Crystal structure, electronic properties, and superconductivity mechanism of La-Phenanthrene

Recently, polycyclic aromatic hydrocarbon (PAH) molecular solids: picene, coronene, dibenzopentacene, phenanthrene among them, have been reported to turn from insulating to metallic and superconducting upon intercalation of electron-donating atoms, such as K, Ba, La. Despite experimental uncertainties, understanding these novel light-element based superconductors is important since both electron phonon coupling and electron electron correlations seem important, as indicated by early theory work. Choosing La-Phenanthrene (La-PA) as our working case, we first search for the theoretical optimal crystal structure and electronic properties by first principles density functional calculations. We single out a stable insulating phase with $P1$ symmetry and, slightly higher in energy, a metastable metallic $P2_{1}$ phase--the same (higher) symmetry of pristine PA, also proposed for La-PA. A tight binding model representing the metallic La-PA electronic structure, its dominant electron phonon coupling with an intermolecular dimerizing mode, and an intramolecular Coulomb $U$ is formulated and discussed. In that model it can be argued that BCS pairing may be essentially unhindered by the Coulomb repulsion. Being symmetry-based, the mechanism could apply to other PAH superconductors as well.