Electronic correlation effects in superconducting picene from ab initio calculations

We show, by means of ab initio calculations, that electron-electron correlations play an important role in potassium-doped picene (K-x-picene), recently characterized as a superconductor with T-c = 18 K. The inclusion of exchange interactions by means of hybrid functionals reproduces the correct gap for the undoped compound and predicts an antiferromagnetic state for x = 3, where superconductivity has been observed. These calculations, which do not require us to assume a value for the interaction strength, indirectly suggest that these materials should have a sizable ratio between the effective Coulomb repulsion U and the bandwidth. This is fully compatible with simple estimates of this ratio. Using these values of U in a simple effective Hubbard model, an antiferromagnetic state is indeed stabilized. Our results highlight the similarity between potassium-doped picene and alkali-doped fulleride superconductors.