The electrodynamics of the spin density wave ground state : optical experiments on (TMTSF)2PF6

Abstract

Conductivity measurements are reported in the organic linear-chain compound (TMTSF)2PF6 , in both the metallic and spin-density-wave states. The components of the complex conductivity were established by measurements in the radio-frequency, micro- and millimeter wave, and infrared spectral ranges. At temperatures above the spin-density-wave transition, a Drude-like metallic behavior was found together with a temperature-independent feature at higher frequencies. An observed Drude scattering rate of 3 cm-1 was found, placing the material well into the clean limit. In the spin-density-wave state, the low field dc resistivity shows an activated behavior similar to a standard semiconductor with a gap value 2Δ/kB≈45 K. The ac response shows a strong frequency dependence, and most importantly, we observe two subgap modes: a very broad one in the radio frequency range, due to internal deformations of the spin density wave, and a narrow mode near 0.1 cm-1, which we interpret as the response of the q=O phason. Furthermore, as expected for a material in the clean limit, we do not see evidence for a single particle gap in the infrared spectral ange. In this paper, we will compare our experimental results with the various models of spindensity-wave dynamics and comment on the current status of the understanding of the dynamical response of spin density waves.