Symmetry, Multipoles and Sliding Ferroelectricity

Recent advances in the fabrication of artificially-stacked van der Waals systems have lead to the discovery of a new class of polar/ferroelectric materials in which the lateral sliding of weakly-bonded layers against each other acts as a "knob" for controlling their ferroelectric properties. A chemically-intuitive understanding of the processes underlying this phenomenon, however, remains an open question. Here, we use honeycomb bilayer materials as a platform to address this question. Our approach combines group-theoretic techniques, the phenomonology of atomic multipoles and first-principles density functional theory to identify the key components of the electron density, in this case dipole and quadrupole-like distortions of lone pair electrons, are responsible for controlling the sliding ferroelectric transition. Looking ahead, we plan on generalizing this approach to gain insight into more complex sliding ferroelectric systems beyond the honeycomb bilayer.