Selection of defect structures in twist-grain-boundary-A phase of chiral liquid crystals

We study the structure of the twist grain boundary in chiral smectic liquid crystals using the Landau-de Gennes model. By considering spatial variation of the smectic order, we distinguish the Melted-Grain-Boundary-A (MGBA) structure with cholesteric-like domains and the Twist-Grain-Boundary-A (TGBA) structure consisting of screw dislocations. The MGBA structure becomes stable near the transition to the cholesteric phase. On approaching the transition, the cholesteric-like domain grows outside the grain boundary. Also, the dislocation spacings of the TGBA structure agree better with experimental results than previous theories.