77Se NMR spin-lattice relaxation rate properties in the (TMTSF)2X series under pressure: cooperative phenomena and SDW transition
Résumé
We present new results on 77Se spin-lattice relaxation in a single crystal of (TMTSF)2PF6 under pressure just above the critical pressure (Pc) (which stabilizes the superconducting state at 1.2 K). Strong deviations to the Korringa law are observed in the metallic regime. A very large enhancement of the relaxation rate at low temperature (below ∼25 K) may be considered as a smooth evolution from one Korringa regime to an other. This behaviour is broadened and reduced in amplitude at higher pressure. These features are quite similar to those already reported for other members of the (TMTSF)2X family (but having a non centrosymmetrical anion (C1O-4, FSO-3)) and strong based on the existence of one-dimensional 2kF spin correlations for repulsion short-range intrachain e--e- interactions. Below Pc the divergence of T1-1 associated with the transition to the insulating magnetic state may be analysed in terms of an antiferromagnetic critical effect with a 3D classical exponent. We propose an analysis of the AF transitions in these systems using a longitudinal nesting mechanism for the Fermi surface, namely (2kF,O,O).
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