Theory of polarization textures in crystal supercells

Abstract

Recently, topologically nontrivial polarization textures have been predicted and observed in nanoscale systems. While these polarization textures are interesting and promising in terms of applications, their topology in general is yet to be fully understood. For example, the relation between topological polarization structures and band topology has not been explored, and polar domain structures are typically considered in topologically trivial systems. In particular, the local polarization in a crystal supercell is not well-defined, and typically calculated using approximations which do not satisfy gauge invariance. Furthermore, local polarization in supercells is typically approximated using calculations involving smaller unit cells, meaning the connection to the electronic structure of the supercell is lost. In this work, we propose a definition of local polarization which is gauge invariant and can be calculated directly from a supercell without approximations. We show using first-principles calculations for commensurate bilayer hexagonal boron nitride that our expressions for local polarization give the correct result at the unit cell level, which is a first approximation to the local polarization in a moiré superlattice. We also illustrate using an effective model that the local polarization can be directly calculated in real space. Finally, we discuss the relation between polarization and band topology, for which it is essential to have a correct definition of polarization textures.

16 pages, 6 figures

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• Ferroelectricity-tuned band topology and superconductivity in two-dimensional materials and related heterostructures
• Twisted bilayer graphene revisited: minimal two-band model for low-energy bands
• Quantum-Geometric Origin of Out-of-plane Stacking Ferroelectricity
• Flat and tunable moire phonons in twisted transition-metal dichalcogenides
• Asymmetric dynamical charges in two-dimensional ferroelectrics
• Twistronics and moiré superlattice physics in 2D transition metal dichalcogenides
• Polarization textures in crystal supercells with topological bands
• Shift photocurrent vortices from topological polarization textures
• 2D Nitride Ordered Alloys: A Novel Class of Ultra-Wide Bandgap Semiconductors