Ferroelectric nematic (NF) liquid crystals are an intriguing polar system for exploring topological defects, and their properties are subject to significant influence by ionic doping. A prior theory based on a modified XY model predicts that string defects with half-integer vortex-antivortex pairs can be excited, while such stable string defects have not been directly observed in polar materials. Here, we report that doping the ferroelectric nematic material RM734 with cationic polymers can facilitate the formation of abundant string defects with butterfly textures. The string defects exhibit a polarization field restricted to 2D plane that is divided by N\'eel type domain walls into domains with either uniform polarization or negative splay deformation in the butterfly wing areas (positive bound charges). We establish a charge double layer model for the string defects: the strings of cationic polymer chains and close packing RM734 molecules form the Stern charge layer, and the small anionic ions and the positive bound charges (due to splay deformation) form the charge diffusion layer. We demonstrate that only cationic polymeric doping is effective due to the coupling between the flexoelectricity and the pear shape of the RM734 molecules. We estimate the line charge density of the strings via measuring the divergence of the polarization and the electrophoretic motion mobility, and obtain good qualitative agreement. We further show that the field-driven polarization reversal undergoes either string rotation or generating and merging with kink walls.
