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Dechiralisation lines induced unwinding in confined SmC* liquid crystals

Equilibrium and dynamic behaviors of dechiralisation lines occurring below SmC* cell boundaries in planar geometry consitute a polemic topic for several decades. We report new observations in the SmC* phase of the strongly polarized liquid crystal CFL08. They are compared with vortex motions in superfluids and type-II superconductors, and we propose a simple heuristic model attempting to explain the intriguing mismatch between bulk helix pitch and inter-lines distance. It results from opposite effects of the helix pitch and surface lines charge value on the equilibrium lines distance. The model assumes that the lines electric interactions dominate the cell equilibrium state. In particular the unwinding transition of the bulk helix is provoked by field-induced displacements of the lines lattices. It permits to relate the lines density and critical fields to intrinsic energy parameters, and to explain the pitch-distance mismatch, together with its almost constant value across the transition. We foresee the helix pitch and unwinding field variations vs. sample width.