¦ van der Waals gap Septuple layer J § J || T 1/2 J 0,1 || J 0,2 || J 0,3 || J 0,4 ||

The recent discovered antiferromagnetic topological insulators in 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 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 between the bulk band gap. Such unexpected surface state remains unchanged across the bulk Né el 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 reveal that the intrinsic magnetic topological insulator hosts a rich platform to realize various topological phases such

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9–35 K), light polarizations and photon energies. We have distinguished both large (60–70 meV) and reduced ( ) gaps at the DP in the ARPES dispersions, which remain open above the Neél temperature ( ). We propose that the gap above remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the “large gap” sample and apparently significantly reduced effective magnetic moment for the “reduced gap” sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calculations surface structural modification can result in a significant modulation of the DP gap.

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† A.M.S. and D.A.E. contributed equally.above-mentioned effects at high temperatures, which opens a quest for materials with large DP gaps.Recently, the first intrinsic antiferromagnetic (AFM) TI MnBi 2 Te 4 (MBT) has been discovered and extensively investigated [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. This layered compound consists of the septuple layer (SL) blocks with a stacking sequence of Te-Bi-Te-Mn-Te-Bi-Te [16,18]. The neighboring blocks are separated by van der Waals (vdW) spacings. Within each SL, Mn atoms are ordered ferromagnetically, while the adjacent SLs are coupled in an AFM fashion [17,19], as shown in Fig. 1(a). Density functional theory (DFT) calculations predict the MBT(0001) surface to exhibit a giant DP gap 2469-9950/2021/104(11)/115168(11)