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 (22ph3^-). In this Letter, we conduct a theory-based study of the superconductivity of 22ph3^- within the framework of BCS theory. We estimate the density of states N(ε_F) on the Fermi level to be 2.2 states per (eVmoleculespin) by using the theoretical intramolecular electron-phonon coupling l_x and the experimental superconducting transition temperature T_c of 18 K. The theoretical value is consistent with the 1.2 states per (eVmoleculespin) determined experimentally for K₃22ph with T_c=18K, indicating the validity of our theoretical treatment and the electron-phonon mechanism for superconductivity. The predicted l_x, 0.206 eV, for 22ph3^- is larger than any value reported for organic superconductors, so picene may have the largest l_x among the superconductors reported so far.