Intrinsic Defect-Induced Exotic Polarization Waves in Van der Waals Layered Topological Insulators

The engineering of inversion symmetry breaking in 2D materials, for example via interlayer twist-angle control, has initiated the '2D age' due to its transformation on the landscape of fundamental research and technological advances. The typical paradigms include nonlinear optical response, topological polar vortices, superconductivity, etc. In parallel, intrinsic structural defects induced inversion symmetry breaking also serves as a lever to create emerging physical phenomena. Here, we report on discovery of antisite defects induced exotic polarization waves in bismuth telluride (Bi₂)_m(Bi₂Te₃)_n topological insulator thin films by using atomic-resolution scanning transmission electron microscopy. Despite weak van der Waals (vdW) bonding, driven by stochastic antisite defects, a quasi-cycloidal polarization order is observed along normal direction of the vdW interface in Bi₂Te₃ phase. As metallic Bi-Bi bilayer is replaced by Bi-Te bilayer via regular Te_Bi antisite defect, hyperbolic polarization waves are observed at septuple layers (SLs) of the Bi₈Te₉ phase. Our first-principles calculations reveal that the face-shared unusual packing of acentric octahedral units and insufficient carrier screening are responsible for the polar metal states. Our findings provide a new degree of freedom to manipulate the physical properties of topological insulators (TIs) and their future device application.