Optical response of the spin-density-wave ground state

We investigated the electrodynamical properties of(TMTSF)(2)PF6 in an extremely wide frequency range from 0.1 to 10(5) cm(-1). While there are no indications of the single-particle gap for the electric field parallel to a by passing through the transition at T-SDW = 12 K, we observe clear signatures of the spin-density-wave gap at 2 Delta approximate to 70 cm(-1) in the direction perpendicular to the chains. We are able to describe the experimental findings with a semi-phenomenological model, starting from mean-field theory. The temperature dependence of the order parameter is extracted.