Environmental stability and anisotropic resistivity of Co-doped Na1-δFe1-xCoxAs

Temperature-dependent resistivity is studied in single crystals of iron-arsenide superconductor Na1-delta Fe1-xCoxAs 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 nonmonotonic behavior with a crossover from nonmetallic resistivity increase on cooling from room temperature down to similar to 80 K to a metallic decrease below this temperature. Both rho(a)(T) and rho(c)(T) show several correlated crossoverlike features at T > 80 K. Despite a general trend towards more metallic behavior of interplane resistivity in Co-doped samples, the temperature of the crossover from insulating to metallic behavior (80 K) does not change much with doping.