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We give a quantitative analysis of the previously published nuclear magneticresonance (NMR) experiments in the k-(ET)2X family of organic charge transfersalts by using the phenomenological spin fluctuation model of Moriya, andMillis, Monien and Pines (M-MMP). For temperatures above T_nmr ~ 50 K, themodel gives a good quantitative description of the data in the metallic phasesof several k-(ET)2X materials. These materials display antiferromagneticcorrelation lengths which increase with decreasing temperature and grow toseveral lattice constants by T_nmr. It is shown that the fact that thedimensionless Korringa ratio is much larger than unity is inconsistent with abroad class of theoretical models (such as dynamical mean-field theory) whichneglects spatial correlations and/or vertex corrections. For materials close tothe Mott insulating phase the nuclear spin relaxation rate, the Knight shiftand the Korringa ratio all decrease significantly with decreasing temperaturebelow T_nmr. This cannot be described by the M-MMP model and the most naturalexplanation is that a pseudogap, similar to that observed in the underdopedcuprate superconductors, opens up in the density of states below T_nmr. Such apseudogap has recently been predicted to occur in the dimerised organic chargetransfer salts materials by the resonating valence bond (RVB) theory. Wepropose specific new experiments on organic superconductors to elucidate theseissues. For example, measurements to see if high magnetic fields or highpressures can be used to close the pseudogap would be extremely valuable.

Antiferromagnetic Spin Fluctuations in the Metallic Phase of Quasi-Two-Dimensional Organic Superconductors