Fabrication and superconductivity ofNaxTaS2crystals

In this paper we report the growth and superconductivity of ${\mathrm{Na}}_x\mathrm{Ta}{\mathrm{S}}_2$ crystals. The structural data deduced from x-ray diffraction pattern shows that the sample has the same structure as $2\mathrm{H}-\mathrm{Ta}{\mathrm{S}}_2$. A series of crystals with different superconducting transition temperatures $\left(T_c\right)$ ranging from $2.5\mathrm{K}\text{to}4.4\mathrm{K}$ were obtained. It is found that the $T_c$ rises with the increase of Na content determined by energy-dispersive x-ray microanalysis(EDX) of scanning electron microscope (SEM) on these crystals. Compared with the resistivity curve of un-intercalated sample $2\mathrm{H}-\mathrm{Ta}{\mathrm{S}}_2$ ($T_c=0.8\mathrm{K}$, $T_{\mathrm{CDW}}\approx 70\mathrm{K}$), no signal of charge density wave (CDW) was observed in samples ${\mathrm{Na}}_{0.1}\mathrm{Ta}{\mathrm{S}}_2$ and ${\mathrm{Na}}_{0.05}\mathrm{Ta}{\mathrm{S}}_2$. However, in some samples with lower $T_c$, the CDW appears again at about $65\mathrm{K}$. Comparison between the anisotropic resistivity indicates that the anisotropy becomes smaller in samples with more Na intercalation (albeit a weak semiconducting behavior along $c$-axis) and thus higher $T_c$. It is thus concluded that there is a competition between the superconductivity and the CDW. With the increase of sodium content, the rise of $T_c$ in ${\mathrm{Na}}_x\mathrm{Ta}{\mathrm{S}}_2$ is caused mainly by the suppression to the CDW in $2\mathrm{H}-\mathrm{Ta}{\mathrm{S}}_2$, and the conventional rigid band model for layered dichalcogenide may be inadequate to explain the changes induced by the slight intercalation of sodium in $2\mathrm{H}-\mathrm{Ta}{\mathrm{S}}_2$.