Inter- to Intra-Layer Resistivity Anisotropy of NdFeAs(O,H) with Various Hydrogen Concentrations

英文摘要：

With molecular beam epitaxy and topotactic chemical reaction, we prepared NdFeAs(O,H) epitaxial thin films with various hydrogen concentrations on 5° vicinal cut MgO substrates. By measuring the resistivities along the longitudinal and transversal directions, the ab plane and the c axis resistivities (\{rho}_ab and \{rho}_c) were obtained. The resistivity anisotropy γ_\{rho}=\{rho}_c \ \{rho}_ab of NdFeAs(O,H) with various hydrogen concentrations was compared with that of NdFeAs(O,F). At the H concentrations which led to superconducting transition temperatures Tc over 40 K, γ_\r{ho} recorded ~100-150 at 50 K. On the other hand, a low γ_\{rho} value of 9 was observed with the mostly doped sample. The exponent \{beta} of the ab plane resistivity obtained by fitting a power law expression \{rho}_{ab}(T)=\{rho}_0+AT^\{beta} to the data was close to unity down to low temperature in the vicinity where the second antiferromagnetic phase locates, which may be related to the quantum critical point discussed at the over-doped side of the phase diagram.