Enhancement of Superconductivity and Evidence of Structural Instability in Intercalated Graphite ${\mathrm{CaC}}_6$ under High Pressure

We measured the temperature dependent resistivity, $\varrho (T)$, of the intercalated graphite superconductor ${\mathrm{CaC}}_6$ as a function of pressure up to 16 GPa. We found a large linear increase of critical temperature, $T_c$, from the ambient pressure value 11.5 K up to 15.1 K, the largest value for intercalated graphite, at 7.5 GPa. At $\approx 8\mathrm{GPa}$, a jump of $\varrho$ and a sudden drop of $T_c$ down to $\approx 5\mathrm{K}$ indicates the occurrence of a phase transition. Our data analysis suggests that a pressure-induced phonon softening related to an in-plane Ca phonon mode is responsible for the $T_c$ increase and that higher pressures $\gtrsim 8\mathrm{GPa}$ lead to a structural transition into a new phase with a low $T_c\lesssim 3\mathrm{K}$.

Figure 1

In-plane resistivity of ${\mathrm{CaC}}_6$ at ambient pressure. The solid line represents the Bloch-Grueneisen fit described in the text. Inset: the $\varrho -{\varrho }_0$ curve in logarithmic scale. The broken line represents a linear $T$ dependence.

Figure 2

Pressure-dependence of the in-plane $\varrho$. Note the jump of ${\varrho }_0$ accompanied by a $T_c$ drop above 8 GPa. Bottom: detail of the superconducting transitions. Note the transition broadening above 7.5 GPa.

Figure 3

Experimental data (points) and Bloch-Grueneisen fits (solid lines) for representative $\varrho (T)$ curves at different pressures in logarithmic scale. The broken line represents a linear dependence. Inset: pressure dependence of the frequency of the Ca (●) and C (■) modes discussed in the text. Lines are a guide to the eye. Error bars are smaller than symbols when not visible.

Figure 4

Pressure dependence of ${\varrho }_0$ and ${\varrho }_{300\mathrm{K}}$ (a) and of $T_c$ (b). The dashed area denotes the phase instability region. Lines are a guide to the eye. In (b), full and open symbols and full and broken lines refer to onset and offset values, respectively. Different symbols refer to different samples.