The topological magnetoelectric (TME) effect is a special version of the magnetoelectric effect, showing quantized response functions. The Z2 topological insulators (TIs) with gapped surface states and time reversal invariant bulk states are predicted to show the TME effect, and are called axion insulators. We present our recent observation of the axion insulator state in a magnetically doped TI ((Bix, Sb1-x)2Te3) sandwich heterostructure, where the top and bottom layers are doped with V and Cr respectively, and are separated by an undoped TI middle layer. Magnetic force microscopy measurements confirm the existence of parallel and antiparallel magnetization of two magnetic layers by sweeping the external magnetic field. Transport measurements unambiguously demonstrate the quantum anomalous Hall (QAH) state (parallel), and the axion insulator state (antiparallel), where we observe a large longitudinal resistance and vanishing longitudinal and Hall conductance. Our findings thus show evidence for a phase of matter distinct from the established QAH state and provide a promising platform for the realization of the TME effect.
Funded by ONR, ARO MURI, and NSF MIP.