Evidence for the formation of a Mott state in potassium-intercalated   pentacene
  We investigate electronic transport through pentacene thin-films intercalated with potassium. From temperature-dependent conductivity measurements we find that potassium-intercalated pentacene shows metallic behavior in a broad range of potassium concentrations. Surprisingly, the conductivity exhibits a re-entrance into an insulating state when the potassium concentration is increased past one atom per molecule. We analyze our observations theoretically by means of electronic structure calculations, and we conclude that the phenomenon originates from a Mott metal-insulator transition, driven by electron-electron interactions. 
M. F. Craciun, G. Giovannetti, S. Rogge, G. Brocks, A. F. Morpurgo and   J. van den Brink