Strong Intramolecular Electron-Phonon Coupling in the Negatively Charged Aromatic Superconductor Picene

Superconductivity was recently discovered in solid potassium-intercalated picene (K₃22ph), in which the picene molecule becomes trianionic (22ph^3-). In this Letter, we conduct a theory-based study of the superconductivity of 22ph^3- within the framework of BCS theory. We estimate the density of states N(εF) on the Fermi level to be 2.2 states per (eV molecule spin) by using the theoretical intramolecular electron-phonon coupling lₓ and the experimental superconducting transition temperature Tc of 18 K. The theoretical value is consistent with the 1.2 states per (eV molecule spin) determined experimentally for K₃22ph with Tc=18 K, indicating the validity of our theoretical treatment and the electron-phonon mechanism for superconductivity. The predicted lₓ, 0.206 eV, for 22ph^3- is larger than any value reported for organic superconductors, so picene may have the largest lₓ among the superconductors reported so far.