Past 10 years and recent progress in the thin-film growth of Fe-based superconductors. Mainly on the achievements made by molecular beam epitaxy growth

This article reviews the developments over the past 10 years in the thin film growth of Fe-based superconductors, including recent topics of interest in this field. At the beginning of the research, there was pessimism that thin films of the highest-T_c compound, LnFeAs(O,F), would not be easy to grow since it is composed of five elements, namely two cations and three anions. The situation is different from cuprates such as Bi₂Sr₂CaCuO₈, which also consists of five elements, but four cations and one anion. Indeed, there were enormous struggles in the initial attempts to grow LnFeAs(O,F) using pulsed laser deposition. But a major breakthrough was achieved using molecular beam epitaxy, and now LnFeAs(O,F) films with T_c^end over 55 K are routinely grown, at least in Japan. The current best films of many of Fe-based superconductors show superconducting properties substantially superior to those of bulk single crystals. Furthermore the simplest material among Fe-based superconductors, FeSe, whose bulk T_c is as low as 8 K, is opening up a new frontier. The T_c of ultra-thin FeSe films are as high as 40 K or possibly it may even reach >100 K in the form of single atomic-layer films. (author)