Electrodynamic properties of (TMTSF)2PF6

We have examined the optical properties (0.1–105 cm−1) of the linear chain compound (TMTSF)2PF6 in the spin-density-wave (SDW) state (T < Tc = 12.1 K) both parallel and perpendicular to the conducting chains. A small mode (m∗mb ∼ 20) develops below Tc near 0.2 cm−1 for light polarized along the chains. This mode has been assigned as the collective mode response of the SDW. However, clear evidence of the single particle gap can only be seen perpendicular to the chains, with no significant deviations from the normal state response along the chains at these frequencies. An analysis of the results perpendicular to the chains reveals the formation of a SDW gap with 2Δopt = 70 cm−1. The position of this feature is slightly temperature dependent and can be seen up to 15 K. The optical gap value found is significantly larger than either the simple BCS weak-coupling value (2ΔMF = 30 cm−1) or the activation energy seen in the dc resistivity measurements (2Δth = 32cm−1). These seemingly disparate results are a direct consequence of the anisotropy of the band structure and are in excellent quantitative agreement with predicted values.