Recent progress in thin-film growth of Fe-based superconductors: superior superconductivity achieved by thin films

Abstract

The discovery of high-Tc 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 Jc 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 Tc ∼ 15 K, LiFeAs with Tc ∼ 18 K, (Ae,K)Fe2As2 (Ae = alkaline earth element) with Tc ∼ 38 K for Ae = Ba, and LnFeAs(O,F) with Tc ∼ 55 K for Ln = Nd or Sm. Increasing the number of elements in compounds, the Tc 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, Tc as high as 58 K exceeding the highest Tc ever reported for bulk samples and high critical current density (Jc) 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 Tc much higher than bulk Tc, though how high Tc 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 Tc by epitaxial strain, etc will also be described.