Helix Alignment, Chevrons, and Edge Dislocations in Twist-Bend Ferroelectric Nematics

Surface alignment and edge dislocations are explored in the recently discovered twist-bend ferroelectric nematic, N_TBF, in which the vector of spontaneous polarization follows an oblique helicoidal trajectory around a polar twist-bend axis. In a planar cell, the polar axis aligns at some angle to the rubbing direction to mitigate the surface electric charge. It is demonstrated that the pseudolayers in planar cells form chevron defects, a hallmark defect of one-dimensionally positionally ordered phases, such as smectic A and smectic C. The polar character of the twist-bend axis prevents the cores of N_TBF edge dislocations from splitting into semi-integer disclinations, in stark contrast to dislocations in paraelectric and ferroelectric chiral nematics. The tilt of pseudolayers around the defect core allows estimation of the elastic penetration length as being close to the pitch of N_TBF. Compression/dilation stresses around the core modify the heliconical tilt angle of molecules as evidenced by a substantial variation in local birefringence. The climb of dislocations exhibits high mobility, allowing the system to equilibrate the temperature-dependent pitch. The uncovered properties facilitate the development of N_TBF materials for electro-optical applications, such as electrically controlled diffraction lattices and structural colors.