The metal‐ferroelectric transition in TTF‐TCNQ

Earlier incorrect proposals for organic superconductors have the virtue of pinpointing the need for a deeper understanding of the metallic state in organic materials. Accordingly, we consider the effects of interstack coupling on the electronic band structure of TTF‐TCNQ, showing that a one‐electron theory predicts semimetallic behavior. We argue, however, that this state is unstable at low T against a ferroelectric distortion perpendicular to the stacking axis, which produces a gap. With increasing T, the gap is filled by states which are localized (in the Anderson sense) by thermal disorder on individual stacks, thereby restoring one‐dimensional metallic conductivity.