The transition from a pseudogapped metal to an insulator: photoemission and optics of (TMTSF)2ReO4

We performed optical and photoemission experiments on the Bechgaard salt (TMTSF)2ReO4, a quasi-one-dimensional (1D) organic conductor that undergoes a metal–insulator (Peierls) transition at Tc∼180 K. As in other compounds of the same family, we observe non-Fermi liquid features in the metallic state, namely a deep pseudogap at the chemical potential. A fully gapped state develops below Tc from this peculiar normal state. Across the transition we successively observe, on the same material, the characteristic spectral signatures of metallic and insulating Bechgaard salts. The characteristic exponent obtained from an analysis of the optical conductivity agrees with the predictions of the 1D Luther–Emery model for a charge-density-wave system.