Proton Spin-Lattice Relaxation Study of Order Fluctuations Above The Smectic A-Isotropic Transition in a Liquid Crystal

Abstract

The proton spin-lattice relaxation time T1 in the isotropic phase of the smectic A liquid crystal diethylazoxybenzoate was measured at several Larmor frequencies in the range from 5 to 60 MHz. The frequency dependent T1 at 25 °C above the smectic A-isotropic phase transition (Tc) is attributed to molecular diffusion. The critical temperature dependent (T1)cf, the spin-lattice relaxation time due to critical order fluctuations, at 5 MHz can be understood by assuming that the effective viscosity has a critical temperature behavior just above Tc. It was deduced that the effective viscosity diverges as ξ2 in the mean field approximation, where ξ is a coherence length describing the short range order.