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The discovery of high-T c superconductivity in Fe-based superconductors pioneered by Kamihara et al (2008 J. Am. Chem. Soc. 130 3296-7) has triggered worldwide research efforts in both science and technology. High-quality epitaxial thin films of Fe-based superconductors are required to develop Fe-based superconducting devices, such as Josephson junctions and superconducting quantum interference devices, and also to develop Fe-based superconducting coated conductors (tapes) with high J c under strong magnetic fields. Epitaxial films are also required for basic research to explore the intrinsic properties of Fe-based superconductors, since sizable bulk single crystals are difficult to grow for some of the Fe-based superconductors such as LnFeAs(O,F) (Ln: lanthanoid element). There are several families of Fe-based superconductors: Fe(Se,Te) with T c ∼15 K, LiFeAs with T c ∼18 K, (Ae,K)Fe 2 As 2 (Ae=alkaline earth element) with T c ∼38 K for Ae=Ba, and LnFeAs(O,F) with T c ∼55 K for Ln=Nd or Sm. Increasing the number of elements in compounds, the T c becomes higher. Simultaneously, thin-film growth becomes more difficult in principle, and it was not imagined at the beginning of the research that thin-film growth of five-element compounds, LnFeAs(O,F), could be realized quickly. At present, T c as high as 58 K exceeding the highest T c ever reported for bulk samples and high critical current density (J c ) over 3 MA cm −2 are obtained in SmFeAs(O,F). In this topical review, we present an overview of the progress in thin-film growth of representative Fe-based superconductors over a decade, including a detailed description of the growth recipes for each family using molecular beam epitaxy or pulsed laser deposition. Furthermore, we present recent hot topics, such as monolayer FeSe with T c much higher than bulk T c , though how high T c can be reached has not been established. Other examples making use of the advantages specific to thin films such as stabilization of quasi-equilibrium phases, enhancement of T c by epitaxial strain, etc will also be described.

Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La 0.18 Sr 0.82 )(Al 0.59 Ta 0.41 )O 3 (LSAT), CaF 2 -buffered LSAT and bare CaF 2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe 0.7 Te 0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe 0.7 Te 0.3films varies significantly. We found that the superconducting transition temperature (T c ) of FeSe 0.7 Te 0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest T c of over 19 K was observed for the film on bare CaF 2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe 0.7 Te 0.3 film and the substrate.

Revealing the universal behaviors of iron-based superconductors (FBS) is important to elucidate the microscopic theory of superconductivity. In this work, we investigate the effect of in-plane strain on the slope of the upper critical field H c2 at the superconducting transition temperature T c (i.e. −dH c2 /dT) for FeSe 0.7 Te 0.3 thin films. The in-plane strain tunes T c in a broad range, while the composition and disorder are almost unchanged. We show that −dH c2 /dT scales linearly with T c , indicating that FeSe 0.7 Te 0.3 follows the same universal behavior as observed for pnictide FBS. The observed behavior is consistent with a multiband superconductivity paired by interband interaction such as sign change s ± superconductivity.

We report on one-step growth of superconducting SmFeAs(O,F) films without post-growth fluorine diffusion. One-step growth is a prerequisite for multilayer deposition, which is required for fabricating sandwich-type Josephson junctions; however, it has not been achieved because of a lack of a suitable fluorine source. We found that FeF2, a ‘soft’ fluoride, decomposes at relatively low temperatures (∼650 °C) and supplies fluorine as well as Fe to films. The use of FeF2 has enabled us to perform reproducible growth of SmFeAs(O,F) films with a systematic change of the film’s properties by varying growth parameters. Now, single-phase single-crystalline SmFeAs(O,F) films with are routinely prepared. Furthermore, the c-axis lattice parameter of the films are varied in a systematic manner with the FeF2 deposition rate, indicating that the F doping level seems to be controllable. The comparison of one-step growth and our previous two-step growth demonstrates that a more homogeneous F concentration is achieved by one-step growth, whereas higher F concentration is achieved by two-step growth.