Anisotropy of the transport properties of NdFeAs(O,F) thin films grown on vicinal substrates

Abstract

NdFeAs(O,F) thin films having different fluorine contents were grown on 5 deg. or 10 deg. vicinal cut MgO and CaF2 single crystalline substrates by molecular beam epitaxy. Structural characterisations by reflection high-energy electron diffraction and x-ray diffraction confirmed the epitaxial growth of NdFeAs(O,F). The resistivities of the ab-plane and along the c-axis were derived from the resistivity measurements in the longitudinal and transversal directions. The c-axis resistivity was always higher than the ab-plane resistivity, resulting from the anisotropic electronic structure. The resistivity anisotropy at 300 K was almost constant in the range of 50-90 irrespective of the F content. On the other hand, the resistivity anisotropy at 56 K showed a strong fluorine dependence: the resistivity anisotropy was over 200 for the films with optimum F contents (superconducting transition temperature Tc around 50 K), whereas the resistivity anisotropy was around 70 for the films in the under-doped regime (Tc between 35 and 45 K). The mass anisotropy are the effective masses along the c-axis and on the ab-plane) close to Tc derived from the anisotropic Ginzburg-Landau approach using the angular-dependency of the ab-plane resistivity was in the range from 2 to 5. On the assumption that the square of the mass anisotropy is equal to the resistivity anisotropy, those values are small compared to the normal state anisotropy.