Abstract :
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.
