With molecular beam epitaxy and a 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 ${\gamma }_{\rho }\equiv {\rho }_c/{\rho }_{ab}$ 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 $T_{\mathrm{c}}$ over 40 K, ${\gamma }_{\rho }$ recorded $\sim 100–150$ at 50 K. On the other hand, a low ${\gamma }_{\rho }$ value of 9 was observed with the highest-doped sample. The exponent β of the $ab$-plane resistivity obtained by fitting a power-law expression ${\rho }_{ab}\left(T\right)={\rho }_0+A{T}^{\beta }$ 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.

• Received 30 December 2021
• Accepted 1 April 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.054802