Inter- to intra-layer resistivity anisotropy of NdFeAs(O,H) with various hydrogen concentrations

Abstract

With molecular beam epitaxy and a topotactic chemical reaction, we prepared NdFeAs(O,H) epitaxial thin films with various hydrogen concentrations on 5\ifmmode^\circ\else\textdegree\fi{} vicinal-cut MgO substrates. By measuring the resistivities along the longitudinal and transversal directions, the $ab$-plane and the $c$-axis resistivities (${\ensuremath{\rho}}_{ab}$ and ${\ensuremath{\rho}}_{c}$) were obtained. The resistivity anisotropy ${\ensuremath{\gamma}}_{\ensuremath{\rho}}\ensuremath{\equiv}{\ensuremath{\rho}}_{c}/{\ensuremath{\rho}}_{ab}$ 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 ${T}_{\mathrm{c}}$ over 40 K, ${\ensuremath{\gamma}}_{\ensuremath{\rho}}$ recorded $\ensuremath{\sim}100--150$ at 50 K. On the other hand, a low ${\ensuremath{\gamma}}_{\ensuremath{\rho}}$ value of 9 was observed with the highest-doped sample. The exponent \ensuremath{\beta} of the $ab$-plane resistivity obtained by fitting a power-law expression ${\ensuremath{\rho}}_{ab}(T)={\ensuremath{\rho}}_{0}+A{T}^{\ensuremath{\beta}}$ to the data was close to unity down to a low temperature in the vicinity where the second antiferromagnetic phase locates, which may be related to the quantum critical point discussed at the overdoped side of the phase diagram.