Substrate effect on structure and superconductivity in SmFeAs(O,F) epitaxial films

Superconducting SmFeAs(O,F) films were prepared on various substrates by molecular beam epitaxy and subsequent fluorine diffusion from an overlayer of SmF3. We have performed a detailed comparison of films on different substrates in the structure and superconductivity. In general, fluoride substrates are more suitable than oxide substrates in obtaining better superconductivity of SmFeAs(O,F) films. The best substrate so far is CaF2, which yielded record high Tc, Tcon(Tcend) = 57.8 K (56.4 K). There is a clear correlation between Tc and epitaxial strain in SmFeAs(O,F) films on CaF2: “the less epitaxial strain, the higher Tc”. The implication of this correlation is briefly discussed.

► Successful MBE growth of high-Tc SmFeAs(O,F) films has been demonstrated.
► A systematic investigation for the substrate dependence has been performed.
► Fluorides are more suitable than oxides as a substrate for SmFeAs(O,F) films.
► CaF2, the best substrate, yielded record high Tc, Tonc (Tcend) = 57.8 K (56.4 K).