Anisotropic resistivity of Na$_{1-delta}$Fe$_{1-x}$Co$_x$As

Temperature-dependent resistivity is studied in single crystals of iron-arsenide superconductor Na$_{1-delta}$Fe$_{1-x}$Co$_x$As for electrical current directions along, $rho_a (T)$, and transverse, $rho_c (T)$, to the Fe-As layers. Doping with Co increases stability of this compound to reaction with the environment and suppresses numerous features in both $rho_a(T)$ and $rho_c(T)$ compared to the stoichiometric NaFeAs. Evolution of $rho_a (T)$ with $x$ follows a universal trend observed in other pnictide superconductors, exhibiting a $T$-linear temperature dependence close to the optimal doping and development of $T^2$ dependence upon further doping. $rho_c (T)$ in parent compound shows a non - monotonic behavior with a crossover from non-metallic resistivity increase on cooling from room temperature down to $sim$ 80 K to a metallic decrease below this temperature. Both $rho_a (T)$ and $rho_c (T)$ show several correlated crossover - like features at $T>$ 80 K. Despite a general trend towards more metallic behavior of inter - plane resistivity in Co-doped samples, the temperature of the crossover from insulating to metallic behavior (80 K) does not change much with doping.