Coherence length and angular susceptibility divergences above A smectic-A to smectic-C phase transition observed by Rayleigh scattering

Using light beating spectroscopy, we analyse the critical angular fluctuations of the molecules above a second order smectic-A to smectic-C transition. Comparing various Fourier components of these fluctuations we put into evidence the quenching of their susceptibility and relaxation time in the critical domain. We deduce an absolue evaluation of the coherence length and of its anisotropy. The results on γ and ν exponents confirm the de Gennes's analogy with the lambda transition of helium-4