Abstract Body

Superconductivity in Fe-based superconductors emerges with chemical substitution with the highest critical temperature (T_c) of 56 K in SmFeAs(O,F). Lattice strain caused by pressure application and partial substitution of isovalent element also causes superconductivity. For example, a high T_c of 31 K has been reported for pressure-applied BaFe2As2^[1], and 31 K for the partial substitution of an isovalent element, P, at the As site^[2]. More recently, Kang et al. have reported the superconductivity in undoped BaFe2As2 by tetrahedral geometry design^[3].

In this study, we report the strain effect on superconducting properties in (Ba,K)Fe₂As₂ epitaxial thin films caused by lattice mismatch. In comparison to the high-pressure experiments, epitaxial strain enables the application of two-dimensional pressure in the in-plane direction, and it is possible to apply not only compressive strain but also tensile strain. (Ba,K)Fe₂As₂ epitaxial thin films were grown using custom-designed molecular-beam epitaxy. Films were grown on various substrates (CaF₂, SrF₂, BaF₂, etc.) at around 400°C^[4]. Real-time rate monitoring of fluxes was enabled by electron impact emission spectrometry (EIES) and atomic absorption spectrometry (AAS): EIES for Ba and Fe, and AAS for K. The films exhibit a sharp c-axis orientation without misorientations (fig.1(a)). T_c of the films was compared with polycrystals^[5] as shown in Figure 1(b). Strong c-axis length dependency on the T_c was confirmed.

References

[1] S. Kimber et al., Nature Mater 8, 471 (2009).
[2] M. Nakajima et al., J. Phys. Soc. Jpn. 81, 104710 (2012).
[3] J.-H. Kang et al., Proc. Natl. Acad. Sci. USA 117, 21170 (2020).
[4] D. Qin et al., Phys. Rev. Materials 5, 014801 (2020).
[5] M. Rotter et al., Angew. Chem. Int. Ed. 47, 7949 (2008).

Acknowledgment

This work was supported by JST CREST Grant Number JPMJCR18J4 and Grant-in-Aid for JSPS Fellows Grant Number JP22J23857.