The Axion Insulator State in Hundred-Nanometer-Thick Magnetic Topological Insulator Sandwich Heterostructures*

One unique feature of the magnetic topological materials is that their electromagnetic response includes a topological coupling θ term, in addition to the ordinary Maxwell’s Lagrangian, leading to the so-called “axion electrodynamics”. An axion insulator is viewed as a 3D topological insulator (TI) with a quantized non-zero θ parameter in the bulk and the surface states gapped by surface magnetization. To date, the axion insulator states have been claimed in both molecular beam epitaxy (MBE)-grown magnetic TI sandwiches and exfoliated MnBi₂Te₄ flakes with “even” number layers. All these samples have a thickness of less than ~12 nm, which is still near the 2D-to-3D boundary. In this work, we use MBE to synthesize magnetic TI sandwich heterostructures with thicknesses up to 106 nm. We find a robust axion insulator state appears in all these magnetic TI heterostructures. Moreover, we find when the thickness of the middle undoped TI layer is less than 3nm, the zero Hall conductance plateau disappears, and thus the samples transition to the standard quantum anomalous Hall (QAH) insulating state. The realization of the hundred-nanometer-thick axion insulators provides an outstanding platform for the exploration of the emergent topological states in 3D magnetic TI multilayers, including the high-order TI phase.