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 T c  showed a distinct substrate dependence, although the same growth condition was employed for all samples. The difference in T c  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 T c  with an onset T c  of 56 K and zero resistance at 52 K. A correlation between T c  and the lattice constant ratio c / 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.