Strong intramolecular electron-phonon coupling in the negatively charged aromatic superconductor picene.

Superconductivity was recently discovered in solid potassium-intercalated picene (K(3)22ph), 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(ε(F)) on the Fermi level to be 2.2 states per (eV molecule spin) 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 (eV molecule spin) determined experimentally for K(3)22ph with T(c)=18 K, indicating the validity of our theoretical treatment and the electron-phonon mechanism for superconductivity. The predicted l(x), 0.206 eV, for 22ph(3-) is larger than any value reported for organic superconductors, so picene may have the largest l(x) among the superconductors reported so far.