Ferroelectrically tunable topological phase transition in In2Se3 thin films

Materials with ferroelectrically switchable topological properties are of interest for both fundamental physics and practical applications. Using first-principles calculations, we find that stacking ferroelectric $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ monolayers into a bilayer leads to polarization-dependent band structures, which yields polarization-dependent topological properties. Specifically, we find that the states with interlayer ferroelectric couplings are quantum spin Hall insulators, while those with antiferroelectric polarizations are normal insulators. We further find that ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ trilayer and quadlayer exhibit nontrivial band topology as long as in the structure the ferroelectric ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ bilayer is antiferroelectrically coupled to ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ monolayers or other ferroelectric ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ bilayer. Otherwise the system is topologically trivial. The reason is that near the Fermi level the band structure of the ferroelectric ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ bilayer has to be maintained for the nontrivial band topology. This feature can be used to design nontrivial band topology for the thicker films by a proper combination of the interlayer polarization couplings. The topological properties can be ferroelectrically tunable using the dipole locking effect. Our study reveals switchable band topology in a family of natural ferroelectrics, which provide a platform for designing new functional devices.