Sliding van der Waals Polytypes

Compared to electronic phase transitions, structural phase transitions of crystals are challenging to control due to the energy cost of breaking dense solid bonds. Recently, however, electric field switching of stacking configuration between honeycomb layers, held together by relatively weak van der Waals (vdW) attractions, was demonstrated. In response to the external fields, the layers slide between commensurate meta-stable configurations with discrete symmetries and distinct lattice orientations. These 2D vdW polytypes host diverse electronic orders such as ferroelectricity and magnetism, providing multiferroic switching via lubricant sliding of incommensurate boundary strips. Ahead, we address recent observations in honeycomb polytypes and identify remaining challenges for extending this conceptual "SlideTronics" mechanism into rapid, local, and practical multiferroic devices. The stacking energies, symmetries, and orbital overlaps that underlie the band structures and internal charge distributions are discussed, along with poly-properties like interfacial-ferroelectricity, ladder-like cumulative polarization, superconductivity, and orbital magnetic orders. Distinct from conventional 3D multiferroic crystals, the 2D vdW assembly and the sliding switching mechanism open poly-opportunities for novel device concepts.