We present a nuclear magnetic resonance study of the low-temperature spin-density-wave (SDW) ground state in the quasi-one-dimensional conductor (TMTSF)${}_2$PF${}_6$ [bis-(tetramethyl-tetraselenafulvalene)hexafluorophosphate]. Using magnetic fields up to 22 T, we have measured the ${}^{77}$Se spin-lattice ${(T}_1^{-1})$ and spin-spin relaxation rates ${(T}_2^{-1})$ above and below the SDW transition ${(T}_{\mathrm{SDW}}\approx 12$ K). Between $T_{\mathrm{SDW}}$ and $\approx 4$ K, we observe anomalously fast and Korringa-like $T_1^{-1}$; below 4 K, $T_1^{-1}$ decays rapidly. A reduction in $T_2^{-1}$ is also observed at 4 K, but the absorption linewidth does not change to within experimental error. We attribute this behavior to an incomplete SDW transition at $T_{\mathrm{SDW}},$ leaving residual carriers at the Fermi level, and a subsequent consummation of the transition at 4 K. Our data below 4 K are consistent with a simple model of the temperature dependence of $T_1^{-1}$ in a fully gapped and low-dimensional conductor.

• Received 14 November 1996

DOI:https://doi.org/10.1103/PhysRevB.56.2585