Environmental stability and anisotropic resistivity of Co-doped 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, ${\rho }_a\left(T\right)$, and transverse, ${\rho }_c\left(T\right)$, to the Fe-As layers. Doping with Co increases stability of this compound to reaction with the environment and suppresses numerous features in both ${\rho }_a\left(T\right)$ and ${\rho }_c\left(T\right)$ compared to the stoichiometric NaFeAs. Evolution of ${\rho }_a\left(T\right)$ 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. ${\rho }_c\left(T\right)$ in parent compound shows a nonmonotonic behavior with a crossover from nonmetallic resistivity increase on cooling from room temperature down to $\sim$80 K to a metallic decrease below this temperature. Both ${\rho }_a\left(T\right)$ and ${\rho }_c\left(T\right)$ 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.

Figure 1

Evolution of the frequency shift signal in the dipper TDR experiment on increasing time of air-exposure treatment in crystals of slightly overdoped Na${}_{1-\delta }$Fe${}_{1-x}$Co${}_x$As, $x=0.08$. Similar to the parent compound (Ref. 12), $T_c$ of the sample increases with exposure time to a maximum and then decreases. Similar effects are observed for other doping levels $x$.

Figure 2

Doping evolution of the frequency shift signal in the dipper TDR experiment during fixed time air-exposure treatment of NaFe${}_{1-x}$Co${}_x$As for 24 h. As can be seen, $T_c$ increase from 12 to 22 K, characteristic of parent NaFeAs samples, is strongly suppressed with Co doping.

Figure 3

Doping evolution of the superconducting $T_c$ in the dipper TDR experiment during air-exposure treatment of NaFe${}_{1-x}$Co${}_x$As. Blue solid dots show $T_c$ of fresh samples, red down triangles $T_c$ of the samples exposed to air for one day. Green up triangles show maximum onset $T_c$ obtained in these experiments.

Figure 4

Doping evolution of the temperature dependence of the in-plane resistivity, ${\rho }_a/{\rho }_a\left(300\text{K}\right)$, for samples of NaFe${}_{1-x}$Co${}_x$As in fresh state after initial sample handling and contact making. For reference we show data from Refs. 38 and 39 for stoichiometric LiFeAs, representative of the overdoped regime.

Figure 5

Doping evolution of the temperature dependence of the interplane resistivity, ${\rho }_c/{\rho }_c\left(300\text{K}\right)$, for samples of NaFe${}_{1-x}$Co${}_x$As in fresh state after initial sample handling and contact making. For reference we show data from Refs. 38 and 39 for stoichiometric LiFeAs, representative of the overdoped regime.

Figure 6

Evolution of the temperature-dependent resistivity in the vicinity of the superconducting transition in magnetic fields applied perpendicular to ($H\parallel c$, left panel) and parallel to ($H\parallel ab$, right panel) the conducting Fe-As plane of the sample of optimally doped NaFe${}_{1-x}$Co${}_x$As, $x=0.025$. The resistive transition temperature, used to plot $H-T$ phase diagram shown in Fig. 7 below, was defined using midpoint criterion.

Figure 7

Temperature and magnetic-field phase diagram of optimally doped NaFe${}_{1-x}$Co${}_x$As, $x=0.025$ for orientation of magnetic field perpendicular and parallel to the conducting Fe-As plane of the crystal.

Figure 8

Top panel: Comparison of the temperature-dependent in-plane and interplane resistivities of parent NaFeAs. For reference we show temperature-dependent Hall constant $R_H\left(T\right)$, right scale, with the data taken from Ref. 27. Bottom panel: Comparison of in-plane and interplane resistivities in a sample with $x=0.025$. The data are plotted on a normalized scale, $\rho \left(T\right)/\rho \left(300\text{K}\right)$.

Figure 9

Comparison of the temperature-dependent in-plane resistivity in samples of NaFeAs doped to the highest $T_c$ by environmental reaction with Apiezon N grease and with Co doping, $x=0.05$. Resistivity data are plotted vs normalized scale, $\rho \left(T\right)/\rho \left(300\text{K}\right)$. Lines show linear extrapolation of the curves to $T\to 0$, revealing difference in residual resistivity of two concentrations of samples.