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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.

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REFeAs(O,F) (RE: rare-earth element) has the highest-Tc (∼58 K) among the iron-based superconductors, but a thin-film growth of REFeAs(O,F) is difficult. This is because it is not only a complex compound consisting of five elements but also requires doping of highly reactive fluorine to achieve superconductivity. We have reported in our previous article that fluorine can be supplied to a film by subliming solid-state fluorides such as FeF2 or SmF3. In this article, we report on the growth of SmFeAs(O,F) using FeF3 as an alternative fluorine source. FeF3 is solid at ambient temperatures and decomposes at temperatures as low as 100–200 °C, and releases fluorine-containing gas during the thermal decomposition. With this alternative fluorine source, we have grown SmFeAs(O,F) films with Tc as high as 55 K. This achievement demonstrates that FeF3 has potential as a fluorine source that can be employed ubiquitously for a thin-film growth of any fluorine containing compounds. One problem specific to FeF3 is that the compound is highly hydroscopic and contains a substantial amount of water even in its anhydrous form. In this article, we describe how to overcome this specific problem.

We have measured the magnetoresistance and de Haas-van Alphen (dHvA) effect in LaRu 2 Al 10 , which is a reference compound of CeRu 2 Al 10 that exhibits an unusual transition at T 0 ' 27 K. The magnetoresistance increases with increasing magnetic field at low temperatures in the wide field directions, suggesting that LaRu 2 Al 10 is a compensated metal without an open orbit. From the dHvA experiments, about three dHvA frequency branches whose cyclotron effective masses are close to 1m 0 (m 0 : rest mass of an electron) were detected. The observed dHvA branches for the main Fermi surface are well explained by the results of band structure calculation based on a full potential linearized augmented-plane-wave (FLAPW) method and local-density approximation (LDA), that demonstrates the multiply connected Fermi surface with no nesting property in LaRu 2 Al 10 .