77Se NMR study of the spin density wave state in (TMTSF)2PF6

The quasi-1D conductor (TMTSF)2PF6 has a Spin Density Wave transition at 12 K. Previous 1H NMR studies [1] have shown that the spin-lattice relaxation rate, T1, is finite and Korringa-like inside the Spin Density Wave phase down to about 4 K below which T1 increases rapidly. We have measured the linewidth, T1 and the spin-spin relaxation time, T2, of the 77Se nuclei which, as calculated by bandstructure, are in closer physical proximity to the electron conduction paths than the 1H and, consequently, more strongly coupled to the conduction electrons. Using fields up to 24 T, we find that while the lineshape is temperature independent inside the SDW phase, the 77Se T1 exhibits the same qualitative behavior as the 1H T1, and T2 increases abruptly at ≈ 4K. We discuss this result in terms of the Fermi surface of the system.