Pressure-induced enhancement of superconductivity and   superconducting-superconducting transition in CaC$\_6$
  We measured the electrical resistivity, $\varrho(T)$, of superconducting CaC$\_6$ at ambient and high pressure up to 16 GPa. For $P \leq$8 GPa, we found a large increase of $T\_c$ with pressure from 11.5 up to 15.1 K. At 8 GPa, $T\_c$ 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 $T\_c$ and more static disorder. 
Andrea Gauzzi (IMPMC), S. Takashima, N. Takeshita, C. Terakura, H.   Takagi, N. Emery (LCSM), C. Herold (LCSM), P. Lagrange ...