Spin fluctuations and the pseudogap in organic superconductors

We show that there are strong similarities in the spin-lattice relaxation of nonmagnetic organic charge-transfer salts and that these similarities can be understood in terms of spin fluctuations. Further, we show that, in all of the κ-phase organic superconductors for which there is nuclear-magnetic-resonance data, the energy scale for the spin fluctuations coincides with the energy scale for the pseudogap. This suggests that the pseudogap is caused by short-range spin correlations. In the weakly frustrated metals κ-(BEDT-TTF)₂Cu[N(CN)₂]Br, κ-(BEDT-TTF)₂Cu(NCS)₂, and κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl (under pressure) the pseudogap opens at the same temperature as coherence emerges in the (intralayer) transport. We argue that this is because the spin correlations are cutoff by the loss of intralayer coherence at high temperatures. We discuss what might happen to these two energy scales at high pressures, where the electronic correlations are weaker. In these weakly frustrated materials the data is well described by the chemical pressure hypothesis (that anion substitution is equivalent to hydrostatic pressure). However, we find important differences in the metallic state of κ-(BEDT-TTF)₂Cu₂(CN)₃, which is highly frustrated and displays a spin liquid-insulating phase. We also show that the characteristic temperature scale of the spin fluctuations in (TMTSF)₂ClO₄ is the same as superconducting critical temperature, which may be evidence that spin fluctuations mediate the superconductivity in the Bechgaard salts.