Nuclear magnetic resonance study of soft-mode dynamics in the nematic phase ofp- azoxyanisole

The critical slowing down of the optical soft mode, or the order-parameter fluctuation, in the nematic phase of methyl-deuterated $p$-azoxyanisole is detected for the first time by combining the proton spin-lattice relaxation times in the laboratory and in the rotating frames. Freed's recent theory of spin relaxation by quasicritical order-parameter fluctuations is used to satisfactorily interpret our NMR measurements. The present study confirms the mean-field prediction of symmetry in critical order-parameter fluctuations at isotropic-nematic transitions.