A MODEL OF CHEVRONS IN SMECTIC C-ASTERISK LIQUID-CRYSTALS - UNIFORM AND TWISTED SOLITON STATES

We consider a simplified model of the chevron structure observed in thin cells of ferroelectric smectic liquid crystals, based on a generalization of the ''uniaxial'' (or ''nematic'') approximation of orientation elasticity. This model allows for the conciliation of the continuous nature of the layers with the matching conditions postulated by Clark, and takes large rotations of the c-director into account. When the layer tilt angle is small compared to the molecular cone angle, we show that the structure of the ''uniform'' states reduces to the ''soliton'' solution found by Nakagawa. Within the same limit, ''twisted'' states are investigated by a perturbation calculation and by qualitative energy considerations. The results suggest the existence of asymmetrical c-director distributions similar to those observed in recent experiments: in these states, the c-director splay remains mainly localized in one half of the cell, the increase of splay energy being compensated by a reduction of the ''folding energy'' localized in the chevron core.