Evidence for the Slow Motion of Organic Molecules in (TMTSF)₂ClO₄ Extracted from the Spin Echo Decay of ⁷⁷Se NMR

Abstract

The ⁷⁷Se nuclear spin echo decay rate (1/T₂) in the organic conductor (TMTSF)₂ClO₂ was found to exhibit a large peak around 30∼32 K, slightly above the structural phase transition temperature T_t=24 K. The peak was more pronounced with increasing field. This result indicates that the slow motion of TMTSF molecules with large amplitude, most probably associated with change in the molecular orientation, develops above T_t, which gives rise to spin echo decay via. the fluctuations of the anisotropic Knight shift. From the analysis of the spin echo decay, it is found that the amplitude of this motion reaches maximum at 30∼32 K and its correlation time is estimated to be 10³∼10⁻⁵ sec depending on temperature. A change was found also in the ⁷⁷Se NMR spectrum in this temperature region which may be interpreted as the motional narrowing effect.