The intrinsic magnetic topological insulators MnBi2X4 (X = Se, Te) are promising candidates in realizing various novel topological states related to symmetry breaking by magnetic order. Although much progress had been made in MnBi2Te4, the study of MnBi2Se4 has been lacking due to the difficulty of material synthesis of the desired trigonal phase. Here, we report the synthesis of multilayer trigonal MnBi2Se4 with alternating-layer molecular beam epitaxy. Atomic-resolution scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM) identify a well-ordered multilayer van der Waals (vdW) crystal with septuple-layer base units in agreement with the trigonal structure. Systematic thickness-dependent magnetometry studies illustrate the layered antiferromagnetic ordering as predicted by theory. Angle-resolved photoemission spectroscopy (ARPES) reveals the gapless Dirac-like surface state of MnBi2Se4, which demonstrates that MnBi2Se4 is a topological insulator above the magnetic ordering temperature. These systematic studies show that MnBi2Se4 is a promising candidate for exploring the rich topological phases of layered antiferromagnetic topological insulators.

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We report the crystal and electronic structures of a non-centrosymmetric quasi-two-dimensional (2D), candidate of topological semimetal AuTe2Br. The Fermi surface of this layered compound consists of 2D-like, topological trivial electron and non-trivial hole pockets which host a Dirac cone along the kz direction. Our transport measurements on the single crystals show highly anisotropic, compensated low-density electrons and holes, both of which exhibit ultrahigh mobility at a level of 10 5 cm 2 V −1 s −1 at low temperature.The highly mobile, compensated carriers lead a non-saturated, parabolic magnetoresistance as large as 3 × 10 5 in single-crystalline AuTe2Br in a magnetic field up to 58 T.

Spin−orbit torque phenomena enable efficient manipulation of the magnetization in ferromagnet/heavy metal bilayer systems for prospective magnetic memory and logic applications. Kagome magnets are of particular interest for spin− orbit torque due to the interplay of magnetic order and the nontrivial band topology (e.g., flat bands and Dirac and Weyl points). Here we demonstrate spin−orbit torque and quantify its efficiency in a bilayer system of topological kagome ferromagnet Fe 3 Sn 2 and platinum. We use two different techniques, one based on the quasistatic magneto-optic Kerr effect (MOKE) and another based on time-resolved MOKE, to quantify spin−orbit torque. Both techniques give a consistent value of the effective spin Hall angle of the Fe 3 Sn 2 /Pt system. Our work may lead to further advances in spintronics based on topological kagome magnets.

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