Here we present microscopic evidence of the persistence of uniaxial A-type antiferromagnetic order to the surface layers of MnBi2Te4 single crystals using magnetic force microscopy. Our results reveal termination-dependent magnetic contrast across both surface step edges and domain walls, which can be screened by thin layers of soft magnetism. The robust surface A-type order is further corroborated by the observation of termination-dependent surface spin-flop transitions, which have been theoretically proposed decades ago. Our results not only provide key ingredients for understanding the electronic properties of the antiferromagnetic topological insulator MnBi2Te4, but also open a new paradigm for exploring intrinsic surface metamagnetic transitions in natural antiferromagnets.Recent progress in topological quantum materials suggest that antiferromagnets may host interesting topological states [1]. For example, it has been proposed that an axion insulator state with topological magnetoelectric response could be realized in an antiferromagentic topological insulator (TI) phase [2,3], where the Z2 topological states are protected by a combination of timereversal symmetry and primitive-lattice translation. The antiferromagnetic TI state adiabatically connects to a stack of quantum Hall insulators with alternating Chern numbers [4], thus providing a promising route to realizing the quantum anomalous Hall (QAH) effect in a stoichiometric material. The prior observation of the QAH effect in magnetically doped TI thin films is limited to extremely low temperature because of the inherent disorder [5][6][7][8][9], though the disorder effects can be partially alleviated by material engineering [10][11][12]. The MnBi2Te4 (MBT) family was predicted and confirmed to be an antiferromagnetic TI that may host QAH and axion-insulator states in thin films with odd and even numbers of septuple layers (SLs) respectively [13][14][15][16][17]. Recent transport measurements on exfoliated thin flakes provide compelling evidence for these predictions [18,19], suggesting gapped topological surface states. On the other hand, recent highresolution angle-resolved photoemission spectroscopy (ARPES) studies reveal gapless (or smallgap) surface states below the antiferromagnetic ordering temperature, suggesting a surface relaxation of the A-type order and/or the formation of nanometer-sized magnetic domains [20][21][22][23]. The antiferromagnetic domain structure of MnBi2Te4 was revealed by imaging of domain walls using magnetic force microscopy (MFM) [24]. The observed domain size is on the order of 10 µm, excluding the speculated nanometer-size domain scenario [22].

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Under pressure, the semiconductor PbSnTe transitions from an insulator to a Weyl metal that displays a large Berry curvature.

The polarization and piezoelectric response of the BiTeX (X=Cl, Br, and I) layered tellurohalides are computed from first principles. The results confirm a mixed ionic-covalent character of the bonding, and demonstrate that the internal structure within each triple layer is only weakly affected by the external stress, while the changes in the charge distribution with stress produce a substantial negative piezoelectric response. This suggests a new mechanism for negative piezoelectric response that should remain robust even in ultra-thin film form in this class of materials. arXiv:1906.07235v1 [cond-mat.mtrl-sci]

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