On-chain electrodynamics of metallic „TMTSF...2X salts: Observation of Tomonaga-Luttinger liquid response

We have measured the electrodynamic response in the metallic state of three highly anisotropic conductors, (TMTSF)2X , where X5PF6, AsF6, or ClO4, and TMTSF is the organic molecule tetramethyltetraselenofulvalene. In all three cases we find dramatic deviations from a simple Drude response. The optical conductivity has two features: a narrow mode at zero frequency, with a small spectral weight, and a mode centered around 200 cm, with nearly all of the spectral weight expected for the relevant number of carriers and single particle bandmass. We argue that these features are characteristic of a nearly one-dimensional halfor quarterfilled band with Coulomb correlations, and evaluate the finite-energy mode in terms of a one-dimensional Mott insulator. At high frequencies (\v.t' , the transfer integral perpendicular to the chains!, the frequency dependence of the optical conductivity s1(v) is in agreement with calculations based on an interacting Tomonaga-Luttinger liquid, and is different from what is expected for an uncorrelated one-dimensional semiconductor. The zero-frequency mode shows deviations from a simple Drude response, and can be adequately described with a frequency-dependent mass and relaxation rate. @S0163-1829~98!05927-X#