Superconductivity for ${\text{CaC}}_6$ to 32 GPa hydrostatic pressure

The dependence of the superconducting transition temperature $T_c$ of ${\text{CaC}}_6$ has been determined as a function of hydrostatic pressure in both helium-loaded gas and diamond-anvil cells to 0.6 GPa and 32 GPa, respectively. Following an initial increase at the rate $+0.39\left(1\right)\text{K}/\text{GPa}$, $T_c$ drops abruptly from 15 to 4 K at $\sim 10\text{GPa}$. Between 18 and 32 GPa, no superconducting transition is observed above 2 K.

Figure 1

X-ray $\left(\text{Cu}K\alpha \right)$ diffraction pattern for ${\text{CaC}}_6$ sample prepared from natural graphite.

Figure 2

(Color online) Real part of the ac susceptibility versus temperature at different pressures to $\sim 0.6\text{GPa}$ in the He-gas cell for ${\text{CaC}}_6$ made from (a) natural graphite or (b) HOPG graphite.

Figure 3

(Color online) Superconducting transition temperature versus pressure to $\sim 0.6\text{GPa}$ for ${\text{CaC}}_6$ using data from Figs. 2a, 2b. $T_c$ is determined from the transition midpoint (see Ref. 14). Numbers give order of measurement. Pressure is changed at room temperature for solid symbols, at 50 K for open symbols.

Figure 4

Real part of the ac susceptibility signal at different pressures in the DAC to 18.3 GPa for ${\text{CaC}}_6$ made from HOPG graphite. Data taken with increasing pressure.

Figure 5

Superconducting transition temperature versus pressure to 32 GPa for ${\text{CaC}}_6$ using data from Fig. 4. $T_c$ is determined from the transition midpoint. Solid line is guide to eyes; dashed line reproduces data from Ref. 6. Above 20 GPa, $T_c$ lies below 2 K. Error bars give $20–80\mathrm{\%}$ transition width.