Multiple Phase Transitions in Single-Crystalline ${\mathrm{Na}}_{1-\delta }\mathrm{FeAs}$

Specific heat, resistivity, susceptibility, and Hall coefficient measurements were performed on high-quality single-crystalline ${\mathrm{Na}}_{1-\delta }\mathrm{FeAs}$. This compound is found to undergo three successive phase transitions at around 52, 41, and 23 K, which correspond to structural, magnetic, and superconducting transitions, respectively. The Hall effect result indicates the development of energy gap at low temperature due to the occurrence of spin-density-wave instability. Our results provide direct experimental evidence of the magnetic ordering in the nearly stoichiometric NaFeAs.

Figure 1

Single crystal x-ray diffraction pattern for ${\mathrm{Na}}_{1-\delta }\mathrm{FeAs}$. The inset shows the photograph of a ${\mathrm{Na}}_{1-\delta }\mathrm{FeAs}$ single crystal (length scale 1 mm).

Figure 2

(a) The in-plane resistivity ${\rho }_{ab}$ for ${\mathrm{Na}}_{1-\delta }\mathrm{FeAs}$ at zero field. (b)–(c) Temperature dependence of the in-plane electrical resistivity for ${\mathrm{Na}}_{1-\delta }\mathrm{FeAs}$ in low temperature region at fixed fields up to 14 T for the $H\parallel ab$ plane and $H\parallel c$ axis. (d) $H_{c2}(T)$ plots for the $H\parallel ab$ plane (closed circle) and $H\parallel c$ axis (closed square), respectively.

Figure 3

(a) Magnetic susceptibility of ${\mathrm{Na}}_{1-\delta }\mathrm{FeAs}$ as a function of temperature with the $H\parallel ab\mathrm{\text{plane}}$ and $H\parallel c$ axis, respectively. The inset shows the expanded view of $\chi$ between 30 and 60 K. (b) Temperature dependence of specific heat divided by temperature $C/T$ for ${\mathrm{Na}}_{1-\delta }\mathrm{FeAs}$. The upper inset shows the fit of $C/T$ vs $T^2$ in the low temperature region. The lower inset shows the plot of $C$ vs $T$ around the structural and magnetic phase transitions.

Figure 4

Temperature dependence of Hall coefficient for ${\mathrm{Na}}_{1-\delta }\mathrm{FeAs}$. The inset shows the verification of the Hall voltage driven by magnetic field where a linear dependence of the transverse voltage on the applied magnetic field is observed up to 3 T at 40 K.