Direct visualization of electronic transport in a quantum anomalous Hall insulator

A quantum anomalous Hall (QAH) insulator is characterized by quantized Hall and vanishing longitudinal resistances at zero magnetic field that are protected against local perturbations and independent of sample details. This insensitivity makes the microscopic details of the local current distribution inaccessible to global transport measurements. Accordingly, the current distributions that give rise to the transport quantization are unknown. In this talk, I will show how we use magnetic imaging to directly visualize the transport current in the QAH regime. As we tune through the QAH plateau by electrostatic gating, we clearly identify a regime in which the sample transports current primarily in the bulk rather than along the edges. Images close to the source and drain of the device show that the current enters and leaves through hot-spots in the corners of the contacts. Furthermore, we image the local response of the equilibrium magnetization to electrostatic gating. Combined, these measurements suggest that the current flows through incompressible regions throughout the QAH regime. Identifying the appropriate microscopic picture of electronic transport in QAH insulators and other topologically non-trivial states of matter is a crucial step toward realizing their potential in next-generation quantum devices.