High magnetic field NMR investigation of the spin density wave phase of TMTSF ₂PF ₆

Abstract
We report proton NMR measurements of the effect of very high magnetic fields up to 44.7 T (1.9 GHz) on the spin density wave (SDW) transition of the organic conductor TMTSF ₂PF ₆. Up to 1.8 GHz, no effect of critical slowing close to the transition is seen on the proton relaxation rate (1/T ₁), which is determined by the SDW fluctuations associated with the phase transition at the NMR frequency. Thus, the correlation time for such fluctuations is less than 1O^-10s. A possible explanation for the absence of longer correlation times is that the transition is weakly first order, so that the full critical divergence is never achieved. The measurements also show a dependence of the transition temperature on the orientation of the magnetic field and a quadratic dependence on its magnitude that agrees with earlier transport measurements at lower fields. The UCLA part of this work was supported by NSF Grant DMR-0072524.