Unusual Ferroelectricity of Trans-Unitcell Ion-Displacement and Multiferroic Soliton in Sodium and Potassium Hydroxides.

We show the first-principles evidence of a hitherto unreported type of ferroelectricity with ultralong ion-displacement in sodium and potassium hydroxides. Even a small number of proton vacancies can completely change the mode of proton-transfer from intra-unitcell to trans-unitcell, giving rise to multiferroic soliton with "mobile" magnetism and a tremendous polarization that can be orders of magnitude higher compared with most perovskite ferroelectrics. Their vertical polarizations of thin-film are robust against a depolarizing field, rendering various designs of two-dimensional ferroelectric field-transistors with nondestructive readout and ultrahigh on/off ratio via sensing the switchable metallic/insulating state.