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Description
The iron-based superconductor is a promising candidate for applications in polycrystalline forms owing to its high current transport performance under high magnetic fields. Especially, doped-BaFe$_2$As$_2$ (Ba122) is particularly interesting in terms of both small anisotropy and small weak-link issues [1]. Epitaxial thin films (ETF) of Ba122-type superconductors have been realized only with Co and P doping by both pulsed laser deposition and molecular beam epitaxy (MBE). Hence, extensive research on enhancing critical current density ($J_c$) has been carried out via introducing artificial pinning centers [2, 3]. However, the fabrication of K-doped Ba122 ETFs has not been realized due to its high volatility and high vapor pressure. Recently, we have succeeded in growing K-doped Ba122 ETFs on CaF$_2$ substrate with high crystallinity by MBE [4]. In the present study, we will report $J_c$ characteristics and nanostructure of the K-doped Ba122 ETFs. Surprisingly high $J_c$ over 10 MA/cm$^2$ was observed at 4 K by magnetic measurement. Note that our thin films showed superior superconducting properties to the pinning enhanced K-doped single crystals by ion irradiation [5]. TEM analyses revealed that low-angle grain boundaries developed in the K-doped Ba122 ETFs grown on CaF$_2$ substrates which could act as flux pinning centers and correspond to the record-high $J_c$.
This work was partly supported by JST CREST (Grant No. JPMJCR18J4) and Advanced Characterization Platform of the Nanotechnology Platform Japan (Grants No. JPMXP09-A-19-KU-1003 and 1004) sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
[1] H. Hosono et al., Materials Today 21, 278 (2018).
[2] L. Fang et al., Nat. Commun. 4, 2655 (2013).
[3] M. Miura et al., Supercond. Sci. Technol. 32, 064005 (2019).
[4] D. Qin et al., Phys. Rev. Materials 5, 014801 (2021).
[5] A. Takahashi et al., J. Phys.: Conf. Ser. 1590, 012015 (2020).
