Strong enhancement of superconductivity at high pressures within the charge-density-wave states of $2H\text{−}{\mathrm{TaS}}_2$ and $2H\text{−}{\mathrm{TaSe}}_2$

We present measurements of the superconducting and charge-density-wave (CDW) critical temperatures $(T_c$ and $T_{\text{CDW}})$ as a function of pressure in the transition metal dichalchogenides $2H\text{−}{\text{TaSe}}_2$ and $2H\text{−}{\mathrm{TaS}}_2$. Resistance and susceptibility measurements show that $T_c$ increases from temperatures below 1 K up to $8.5$ K at $9.5$ GPa in $2H\text{−}{\mathrm{TaS}}_2$ and $8.2$ K at 23 GPa in $2H\text{−}{\mathrm{TaSe}}_2$. We observe a kink in the pressure dependence of $T_{\text{CDW}}$ at about 4 GPa that we attribute to the lock-in transition from incommensurate CDW to commensurate CDW. Above this pressure, the commensurate $T_{\text{CDW}}$ slowly decreases, coexisting with superconductivity within our full pressure range.

Figure 1

Upper panels: Susceptibility of $2H\text{−}{\mathrm{TaSe}}_2$ (left panel) and $2H\text{−}{\mathrm{TaS}}_2$ (right panel) as a function of temperature for several applied pressures (5.3, 8.1, 11.7, 16.7, and 19.4 GPa for $2H\text{−}{\mathrm{TaSe}}_2$ and 3.1, 5.8, 7.9, 9.8, 14.8, and 17.1 GPa for $2H\text{−}{\mathrm{TaS}}_2)$. Susceptibility has been normalized to the value found at 10 K, and the low-temperature value has been modified to give zero. Lower panels: Resistance as a function of temperature for $2H\text{−}{\mathrm{TaSe}}_2$ (left panel) and for $2H\text{−}{\mathrm{TaS}}_2$ (right panel) for several applied pressures (from 7.5 to 15 by 1.5 GPa steps and then every GPa up to 19 GPa for $2H\text{−}{\mathrm{TaSe}}_2$ and every GPa from 1 to 20 GPa for $2H\text{−}{\mathrm{TaS}}_2)$. The resistance has been normalized to its value at 10 K.

Figure 2

Derivative of the resistance close to the CDW ordering temperature for respectively $2H\text{−}{\mathrm{TaSe}}_2$ (left panel) and $2H\text{−}{\mathrm{TaS}}_2$ (right panel) for different pressures [1, 2, 3, 4, 5, 6, 7.5, 9, 10.5, 12, 13.5, 15, 16, 17, 18, and 19 GPa (left) and every GPa from 1 to 17 GPa (right)]. Curves are shifted in the $y$ axis for clarity. An arrow is used to mark the position where we take $T_{\text{CDW}}$ to give the pressure dependence discussed in Fig. 3

Figure 3

Phase diagram of the superconducting transition obtained from susceptibility (blue solid circles), resistance (light blue lozenges), and of the CDW transition (red triangles). Solid triangles at low pressures have been obtained from Smith et al. [25]. Typical error bars are shown. Blue lines are guides to the eyes. Green dashed lines in the top panel are the positions of the CDW as found in previous works for $2H\text{−}{\mathrm{TaSe}}_2$ [26, 28, 29]. Inset: Proposed phase diagram for both compounds. Black dashed lines are extrapolations of the low-pressure range data (see text).