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Physical review letters 129(24), 246404 (2022). doi:10.1103/PhysRevLett.129.246404, The electronic structure of Weyl semimetals features Berry flux monopoles in the bulk and Fermi arcs at the surface. While angle-resolved photoelectron spectroscopy (ARPES) is successfully used to map the bulk and surface bands, it remains a challenge to explicitly resolve and pinpoint these topological features. Here we combine state-of-the-art photoemission theory and experiments over a wide range of excitation energies for the Weyl semimetals TaAs and TaP. Our results show that simple surface-band-counting schemes, proposed previously to identify nonzero Chern numbers, are ambiguous due to pronounced momentum-dependent spectral weight variations and the pronounced surface-bulk hybridization. Instead, our findings indicate that dichroic ARPES provides an improved approach to identify Fermi arcs but requires an accurate description of the photoelectron final state., Published by APS, College Park, Md.

The Rashba effect is fundamental to the physics of two-dimensional electron systems and underlies a variety of spintronic phenomena. It has been proposed that the formation of Rashba-type spin splittings originates microscopically from the existence of orbital angular momentum (OAM) in the Bloch wave functions. Here, we present detailed experimental evidence for this OAM-based origin of the Rashba effect by angle-resolved photoemission (ARPES) and two-photon photoemission (2PPE) experiments for a monolayer AgTe on Ag(111). Using quantitative low-energy electron diffraction (LEED) analysis we determine the structural parameters and the stacking of the honeycomb overlayer with picometer precision. Based on an orbital-symmetry analysis in ARPES and supported by first-principles calculations, we unequivocally relate the presence and absence of Rashba-type spin splittings in different bands of AgTe to the existence of OAM.

The layered van der Waals antiferromagnet MnBi$_2$Te$_4$ has been predicted to combine the band ordering of archetypical topological insulators such as Bi$_2$Te$_3$ 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 MnBi$_2$Te$_4$(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy 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., Revised version

Chemistry of materials 31(8), 2795 - 2806 (2019). doi:10.1021/acs.chemmater.8b05017, High-quality single crystals of MnBi2Te4 are grown for the first time 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 (Mn$_{0.85(3)}$Bi$_{2.10(3)}$Te$_4$). Thermochemical studies complemented with high-temperature X-ray diffraction establish a limited high-temperature range of phase stability and metastability at room temperature. Nevertheless, the synthesis of MnBi$_2$Te$_4$ can be scaled-up as powders can be obtained at subsolidus temperatures and quenched at room temperature. Bulk samples exhibit long-range antiferromagnetic ordering below 24 K. The Mn(II) out-of-plane magnetic state is confirmed by the magnetization, X-ray photoemission, X-ray absorption, and linear dichroism measurements. The compound shows a metallic type of resistivity in the range 4.5–300 K and is an n-type conductor that reaches a thermoelectric figure of merit up to ZT = 0.17. Angle-resolved photoemission experiments show a surface state forming a gapped Dirac cone, thus strengthening MnBi$_2$Te$_4$ as a promising candidate for the intrinsic magnetic topological insulator, in accordance with theoretical predictions. The developed synthetic protocols enable further experimental studies of a crossover between magnetic ordering and nontrivial topology in bulk MnBi$_2$Te$_4$., Published by American Chemical Society, Washington, DC

Magnetic topological insulators are narrow-gap semiconductor materials that combine non-trivial band topology and magnetic order. Unlike their nonmagnetic counterparts, magnetic topological insulators may have some of the surfaces gapped, which enables a number of exotic phenomena that have potential applications in spintronics, such as the quantum anomalous Hall effect and chiral Majorana fermions. So far, magnetic topological insulators have only been created by means of doping nonmagnetic topological insulators with 3d transition-metal elements; however, such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temperatures. An intrinsic magnetic topological insulator—a stoichiometric well ordered magnetic compound—could be an ideal solution to these problems, but no such material has been observed so far. Here we predict by ab initio calculations and further confirm using various experimental techniques the realization of an antiferromagnetic topological insulator in the layered van der Waals compound MnBiTe. The antiferromagnetic ordering that MnBiTe shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to a ℤ topological classification; ℤ = 1 for MnBiTe, confirming its topologically nontrivial nature. Our experiments indicate that the symmetry-breaking (0001) surface of MnBiTe exhibits a large bandgap in the topological surface state. We expect this property to eventually enable the observation of a number of fundamental phenomena, among them quantized magnetoelectric coupling and axion electrodynamics. Other exotic phenomena could become accessible at much higher temperatures than those reached so far, such as the quantum anomalous Hall effect and chiral Majorana fermions., We acknowledge support by the Basque Departamento de Educacion, UPV/EHU (grant number IT-756-13), the Spanish Ministerio de Economia y Competitividad (MINECO grant number FIS2016-75862-P), and the Academic D.I. Mendeleev Fund Program of Tomsk State University (project number 8.1.01.2018). Support from the Saint Petersburg State University grant for scientific investigations (grant ID 40990069), the Russian Science Foundation (grants number 18-12-00062 for part of the photoemission measurements and 18-12-00169 for part of the calculations of topological invariants and tight-binding bandstructure calculations), the Russian Foundation for Basic Research (grant number 18-52-06009), and the Science Development Foundation under the President of the Republic of Azerbaijan (grant number EİF-BGM-4-RFTF-1/2017-21/04/1-M-02) is also acknowledged. M.M.O. acknowledges support by the Diputación Foral de Gipuzkoa (project number 2018-CIEN-000025-01). I.I.K. and A.M.S. acknowledge partial support from the CERIC-ERIC consortium for the stay at the Elettra synchrotron. The ARPES measurements at HiSOR were performed with the approval of the Proposal Assessing Committee (proposal numbers 18AG020, 18BU005). The support of the German Research Foundation (DFG) is acknowledged by A.U.B.W., A.I. and B.B. within Collaborative Research Center 1143 (SFB 1143, project ID 247310070); by A.Z., A.E. and A.I. within Special Priority Program 1666 Topological Insulators; by H.B. and F.R. within Collaborative Research Center 1170; and by A.Z. and A.I. within the ERANET-Chemistry Program (RU 776/15-1). H.B., A.U.B.W., A.A., V.K., B.B., F.R. and A.I. acknowledge financial support from the DFG through the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter – ct.qmat (EXC 2147, project ID 39085490). A.E. acknowledges support by the OeAD grant numbers HR 07/2018 and PL 03/2018. This work was also supported by the Fundamental Research Program of the State Academies of Sciences, line of research III.23. A.K. was financially supported by KAKENHI number 17H06138 and 18H03683. I.P.R. acknowledges support by the Ministry of Education and Science of the Russian Federation within the framework of the governmental program Megagrants (state task number 3.8895.2017/P220). E.V.C. acknowledges financial support by the Gobierno Vasco-UPV/EHU project (IT1246-19). S.K. acknowledges financial support from an Overseas Postdoctoral Fellowship, SERB-India (OPDF award number SB/OS/PDF-060/2015-16). J.S.-B.acknowledges financial support from the Impuls-und Vernetzungsfonds der Helmholtz-Gemeinschaft under grant number HRSF-0067 (Helmholtz-Russia Joint Research Group). The calculations were performed in Donostia International Physics Center, in the research park of Saint Petersburg State University Computing Center (http://cc.spbu.ru), and at Tomsk State University

The monopnictides TaAs and TaP are well-established Weyl semimetals. Yet, a precise assignment of Fermi arcs, accommodating the predicted chiral charge of the bulk Weyl points, has been difficult in these systems, and the topological character of different surface features in the Fermi surface is not fully understood. Here, employing a joint analysis from linear dichroism in angle-resolved photoemission and first-principles calculations, we unveil the orbital texture on the full Fermi surface of TaP(001). We observe pronounced switches in the orbital texture at the projected Weyl nodes, and show how they facilitate a topological classification of the surface band structure. Our findings establish a critical role of the orbital degrees of freedom in mediating the surface-bulk connectivity in Weyl semimetals.