NMR in Commensurate and Incommensurate Spin Density Waves

We present 13 C NMR lineshapes and spin lattice relaxation measurements of (TMTTF) 2 Br and (TMTSF) 2 PF 6 in the spin density wave (SDW) state. We confirm that (TMTSF) 2 PF 6 has an incommensurate SDW and show that the phason mode is dominant in the relaxation. We prove that (TMTTF) 2 Br is commensurate and estimate the SDW amplitude. No phason contribution to the relaxation is observed in this compound. The implications for theory are discussed.