intrinsic grain boundary transport of Ba1−xKxFe2As2.

Enhanced critical current density in K-doped Ba122 polycrystalline bulk superconductors via fast densification

“…The residual resistivity ratio ( RRR = ρ (300 K)/ ρ (40 K)) increased with the sintering temperature, and the samples sintered at 500°C, 550°C, 600°C, 650°C, 700°C, and 900°C showed the RRR values of 2.22, 3.23, 4.25, 4.91, 6.79, and 7.95, respectively. The RRR values reported in the literature are 4.59 ( Chen et al., 2009 ), 5.55 ( Rotter et al., 2008b ), 6.97 ( Mu et al., 2009 ), 7.06 ( Luo et al., 2008 ), and 9.94 ( Nakajima et al., 2015 ) for single crystals (in-plane), 6.80 ( Qin et al., 2021 ) for thin films (in-plane), and 5.00 ( Liu et al., 2019 ) and 6.58 ( Weiss et al., 2012 ) for polycrystalline materials. The RRR value of the sample sintered at >700°C is equivalent to those of single crystals and thin films and higher than those of polycrystalline materials.…”

“…The intergranular currents of Ba122 polycrystalline materials are considerably lower than the intragranular current of a Ba122 single crystal because of intrinsic weak-link, extrinsic structural defects, and compositional variations ( Kametani et al., 2020 ). The J c values of the intragranular currents of K-doped Ba122 single crystals and thin films are >10 6 A/cm 2 at 4 or 5 K under self-field conditions ( Ishida et al., 2017 ; Qin et al., 2021 ), while those of the intergranular currents of K-doped Ba122 polycrystalline bulks synthesized under ambient pressure are ∼10 4 A/cm 2 at 4.2 K under self-field conditions ( Weiss et al., 2013 ).…”

Chiral symmetry protected topological nodal superconducting phase and Majorana Fermi arc

“…The superconducting TSSs could be realized in the thin films of the iron-based superconductors through the molecular beam epitaxy method [71] or exfoliation; [72] in either method, the substrate will always be present. Then we shall consider if the topological nodal superconducting phase was stable in the presence of substrate.…”

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

“…All three groups reported the c-axis oriented films, however, truly epitaxial (both c-axis oriented and in-plane aligned) film was only achieved in K122 [18]. In this study, we employed MgO(001) as an oxide substrate, yet it turned out that K-doped Ba122 films directly grown on MgO(001) are not epitaxially grown [14]. To realize ideal epitaxy, proper buffer layers have to be introduced in between lower MgO substrate and upper Kdoped Ba122 layers.…”

“…Differently from Co-and P-doped Ba122, however, the volatility of K provides a serious problem in the epitaxial growth of K-doped Ba122. Recently, we have reported the successful epitaxial growth of K-doped Ba122 by employing fluoride substrates (CaF 2 , SrF 2 , and BaF 2 ) [14].…”