  Pressure can tune material's electronic properties and control its quantum
state, making some systems present disconnected superconducting region as
observed in iron chalcogenides and heavy fermion CeCu2Si2. For CaC6
superconductor (Tc of 11.5 K), applying pressure first Tc increases and then
suppresses and the superconductivity of this compound is eventually disappeared
at about 18 GPa. Here, we report a theoretical finding of the re-emergence of
superconductivity in heavily compressed CaC6. The predicted phase III (space
group Pmmn) with formation of carbon nanofoam is found to be stable at wide
pressure range with a Tc up to 14.7 K at 78 GPa. Diamond-like carbon structure
is adhered to the phase IV (Cmcm) for compressed CaC6 after 126 GPa, which has
bad metallic behavior, indicating again departure from superconductivity.
Re-emerged superconductivity in compressed CaC6 paves a new way to design
new-type superconductor by inserting metal into nanoporous host lattice.
