Helical Chern insulator phase with broken time-reversal symmetry in MnBi$_2$Te$_4$

A perpetual quest in the field of topological quantum matter is to search for electronic phases with unprecedented band topology and transport phenomena. The most prominent example is the discovery of topological insulators, in which band inversion leads to topologically nontrivial bulk electronic structure and metallic boundary states. In two-dimensional topological insulators with time-reversal symmetry, a pair of helical edge states gives rise to the quantum spin Hall effect. When the time-reversal symmetry is broken by magnetic order, only one chiral edge mode remains and the quantum anomalous Hall effect emerges in zero magnetic field. This quantum Hall phase without Landau levels, first observed in magnetically doped topological insulators, is now called the Chern insulator. The recently discovered MnBi2Te4 combines intrinsic magnetism and nontrivial topology in one material, providing an ideal platform for exploring novel topological phases. Here, we investigate the transport properties of exfoliated MnBi2Te4 in exceedingly high magnetic fields up to 60 T. By varying the gate voltage, we observe systematic and yet uniquely complex evolution of quantized Hall plateaus with Chern numbers from C = -3 to +2. More surprisingly, a novel phase characterized by an extremely broad zero Hall plateau emerges as the most robust ground state in the high field limit. Theoretical calculations reveal that this C = 0 phase arises from the coexistence of a connate Chern band with C = -1 and a Zeeman-effect-induced Chern band with C = +1, as corroborated by nonlocal transport measurements. This helical Chern insulator phase with broken time-reversal symmetry represents an unexpected new member of the quantum Hall family, and manifests a new route to change the band topology by using magnetic field.