EXPERIMENTAL MEASURE OF THE ANISOTROPY OF THE BRAGG SINGULARITY IN LYOTROPIC SMECTICS

We report small-angle-neutrons-scattering experiments on the structure factor of oriented dilute lyotropic smectics-A stabilized by the undulation interaction. The power laws of these Landau-Peierls systems in the vicinity of the Bragg singularity, predicted by Caille, have been checked experimentally along the two orthogonal directions (wave vectors parallel (parallel-to) or perpendicular (perpendicular-to) to the director of the phase) for the first time. We show that the decay is twice faster in the q perpendicular-to direction than in the other one. We deduce a characteristic exponent X consistent with the undulation interaction.