Superconductivity in alkali-earth metals doped phenanthrene

We discover superconductivity in alkali-earth metals doped phenanthrene. The superconducting critical temperatures \emph{T}$_c$ are 5.6 K and 5.4 K for Sr$_{1.5}$phenanthrene and Ba$_{1.5}$phenanthrene, respectively. The shielding fraction of Ba$_{1.5}$phenanthrene exceeds 65%. The Raman spectra show 8 cm$^{-1}$/electron and 7 cm$^{-1}$/electron downshifts for the mode at 1441 cm$^{-1}$ due to the charge transfer to organic molecules from the dopants of Ba and Sr. Similar behavior has been observed in A$_3$phenanthrene and A$_3$C$_{60}$(A = K and Rb). The positive pressure effect in Sr$_{1.5}$phenanthrene and Ba$_{1.5}$phenanthrene together with the lower $T_c$ with larger lattice indicates unconventional superconductivity in this organic system.