Spin-density-wave gap in the Bechgaard salts (TMTSF)2X

The electrodynamic response of the organic linear chain Bechgaard salts (TMTSF)(2)X (X = PF6 and ClO4) has been investigated over a very broad spectral range by applying a variety of optical spectroscopic tools. We focus our attention on the spin-density-wave ground state and, in particular, on its manifestation in the optical properties, with polarization perpendicular to the chains. Along this direction, we have clearly identified the spin-density-wave gap in the optical spectra of both compounds. We discuss the single particle excitation across the gap within a formalism similar to the one worked out for the superconducting state, but with Bardeen-Cooper-Schrieffer case I coherence factors, as well as an approach based on the two-dimensional Hubbard model. [S0163-1829(99)05235-2].