Extremely high magnetoresistance (XMR) in the lanthanum monopnictides LaX (X = Sb, Bi) has recently attracted interest in these compounds as candidate topological materials. However, their perfect electron-hole compensation provides an alternative explanation, so the possible role of topological surface states requires verification through direct observation. Our angle-resolved photoemission spectroscopy (ARPES) data reveal multiple Dirac-like surface states near the Fermi level in both materials. Intriguingly, we have observed circular dichroism in both surface and near-surface bulk bands. Thus the spin-orbit coupling-induced orbital and spin angular momentum textures may provide a mechanism to forbid backscattering in zero field, suggesting that surface and near-surface bulk bands may contribute strongly to XMR in LaX. The extremely simple rock salt structure of these materials and the ease with which high-quality crystals can be prepared suggests that they may be an ideal platform for further investigation of topological matter.

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Intrinsic magnetic topological insulator MnBi 2 Te 4 is the key to realizing the quantum anomalous Hall effect and other related quantum phenomena at a sufficiently high temperature for their practical electronic applications. The research progress on the novel material, however, is severely hindered by the extreme difficulty in preparing its high-quality thin films with well-controlled composition and thickness. Combining molecular beam epitaxy (MBE) and in situ angle-resolved photoemission spectroscopy (ARPES), we have systematically studied the growth conditions and kinetics of MnBi 2 Te 4 thin films prepared by simple co-evaporation of Mn, Bi and Te. The transition and competition between the Mn-doped Bi 2 Te 3 and MnBi 2 Te 4 phases under different growth conditions have been mapped in detail, which gives the recipe and principles of growing high-quality MnBi 2 Te 4 thin films. Particularly, to obtain high quality MnBi 2 Te 4 films, it is crucial to raise the growth temperature as high as allowed by the nucleation of the films to minimize density of Mn substitutional atoms on Bi sites. The ARPES data also map the kinetic process in the nucleation and ripening of MnBi 2 Te 4 islands. These results offer the essential information for designing and optimizing the MBE growth procedure of MnBi 2 Te 4 -like compounds to achieve the exotic topological quantum effects.