We report frequency-dependent conductivity measurements in the organic conductor tetramethyltetraselenofulvalene hexafluorophosphate [${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$P${\mathrm{F}}_6$]. The conductivity $\sigma \left(\omega \right)$ increases with increasing frequency $\omega$ in the spin-density-wave (SDW) state, providing direct evidence for a collective response due to SDW's. A comparison is made with similar observations in charge-density-wave systems.

• Received 8 September 1981

DOI:https://doi.org/10.1103/PhysRevB.25.1443