Optical evidence of the layered array of grain boundaries in TGBA and TGBC mesophases

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Abstract
We report optical pitch measurements on chiral mesophases : cholesteric, twist grain boundary smectic (TGBA and TGBC) and helicoidal smectic C. The nature of the phase transitions is discussed. A new series of defect lines parallel to the usual Grandjean-Cano steps is observed in the TGB phases only. They are interpreted as edge dislocations of the layered array of smectic slabs stacked along the TGB screw axis and thus as a direct visual evidence of the Renn-Lubensky structure. Their origin and stability are discussed in an elastic model of strain of a TGB sample. Their core is described as a wall constituted of edge dislocation lines

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