Pressure-induced enhancement of superconductivity and superconducting-superconducting transition in CaC$\mathrm{\_}6$

We measured the electrical resistivity, $\varrho (T)$, of superconducting CaC$\mathrm{\_}6$ at ambient and high pressure up to 16 GPa. For $P\le$8 GPa, we found a large increase of $T\mathrm{\_}c$ with pressure from 11.5 up to 15.1 K. At 8 GPa, $T\mathrm{\_}c$ drops and levels off at 5 K above 10 GPa. Correspondingly, the residual $\varrho$ increases by $\approx$ 200 times and the $\varrho (T)$ behavior becomes flat. The recovery of the pristine behavior after depressurization is suggestive of a phase transition at 8 GPa between two superconducting phases with good and bad metallic properties, the latter with a lower $T\mathrm{\_}c$ and more static disorder.