Optical experiments were conducted on a series of organic linear chain conductors with different values of the interchain single-electron transfer integral tb, which quantifies the degree of anisotropy. Electron-electron interactions together with Umklapp scattering resulted in a correlation gap and an insulating state for small tb. An insulator-to-metal transition was observed when tb exceeded a critical value, on the order of the correlation gap Egap. The absence of a plasma edge on the insulator side of the transition for polarization perpendicular to the chains suggests that the electrons are confined to the chains. The optical features of the metallic state, when contrasted with the magnetic properties, are suggestive of spin-charge separation.

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temperature. In contrast to the selenium analogs TMTSF which are one-dimensional metals, the sulfur salts are semiconductors with localized spins on the TMTTF dimers. Taking into account the thermal expansion of the crystals at high temperature (TϾ20 K) the ESR intensity of all sulfur compounds can be described as a spin-1/2 antiferromagnetic Heisenberg chain with exchange constants 420рJ р500 K. Although the TMTSF compounds are one-dimensional organic metals down to 10 K, the temperature dependence of the spin susceptibility can also be described within the framework of the Hubbard model in the limit of strong Coulomb repulsion with JϷ1400 K. By modeling (TMTTF) 2 ClO 4 as an alternating spin chain, the change of the alternation parameter at the first-order phase transition (T AO ϭ72.5 K) indicates a tetramerization of the chain. (TMTTF) 2 PF 6 undergoes a spin-Peierls transition at T SP ϭ19 K which can be well described by Bulaevskii's model with a singlet-triplet gap ⌬ (0)ϭ32.3 K. We find evidence of antiferromagnetic fluctuations at temperatures well above the magnetic ordering in (TMTTF) 2 Br, (TMTSF) 2 PF 6 , and (TMTSF) 2 AsF 6 which follow the critical behavior expected for three-dimensional ordering. (TMTTF) 2 PF 6 and (TMTTF) 2 Br show one-dimensional lattice fluctuations.