MBE growth of Fe-based superconducting films

We report MBE growth of the iron-based superconductors, Sr _1-xK_xFe₂As₂, Ba _1-xK_xFe₂As₂, and SmFeAs(O,F). In the growth of Sr_1-xK_xFe ₂As₂ and Ba_1-xK_xFe ₂As₂ films, the key to incorporating volatile K in films is low-temperature (300-350 °C) growth in reduced As flux. The highest T_c so far obtained are Tcon(Tcend)=33.4K (31.0 K) and 38.3 K (35.5 K) for Sr_1-xK_xFe₂As₂ and Ba_1-xK_xFe₂As₂, respectively. In the growth of superconducting SmFeAs(O,F), we have adopted two approaches. In the first approach, we first grew F-free SmFeAsO films, and subsequently introduced F to the films via F diffusion from an overlayer of SmF₃ or NdF₃. In the second approach, we attempted the growth of as-grown superconducting SmFeAs(O,F) films by coevaporating Sm, SmF₃, Fe, and As. In both the approaches, the growth temperature was as high as 650 °C. So far better results have been obtained by the first F diffusion method. The films prepared by F diffusion showed Tcon (Tcend) = 52 K (48.6 K) whereas the as-grown films showed Tcon = 47 K but with a long transition tail.