Unusual doping dependence of superconductivity in ${\text{Na}}_y\text{FeAs}$

Superconductivity and phase relationships were explored in the Na-Fe-As system. The PbFCl-type 111 phase is stable only within a Na stoichiometry range of 1.00 to $\sim 0.85$, and exhibits bulk superconductivity within an even narrower range around 0.90 in ${\text{Na}}_{0.9}\text{FeAs}$. In particular, stoichiometric NaFeAs is not a bulk superconductor. The onset of the superconducting transition varies in a totally different way and the highest $T_c$ occurs in multiphase samples with a nominal composition of $\text{Na}:\text{Fe}:\text{As}=0.5:1:1$, where the superconductive volume-fraction is almost zero. Such doping dependency is rather surprising and in disagreement with most expectations.

Figure 1

The powder XRD of a $y=1.0$ sample. The vertical bars at the bottom are the line positions of the $P4/nmm$ cell.

Figure 2

The $4\pi {\chi }_{\text{ZFC}}$ at zero-field-cooling (open triangles) and the field-cooling (solid triangles) $4\pi {\chi }_{\text{FC}}$ at 10 Oe. (a): $y=1.0$; (b) $y=0.9$; and (c) $y=0.5$. The $4\pi {\chi }_{\text{ZFC}}$ of the $y=0.9$ sample (x’s) is also coplotted in Fig. 1c.

Figure 3

a) $M\left(1.8\text{K}\right)\text{−}M\left(25\text{K}\right)$ vs $H$ for the $x=0.9$ sample (open triangles) and the $x=0.8$ sample ($+$’s). b) solid circles: $-4\pi \left({\chi }_{\text{FC}}-{\chi }_{\text{FC},30\text{K}}\right)$; open triangles: $-4\pi \left({\chi }_{\text{ZFC}}-{\chi }_{\text{ZFC},30\text{K}}\right)$; and the black line: the likely superconducting volume-fraction $V_S$ (see text).

Figure 4

$R\left(T,H\right)$’s of the $y=0.9$ sample. The lines from left to right are at $H=7$, 3, 5, 1, and 0 T, respectively. The grey band is the estimated experimental uncertainty due to the shifts (see text). Inset: The zero-field resistance monitored over the measurement period. The irregular jumps (one of them is indicated by an arrow) between 230 and 300 K suggest the possible Na migration and the $R$ shift associated.

Figure 5

(a) The zero-field resistance of Na-Fe-As samples with $y=0.9$ (dashed line) and $y=1.0$ (solid line). (b) Thermoelectric power of the $y=1.0$ (solid line) and the $y=0.9$ (dashed line) samples.