Simultaneous vanishing of nematic electronic state and structural orthorhombicity in ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ single crystals

We have carried out in-plane resistivity measurements under a uniaxial pressure on ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ single crystals. A clear distinction of the in-plane resistivity ${\rho }_a$ and ${\rho }_b$ with the uniaxial pressure along the $b$ axis was discovered in the parent and underdoped regime with a doping level of up to about $x=0.025\pm 0.002.$ From the deviating point of ${\rho }_a$ and ${\rho }_b$, and the unique kinky structure of resistivity, together with the published data, we determined the temperatures for the nematic, structural, and antiferromagnetic transitions. It is clearly shown that the nematic electronic state vanishes simultaneously with the structural transition. The antiferromagnetic state disappears, however, at a lower doping level. Our results, in combination with the data on ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2\mathrm{As}}_2$, indicate a close relationship between nematicity and superconductivity.

Figure 1

(a) Temperature dependence of the in-plane resistivity for twinned ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ single crystals. (b) Temperature dependence of the normalized in-plane resistance of the parent phase NaFeAs. Here ${\overline{R}}_a$ and ${\overline{R}}_b$ represent the normalized in-plane resistance along the orthorhombic $a$ and $b$ axes measured using the Montgomery method. The ${\overline{R}}_{\text{twin}}$ is measured on the same sample without uniaxial pressure. The insets show the homemade detwinning device (left) and the Laue x-ray diffraction pattern for one sample (right).

Figure 2

(a)–(f) Temperature dependence of the normalized in-plane resistance for ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ single crystals ($x=0.01$, 0.02, 0.022, 0.023, 0.025, 0.03). ${\overline{R}}_a$ and ${\overline{R}}_b$ represent the normalized in-plane resistance along the orthorhombic $a$ and $b$ axes measured using the Montgomery method. ${\overline{R}}_{\text{twin}}$ is measured on the same sample without uniaxial pressure.

Figure 3

Derivative of the normalized resistance ${\overline{R}}_b$ for all samples investigated here. The arrows indicate the positions where we determine the $T_s$ and $T_{\text{AF}}$.

Figure 4

Difference between the normalized resistance along the $b$ and $a$ axes. The temperature associated with the nematicity ($T_{\text{nem}}$) is determined using the crossing point of the normal state background line and the extrapolated linear line of the steep transition where ${\overline{R}}_b-{\overline{R}}_a$ changes dramatically.

Figure 5

Phase diagram containing the nematicity ($T_{\text{nem}}$), structural ($T_s$), and antiferromagnetic ($T_{\text{AF}}$) temperatures. The data obtained from other measurement techniques are collected and shown in the figure as well. For the sample with $x=0.025$, as highlighted by the vertical frame, we did not see any feature of structural transition and nematicity in the normal state. The HRPD measurements give one point for the structural transition at about 3 K at a doping level of $x=0.025$. The dashed lines are guides to the eyes.