PENETRATION LENGTH OF FERROSMECTICS

We report the first quantitative measurements of the smectic penetrati on length lambda = square-root K/BBAR, with K and BAR the curvature an d compression elastic constants, for ferrosmectics, which are doped ly otropic smectics. We use a method based on the observation of defects created in a cell of varying thickness. In particular, we determine th e evolution of lambda with the volume fraction of doping particles, ph i. The main experimental features are that lambda is constant at small phi then decreases, and that this diminution is enhanced at large swe lling of the phases. The results and their consequences on K and BBAR are discussed ; they support and complement previous neutron scatterin g studies. We also observe edge dislocations with very small Burgers v ector, that were not expected in such swollen phases. A microscopic mo del is proposed to evaluate lambda from such defects.