Synthesis, Magnetic Properties, and Electronic Structure of Magnetic Topological Insulator MnBiSe.

The intrinsic magnetic topological insulators MnBiTe and MnBiSe support novel topological states related to symmetry breaking by magnetic order. Unlike MnBiTe, the study of MnBiSe has been inhibited by the lack of bulk crystals, as the van der Waals (vdW) crystal is not the thermodynamic equilibrium phase. Here, we report the layer-by-layer synthesis of vdW MnBiSe crystals using nonequilibrium molecular beam epitaxy. Atomic-resolution scanning transmission electron microscopy and scanning tunneling microscopy identify a well-ordered vdW crystal with septuple-layer base units. The magnetic properties agree with the predicted layered antiferromagnetic ordering but disagree with its predicted out-of-plane orientation. Instead, our samples exhibit an easy-plane anisotropy, which is explained by including dipole-dipole interactions. Angle-resolved photoemission spectroscopy reveals the gapless Dirac-like surface state, which demonstrates that MnBiSe is a topological insulator above the magnetic-ordering temperature. These studies show that MnBiSe is a promising candidate for exploring rich topological phases of layered antiferromagnetic topological insulators.