Temperature-dependent resistivity in single crystals Na$_{1-\delta}$Fe$_{1-x}$Co$_x$

Stoichiometric NaFeAs superconductor is representative of the slightly underdoped part of the doping phase diagram, with a sequence of tetragonal-to-orthorhombic, $T_{s} \approx$ 60 K, magnetic, $T_{m}$=45 K, and superconducting, $T_c$=12~K transitions. Doping level in the compound can be tuned with Co substitution of Fe, acting as electron donor. This doping suppresses structural and magnetic instabilities and induces superconductivity with $T_c$ up to 25~K. Doping with Co allows for studying complete doping phase diagram. We performed systematic measurements of the temperature-dependent in-plane, $\rho _a(T)$, and inter-plane, $\rho _c(T)$, electrical resistivities in the compounds. At optimal doping, both $\rho _a(T)$ and $\rho _c(T)$ show close to $T$-linear temperature dependence above the superconducting $T_c$. With doping this dependence gradually evolves towards $T^2$. At much higher temperatures a slope-change is observed in $\rho _a(T)$, which we relate with onset of carrier activation over a pseudogap.