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

Temperature-dependent resistivity is studied in single crystals of iron-arsenide superconductor Na$_{1-δ}$Fe$_{1-x}$Co$_x$As for electrical current directions along, $ρ_a (T)$, and transverse, $ρ_c (T)$, to the Fe-As layers. Doping with Co increases stability of this compound to reaction with the environment and suppresses numerous features in both $ρ_a(T)$ and $ρ_c(T)$ compared to the stoichiometric NaFeAs. Evolution of $ρ_a (T)$ 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. $ρ_c (T)$ 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 $ρ_a (T)$ and $ρ_c (T)$ 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.

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