Confinement and magnetic-field effect on chiral ferroelectric nematic liquid crystals in Grandjean-Cano wedge cells

We explore the structure and magnetic-field response of edge dislocations in Grandjean-Cano wedge cells filled with chiral mixtures of the ferroelectric nematic mesogen DIO. Upon cooling, the ordering changes from paraelectric in the cholesteric phase 4pt0exN* to antiferroelectric in the smectic SmZA* and to ferroelectric in the cholesteric NF*. Dislocations of the Burgers vector b equal to the helicoidal pitch P are stable in all three phases, while dislocations with b=P/2 exist only in the 4pt0exN* and SmZA*. The b=P/2 dislocations split into pairs of τ^-1/2λ+1/2 disclinations, while the thick dislocations b=P are pairs of nonsingular λ^-1/2λ+1/2 disclinations. The polar order makes the τ^-1/2 disclinations unstable in the NF* phase, as they should be connected to singular walls in the polarization field. We propose a model of transformation of the composite τ^-1/2 line-wall defect into a nonsingular λ^-1/2 disclination, which is paired up with a λ+1/2 line to form a b=P dislocation. The SmZA* behavior in the in-plane magnetic field is different from that of the NF* and N*: the dislocations show no zigzag instability, and the pitch remains unchanged in the magnetic fields up to 1 T. The behavior is associated with the finite compressibility of smectic layers.