Fabrication and superconductivity of ${\mathrm{Na}}_x\mathrm{Ta}{\mathrm{S}}_2$ crystals

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 $2\mathrm{H}-\mathrm{Ta}{\mathrm{S}}_2$.

Figure 1

An enlarged view of the ${\mathrm{Na}}_{0.1}\mathrm{Ta}{\mathrm{S}}_2$ crystal. The intercalated sodium content is determined by energy dispersive x-ray microanalysis (EDX) of SEM.

Figure 2

An enlarged view of the ${\mathrm{Na}}_{0.1}\mathrm{Ta}{\mathrm{S}}_2$ crystal at a corner. It is obvious that the samples have a layered structure.

Figure 3

X-ray diffraction pattern of a ${\mathrm{Na}}_x\mathrm{Ta}{\mathrm{S}}_2$ crystal. The $c$-axis lattice constant determined here is about $c=12.082\mathrm{\AA }$ which is quite close to that of $2\mathrm{H}-\mathrm{Ta}{\mathrm{S}}_2$. With increasing the Na content no obvious change of the $c$-axis constant has been observed in our samples.

Figure 4

AC diamagnetism ($H_{\mathrm{AC}}=1\mathrm{Oe}$, $f=333\mathrm{Hz}$) for a series of ${\mathrm{Na}}_x\mathrm{Ta}{\mathrm{S}}_2$ crystals. Analysis using EDX reveals that the sodium content is slightly different among the samples.

Figure 5

EDX spectrum of the ${\mathrm{Na}}_{0.05}\mathrm{Ta}{\mathrm{S}}_2$ crystal. The characteristic peak of sodium around $1.1\mathrm{keV}$ is obvious.

Figure 6

AC susceptibility $(H_{\mathrm{AC}}=1\mathrm{Oe},f=333\mathrm{Hz})$ under different dc magnetic fields for the crystal ${\mathrm{Na}}_{0.1}\mathrm{Ta}{\mathrm{S}}_2$. Here the dc magnetic field is applied along the $c$-axis.

Figure 7

In-plane resistance ($ab$ plane) vs temperature for one piece of crystal ${\mathrm{Na}}_{0.1}\mathrm{Ta}{\mathrm{S}}_2$ under zero field. The transition width is less than $0.5\mathrm{K}$.

Figure 8

The superconducting transition measured at different magnetic fields when the field is applied (a) perpendicular to and (b) parallel to the $c$-axis. From the midpoint of the resistive curve one can determine the upper critical field $H_{c2}\left(T\right)$.

Figure 9

The upper critical field determined from the midpoint of the transition curve. The solid and dashed lines here are theoretical curves of $H_{c2}\left(T\right)=H_{c2}\left(0\right)[(1-t^2)∕(1+t^2)]$ with $H_{c2}^{\Vert c}\left(0\right)=2.5T$ and $H_{c2}^{\Vert ab}\left(0\right)=16T$.

Figure 10

Comparison of resistivity between $2\mathrm{H}-\mathrm{Ta}{\mathrm{S}}_2$ and ${\mathrm{Na}}_{0.1}\mathrm{Ta}{\mathrm{S}}_2$. For clarity, here we show only the data from $2\mathrm{K}\text{to}90\mathrm{K}$. Resistivity of $2\mathrm{H}-\mathrm{Ta}{\mathrm{S}}_2$ is adopted from Ref. 11.

Figure 11

Comparison of resistivity between $2\mathrm{H}-\mathrm{Ta}{\mathrm{S}}_2$ and ${\mathrm{Na}}_{0.05}\mathrm{Ta}{\mathrm{S}}_2$. For clarity we show here only the data from $2\mathrm{K}\text{to}90\mathrm{K}$. Resistivity of $2\mathrm{H}-\mathrm{Ta}{\mathrm{S}}_2$ is adopted from Ref. 11.

Figure 12

Temperature dependence of (a) in-plane and (b) off-plane resistivity for the sample ${\mathrm{Na}}_{0.1}\mathrm{Ta}{\mathrm{S}}_2$. One can see that the in-plane resistivity shows a metallic behavior, but the off-plane one has an insulating behavior above $T_c$. The anisotropy of resistivity is about 63.6 at $10\mathrm{K}$ and 5.3 at $300\mathrm{K}$. On both curves we cannot see any trace of CDW. A linear behavior of the in-plane resistivity is observed in wide temperature range. This resembles that in optimally doped high-$T_c$ cuprate superconductors.

Figure 13

Temperature dependence of (a) in-plane and (b) off-plane resistivity of a sample ${\mathrm{Na}}_x\mathrm{Ta}{\mathrm{S}}_2$ with $x⩽0.05$ and $T_c=2.5\mathrm{K}$. A clear kink corresponding to the CDW transition can be seen here. One can also see that the in-plane resistivity shows metallic behavior in wide region of temperature, but the off-plane resistivity first shows a metallic behavior above $T_{\mathrm{CDW}}$ and an insulating behavior below $T_{\mathrm{CDW}}=65\mathrm{K}$. The anisotropy of resistivity is about 5000 at $10\mathrm{K}$ and 300 at $300\mathrm{K}$.