  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.

Soft Condensed Matter