ON-CHAIN ELECTRODYNAMICS OF METALLIC (TMTSF)(2)X SALTS - OBSERVATION OF TOMONAGA-LUTTINGER LIQUID RESPONSE

We have measured the electrodynamic response in the metallic state of three highly anisotropic conductors, (TMTSF)(2)X, where X=PF6, AsF6, o r ClO4, and TMTSF is the organic molecule tetramethyltetraselenofulval ene. In all three cases; we find dramatic deviations from a simple Dru de response. The optical conductivity has two features: a narrow mode at zero frequency, with a small spectral weight, and a mode centered a round 200 cm(-1), 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 ha lf- or quarter-filled band with Coulomb correlations, and evaluate the finite-energy mode in terms of a one-dimensional Mott insulator. At h igh frequencies ((h) over bar omega>t(perpendicular to), the transfer integral perpendicular to the chains), the frequency dependence of the optical conductivity sigma(1)(omega) is in agreement with calculation s based on an interacting Tomonaga-Luttinger liquid, and is different from what is expected for an uncorrelated one-dimensional semiconducto r. The zero-frequency mode shows deviations from a simple Drude respon se, and can be adequately described with a frequency-dependent mass an d relaxation rate. [S0163-1829(98)05927-X].