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

Crystal growth of MnBi 2 Te 4 has delivered the first experimental corroboration of the 3D antiferromagnetic topological insulator state. Our present results confirm that the synthesis of MnBi 2 Te 4 can be scaled-up and strengthen it as a promising experimental platform for studies of a crossover between magnetic ordering and non-trivial topology. High-quality single crystals of MnBi 2 Te 4 are grown by slow cooling within a narrow range between the melting points of Bi 2 Te 3 (586 °C) and MnBi 2 Te 4 (600 °C). Single crystal X-ray diffraction and electron microscopy reveal ubiquitous antisite defects in both cation sites and, possibly, Mn vacancies. Powders of MnBi 2 Te 4 can be obtained at subsolidus temperatures, and a complementary thermochemical study establishes a limited high-temperature range of phase stability. Nevertheless, quenched powders are stable at room temperature and exhibit long-range antiferromagnetic ordering below 24 K. The expected Mn(II) out-of-plane magnetic state is confirmed by the magnetization, X-ray photoemission, X-ray absorption and linear dichroism data. MnBi 2 Te 4 exhibits a metallic type of resistivity in the range 4.5-300 K. The compound is an n-type conductor that reaches a thermoelectric figure of merit up to ZT = 0.17. Angle-resolved photoemission experiments provide evidence for a surface state forming a gapped Dirac cone.

A novel method is demonstrated to tailor the electronic structure of high-Z surface alloys showing a giant Rashba splitting by means of electron doping. This aims for new model systems to study unconvential spin topologies and for possible applications in the field of spintronics. We give new insights into the origin of the enhanced Rashba effect in the two specific alloys Bi/Cu(111) and Bi/Ag(111) employing angle-resolved photoelectron spectroscopy and firstprinciples calculations.

We have studied by angle-resolved photoelectron spectroscopy the ordered XCu 2 surface alloys formed by X = Sb and Bi on a Cu͑111͒ substrate. We found clear analogies with the corresponding XAg 2 alloys formed by the same elements on Ag͑111͒. The electronic states near the Fermi level are similarly split by the spin-orbit ͑SO͒ interaction with the smaller splitting in XCu 2 reflecting the smaller atomic SO parameter of Cu vs Ag. The charge transfer from the Bi and Sb adatoms to the substrate atoms and their outward relaxation are different for two substrates; both determine the Fermi energy of the two-dimensional electron gas.

The layered van der Waals antiferromagnet MnBi2Te4 has been predicted to combine the band ordering of archetypical topological insulators like Bi2Te3 with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi2Te4(0001) single crystals by use of spin-and angle-resolved photoelectron spectroscopy (ARPES) experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.The hallmark of a topological insulator is a single spinpolarized Dirac cone at the surface which is protected by time reversal-symmetry and originates from a band inversion in the bulk [1,2]. Notably, breaking time-reversal symmetry by magnetic order does not necessarily destroy the non-trivial topology but instead may drive the system into another topological phase. One example is the quantum anomalous Hall (QAH) state that has been observed in magnetically doped topological insulators [3]. The QAH state, in turn, may form the basis for yet more exotic electronic states, such as axion insulators [4,5] and chiral Majorana fermions [6]. Another example is the antiferromagnetic topological insulator state which is protected by a combination of time-reversal and lattice translational symmetries [7].Magnetic order in a topological insulator has mainly been achieved by doping with 3d impurities [3,8], which however inevitably gives rise to increased disorder. By contrast, the layered van der Waals material MnBi 2 Te 4 [9, 10] has recently been proposed to realize an intrinsic magnetic topological insulator [11][12][13][14], i.e. a compound that features magnetic order and a topologically non-trivial bulk band structure at the arXiv:1903.11826v2 [cond-mat.str-el]