Strongly enhanced topological quantum phases in dual-surface AlO${}_x$-encapsulated MnBi${}_2$Te${}_4$

The topological quantum phases in antiferromagnetic topological insulator MnBi${}_2$Te${}_4$ hold promise for next-generation spintronics, but their experimental realization has been constrained by challenges in preparing high-quality devices. In this work, we report a new wax-assisted exfoliation and transfer method that enables the fabrication of MnBi${}_2$Te${}_4$ heterostructures with both surfaces encapsulated by AlO${}_x$. This strategy strongly enhances the topological quantum phases in MnBi${}_2$Te${}_4$ flakes. We observe the robust axion insulator state in even-layer device with wide zero Hall plateau and high longitudinal resistivity, and the quantum anomalous Hall effect in odd-layer device with large hysteresis and sharp plateau transition. These results demonstrate that the combination of wax exfoliation and AlO${}_x$ encapsulation provides great potentials for exploring novel topological quantum phenomena and potential applications in MnBi${}_2$Te${}_4$ and other two-dimensional materials.