Nonlinear Charge Transport in Magnetic Topological Insulator Sandwiches

The wavefunction geometry has been used to describe the electron movement in solid-state systems. One of its key components is the quantum metric, which quantifies the distance between neighboring wavefunctions. The quantum metric has recently been identified as the origin of the nonlinear Hall effect in exfoliated antiferromagnetic topological insulator (TI) MnBi₂Te₄ flakes. All these samples are highly metallic and far from well-quantized Chern insulator and axion insulator regimes. In this work, we employed molecular beam epitaxy (MBE) to synthesize magnetically doped TI sandwiches. We observed the nonlinear anomalous Hall effect and the non-reciprocal longitudinal response near the quantum anomalous Hall (QAH) insulator and axion insulator regimes. We found that both the transverse and longitudinal nonlinear responses flip signs when the magnetic order is reversed in the QAH or axion insulator states, which agrees well with our theoretical calculations. The magnetic TI sandwiches provide a promising platform for exploring nonlinear transport and developing magnetic nonlinear devices.