Substrate dependence of the superconducting properties of NdFeAs(O,F) thin films

NdFeAs(O,F) superconducting thin films were grown on MgO, GaAs, CaF2, and LaAlO3 single crystal substrates by molecular beam epitaxy. All thin films exhibited a clear superconducting transition, while the critical temperature TcTc showed a distinct substrate dependence, although the same growth condition was employed for all samples. The difference in TcTc can be attributed to the lattice strain effect and how susceptible the substrate is to fluorine. The thin film grown on CaF2 substrate showed the highest TcTc with an onset TcTc of 56 K and zero resistance at 52 K. A correlation between TcTc and the lattice constant ratio c/ac/a was observed similar to what was reported for other iron-based superconductors, suggesting an intimate relation between the crystal structure and the superconducting properties.

► NdFeAs(O,F) superconducting thin films were grown on various substrates by MBE.
► TcTc depended on the substrate, although the same growth condition was employed.
► The dependence cannot be understood in terms of a simple lattice strain effect.
► The thin film grown on CaF2 exhibited a high TcTc of 56 K (onset).
► A correlation between TcTc and the lattice constant ratio c/ac/a was observed.