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Antiferromagnetic Spin Fluctuations and the Pseudogap in the Paramagnetic Phases of Quasi-Two-Dimensional Organic Superconductors

We give a quantitative analysis of the published results of nuclear magnetic resonance (NMR) experiments for k-(ET)2X family of organic charge transfer salts by using the phenomenological spin fluctuation model of Moriya, and Millis, Monien and Pines (M-MMP). For temperatures above T_NMR ~ 50 K, the model gives a good quantitative description of the data for the paramagnetic metallic phase of several k-(ET)2X materials, with an antiferromagnetic correlation length which increases with decreasing temperature; growing to several lattice constants by T_NMR. It is shown that the fact that the dimensionless Korringa ratio is much larger than unity is inconsistent with a broad class of theoretical models (such as dynamical mean-field theory) which neglect spatial correlations and/or vertex corrections. For materials close to the Mott insulating phase, 1/T1, Ks, and K all decrease significantly with decreasing temperature below T_NMR. This cannot be described by the M-MMP model and the most natural explanation is that a pseudogap opens up in the density of states below T_NMR, as in, for example, the cuprates. We show that the NMR measurements reported for k-(ET)2Cu2(CN)3 are qualitatively inconsistent with this material having a ground state with long range magnetic order. A pseudogap in the metallic state of organic superconductors is an important prediction of the resonating valence bond theory of superconductivity. We propose specific new experiments on organic superconductors to elucidate the nature, origin, and momentum dependence of the pseudogap and its relationship to superconductivity.