Three-dimensional quantum Anomalous Hall effect in hundred-nanometer-thick magnetic topological insulator trilayers

Magnetic topological states refer to a class of exotic phases in magnetic materials with their non-trivial topological property determined by magnetic spin configurations. The quantum anomalous Hall (QAH) state, a zero magnetic field manifestation of the integer quantum Hall state, is one example of magnetic topological states. To date, most of the experimental efforts on the QAH insulators still focus on the magnetic TI films/heterostructures with thicknesses less than 12nm, which are still in the 2D regime or near the 2D-to-3D boundary. In this work, we optimize the molecular beam epitaxy (MBE) growth conditions and parameter space of magnetic TI sandwiches and succeed in realizing the QAH state in magnetic TI sandwiches with the middle undoped TI layer up to 100 quintuple layers (QL). We find that the magnetic TI sandwiches with the middle undoped TI layers from 4 to 100 QLs show the perfect quantization and similar transport behaviors under different gate voltages. These observations suggest the negligible role of the side surface states in our magnetic TI sandwich samples. For the thick magnetic TI sandwiches with Cr- and V-co-doping in both the top and bottom surface layers, we find that the peak value of the longitudinal resistance near the coercive field is greatly boosted by tuning the bottom gate, implying the feasibility of the realization of the gate-induced QAH to axion insulator state phase transition in a single magnetic TI sandwich sample. The successful synthesis of thick QAH insulators and the possible realization of the gate-induced QAH to axion insulator phase transition pave the way for energy-efficient electronic and spintronic devices and topological quantum computations.