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 N * to antiferroelectric in the smectic SmZ * A and to ferroelectric in the cholesteric N * F . 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 N * and SmZ * A . The b = P /2 dislocations split into pairs of tau - 1 /2 lambda + 1 /2 disclinations, while the thick dislocations b = P are pairs of nonsingular lambda - 1 /2 lambda + 1 /2 disclinations. The polar order makes the tau - 1 /2 disclinations unstable in the N * F phase, as they should be connected to singular walls in the polarization field. We propose a model of transformation of the composite tau - 1 /2 line -wall defect into a nonsingular lambda - 1 /2 disclination, which is paired up with a lambda + 1 /2 line to form a b = P dislocation. The SmZ * A behavior in the in -plane magnetic field is different from that of the N * F 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.