Tuning the charge density wave and superconductivity in ${\text{Cu}}_x{\text{TaS}}_2$

We report the characterization of layered $2H$-type ${\text{Cu}}_x{\text{TaS}}_2$ for $0\le x\le 0.12$. The charge density wave (CDW), at 70 K for ${\text{TaS}}_2$, is destabilized with Cu doping. The sub-1 K superconducting transition in undoped $2H{\text{-TaS}}_2$ jumps quickly to 2.5 K at low $x$, increases to 4.5 K at the optimal composition ${\text{Cu}}_{0.04}{\text{TaS}}_2$, and then decreases at higher $x$. The electronic contribution to the specific heat, first increasing and then decreasing as a function of Cu content, is $12\text{mJ}{\text{mol}}^{-1}{\text{K}}^{-2}$ at ${\text{Cu}}_{0.04}{\text{TaS}}_2$. Electron-diffraction studies show that the CDW remains present at the optimal superconducting composition but with both a changed $q$ vector and decreased coherence length. We present an electronic phase diagram for the system.

Figure 1

(Color online) Variation of the crystallographic cell parameters with copper content in $2H{\text{-Cu}}_x{\text{TaS}}_2$. Standard deviations on the cell parameters are smaller than the points.

Figure 2

(Color online) Temperature dependent resistivity on polycrystalline pellets of ${\text{Cu}}_x{\text{TaS}}_2$. Inset: detail of superconducting transitions in ${\text{Cu}}_x{\text{TaS}}_2$ for $x=0.01$, 0.04, and 0.08.

Figure 3

(Color online) Characterization of the superconducting transitions through measurement of the temperature dependent dc magnetizations in ${\text{Cu}}_x{\text{TaS}}_2$.

Figure 4

(Color online) Characterization of the superconducting transitions and electronic states in ${\text{Cu}}_x{\text{TaS}}_2$ for different $x$ through measurement of the temperature-dependent specific heat. Lower inset: composition dependence of the electronic contribution to the specific heat in ${\text{Cu}}_x{\text{TaS}}_2$. Upper inset: $T_c$ values measured by specific heat and magnetization for ${\text{Cu}}_x{\text{TaS}}_2$, with the $x=0$ value taken from the literature (Refs. 18, 19).

Figure 5

(Color online) Detailed characterization of the superconducting transition in the optimal superconducting composition ${\text{Cu}}_{0.04}{\text{TaS}}_2$ through specific heat. Inset: the $H=0$ electronic contribution to the specific heat at the temperatures near the superconducting transition. The solid line shows the entropy conserving construction, with a fit to a $T^2$ polynomial at low temperatures.

Figure 6

(Color online) Characterization of the superconducting state in ${\text{Cu}}_{0.04}{\text{TaS}}_2$. (a) $M\left(H\right)$ isotherms, with the low-$H$ part expanded in the inset. The estimated $H_{c2}$ and $H_{c1}$ values are indicated by large triangles and small vertical arrows, respectively. (b) Estimated $H\text{−}T$ phase diagram with $H_{c1}$ values (left axis) and $H_{c2}$ values (right axis) as determined from $M\left(T,H\right)$ (full symbols) and $C_P\left(T,H\right)$ (open symbols) data. The dotted line represents the linear fit close to $T_c$ according to the WWH formula.

Figure 7

(Color online) The temperature dependence of the Seebeck coefficients for ${\text{Cu}}_x{\text{TaS}}_2$. Upper panel: general behavior of the Seebeck coefficients over a wide temperature and $x$ range. Lower panel: detail of the behavior for $x=0$, 0.01, 0.03, and 0.04 in the vicinity of the CDW transition. The lines are guides for the eyes.

Figure 8

[(a)–(d)] Temperature evolution of the electron-diffraction (ED) patterns in the $hk0$ plane for ${\text{Cu}}_{0.04}{\text{TaS}}_2$. Bright spots are the diffraction peaks from the hexagonal crystal structure. Diffuse scattering is present at high temperatures, but at 55 and 25 K, the diffraction peaks from the CDW are clearly observed. (e) $hk0$ plane ED pattern at 11 K of $2H{\text{-TaS}}_2$. The weak sharp CDW reflections are clearly shown between the fundamental reflections. (f) and (g) are the intensity profiles of the fundamental and the CDW reflections measured [along the dashed line in Fig. 8e] from the ED patterns in Figs. 8a, 8e for ${\text{Cu}}_{0.04}{\text{TaS}}_2$ and ${\text{TaS}}_2$, respectively. Quantitative fits of the CDW peak intensity profiles yield a CDW $q=\left(0.341\pm 0.003\right)a^{\ast }$ for ${\text{TaS}}_2$ and a CDW $q=\left(0.358\pm 0.004\right)a^{\ast }$ for ${\text{Cu}}_{0.04}{\text{TaS}}_2$. The coherence length of the CDW is greater than 50 nm for ${\text{TaS}}_2$ and only $\sim 4–5\text{nm}$ for ${\text{Cu}}_{0.04}{\text{TaS}}_2$.

Figure 9

(Color online) Electronic phase diagram for ${\text{Cu}}_x{\text{TaS}}_2$. Inset: the $2H{\text{-TaS}}_2$ crystal structure. Copper is inserted between the ${\text{TaS}}_2$ planes.