Thermal Fluctuations and Proton Spin-lattice Relaxation in Nematic Liquid Crystals

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Abstract
Measurements of the spin-lattice relaxation time T1 in both nematic and isotropic phases of normal (PAA) and methyl-deuterated (PAA-CD3) P-azoxyanisole are reported and discussed. For both materials, T1 varies linearly with temperature in the isotropic phase, just above the transition. In order to explain this behaviour two processes are discussed: diffusion and thermal fluctuations in the local order parameter. It is shown crudely that diffusion, in this region, is to be expected as a “correlated” process. Recent measurements of D are criticized within this context, but no definite conclusion can be drawn. A simple theoretical approach of the fluctuations in the order parameter is proposed which accounts for the observed behaviour of T 1(T). In the nematic phase of PAA-CD3, T 1 does not depend on temperature. The frequency dependence is the same as for PAA. Our results show the relative importance of short range phenomena and allow us to question the meaningfulness of earlier discussions of experimental data on the unique basis of Pincus's formula.

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