Molecular beam epitaxy growth of superconducting Ba _1-xK _xFe ₂As ₂ and SmFeAs(O,F) films

We report the molecular beam epitaxy (MBE) growth of the iron-based superconductors, Ba _1-xK _xFe ₂As ₂ and SmFeAs(O,F). In the growth of Ba _1-xK _xFe ₂As ₂ films, the key to incorporating volatile K in films is low-temperature (≥350°C) growth in reduced As flux. The highest T _c thus far obtained is T _c ^on (T _c ^end) = 38.0 K (35.8 K). In the growth of superconducting SmFeAs(O,F), we adopted two methods. In the first method, we first grew pristine SmFeAsO films, and subsequently introduced F into the films by diffusion from an overlayer of SmF ₃. In the second method, we grew as-grown superconducting SmFeAs(O,F) films by coevaporating Sm, SmF ₃, Fe, and As. Thus far, better results have been obtained by the first F diffusion method. The films prepared by F diffusion showed T _c ^on (T _c ^end) = 56.5K (55.3K), whereas the as-grown films showed T _c ^on (T _c ^end) = 51.5K (48.0K).