Nuclear relaxation and antiferromagnetic critical effects in organic conductors

We present and analyze ¹H relaxation data of the spin-density-wave transition on oriented ditetramethyltetraselenafulvalenium phosphorus hexafluoride [(TMTSF)₂{PF}₆$] single crystals at ambient pressure. A detailed analysis of the methyl-group rotation contribution to the nuclear relaxation rate ${T}₁^{{{-}}1}$ is made in the temperature domain precursor to the transition. We show that the singular profile of ${T}₁^{{{-}}1}$ can be well described as an antiferromagnetic critical effect with a three-dimensional classical exponent. The scaling form of the critical quantity (${T}₁T{)}^{{{-}}1}$ in low-dimensional antiferromagnets with a nesting vector exhibiting a dimensionality crossover in temperature is derived and discussed in connection with the data.