N.M.R. proton lineshape in (TMTSF)2X : incommensurability of nesting vector and order parameter

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Abstract
For the first time, in the SDW state of the organic conductors (TMTSF) 2PF 6 and (TMTSF)2ClO4, the nesting vector Q is determined from a detailed analysis of the methyl proton N.M.R. lineshape for various magnetic field orientations. The amplitude δ of the order parameter is determined with a precision superior to that of previous works which had obtained no information on the nesting vector and neglected the hyperfine coupling. In the paramagnetic metallic state, the fast rotational tunnelling of the two unequivalent methyl groups splits the line into one central line and two pairs of satellites with shifts depending on field orientation, in good agreement with theory. In the SDW state, the local fields due to the ordered magnetic structure lead to an important broadening of the line. By a careful analysis of the lineshape and its evolution in terms of field orientation, we prove, for both compounds, that the SDW is incommensurate; we are able to determine the local fields at each methyl site and separate the dipolar contribution from the hyperfine contact term; we deduce both the amplitude δ and wave vector Q of the SDW. The amplitude δ = 8.5 % ± 2 % (in unit μB per molecule) for PF6 is much larger than previous estimates from N.M.R. broadening. The b* component of the nesting vector Qb = (0.20 + 0.05) b* (b* reciprocal lattice basis vector) for PF6 is in contradiction with simple theories leading to Qb = 0 or Q b = 0.5 b* but agrees with the best nesting conditions obtained with realistic tight binding band calculations. The Q vector is different in (TMTSF)2ClO4, in agreement with the theoretical prediction that Q depends on the nature of anion and experimental conditions such as pressure