Gapless Surface Dirac Cone in Antiferromagnetic Topological Insulator MnBi2Te4

The recently discovered antiferromagnetic topological insulators in the Mn-Bi-Te family with intrinsic magnetic ordering have rapidly drawn broad interest since its cleaved surface state is believed to be gapped, hosting the unprecedented axion states with a half-integer quantum Hall effect. Here, however, we show unambiguously by using high-resolution angle resolved photoemission spectroscopy that a gapless Dirac cone at the (0001) surface of MnBi2Te4 exists inside the bulk band gap. Such an unexpected surface state remains unchanged across the bulk Neel temperature, and is even robust against severe surface degradation, indicating additional topological protection. Through symmetry analysis and ab initio calculations we consider different types of surface reconstruction of the magnetic moments as possible origins giving rise to such linear dispersion. Our results unveil the experimental topological properties of MnBi2Te4, revealing that the intrinsic magnetic topological insulator hosts a rich platform to realize various topological phases by tuning the magnetic or structural configurations, and thus push forward the comprehensive understanding of magnetic topological materials.