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Topological phases of matter offer exciting possibilities to realize lossless charge and spin information transport on ultrafast time scales. However, this requires detailed knowledge of their nonequilibrium properties. Here, we employ time-, spin- and angle-resolved photoemission to investigate the ultrafast response of the Sb(111) spin-polarized surface state to femtosecond-laser excitation. The surface state exhibits a giant mass enhancement which is observed as a kink structure in its energy-momentum dispersion above the Fermi level. The kink structure, originating from the direct coupling of the surface state to the bulk continuum, is characterized by an abrupt change in the group velocity by ~70%, in agreement with our GW-based band structure calculations. Our observation of this connectivity in the transiently occupied band structure enables the unambiguous experimental verification of the topological nature of the surface state. The influence of bulk-surface coupling is further confirmed by our measurements of the electron dynamics, which show that bulk and surface states behave as a single thermalizing electronic population with distinct contributions from low-k electron-electron and high-k electron-phonon scatterings. These findings are important for future applications of topological semimetals and their excitations in ultrafast spintronics. Understanding the coupling between spin-polarised topological surface states and the bulk provides insight into ultrafast spin dynamics. Here, this coupling is shown to be accompanied by a large mass enhancement in the Sb(111) surface electronic structure, leading to unusual dynamics.

Ferromagnetic topological insulators exhibit the quantum anomalous Hall effect, which is potentially useful for high-precision metrology, edge channel spintronics, and topological qubits. The stable 2+ state of Mn enables intrinsic magnetic topological insulators. MnBi2Te4 is, however, antiferromagnetic with 25 K Néel temperature and is strongly n-doped. In this work, p-type MnSb2Te4, previously considered topologically trivial, is shown to be a ferromagnetic topological insulator for a few percent Mn excess. i) Ferromagnetic hysteresis with record Curie temperature of 45–50 K, ii) out-of-plane magnetic anisotropy, iii) a 2D Dirac cone with the Dirac point close to the Fermi level, iv) out-of-plane spin polarization as revealed by photoelectron spectroscopy, and v) a magnetically induced bandgap closing at the Curie temperature, demonstrated by scanning tunneling spectroscopy (STS), are shown. Moreover, a critical exponent of the magnetization β ≈ 1 is found, indicating the vicinity of a quantum critical point. Ab initio calculations reveal that Mn–Sb site exchange provides the ferromagnetic interlayer coupling and the slight excess of Mn nearly doubles the Curie temperature. Remaining deviations from the ferromagnetic order open the inverted bulk bandgap and render MnSb2Te4 a robust topological insulator and new benchmark for magnetic topological insulators., The authors thank Ondrej Man for help with lamella preparation for STEM and gratefully acknowledge financial support from the Austrian Science Funds (FWF, I4493-N: P30960-N27), the Impuls- und Vernetzungsfonds der Helmholtz-Gemeinschaft (Grant No. HRSF-0067, Helmholtz-Russia Joint Research Group), the CzechNanoLab project LM2018110 funded by MEYS CR, the CEITEC Nano Research Infrastructure, the Swedish Research Council (Project No. 821-2012-5144), the Swedish Foundation for Strategic Research (Project No. RIF14-0053), the Spanish Ministerio de Ciencia e Innovación (Project No. PID2019-103910GB-I00), Tomsk State University (Project No. 8.1.01.2018), and Saint Petersburg State University (Project No. 73028629). The work was also funded by the Deutsche Forschungsgemeinschaft within SPP1666 Topological Insulators and Germany's Excellence Strategy—Cluster of Excellence Matter and Light for Quantum Computing (ML4Q) EXC 2004/1 — 390534769, and the Graphene Flagship Core 3. G.Bihlmayer gratefully acknowledges computing resources on the supercomputer JURECA at the Jülich Supercomputing Centre., Open access funding enabled and organized by Projekt DEAL.

We show that, although the equilibrium band dispersion of the Shockley-type surface state of two-dimensional Au(111) quantum films grown on W(110) does not deviate from the expected free-electron-like behavior, its nonequilibrium energy-momentum dispersion probed by time- and angle-resolved photoemission exhibits a remarkable kink above the Fermi level due to a significant enhancement of the effective mass. The kink is pronounced for certain thicknesses of the Au quantum well and vanishes in the very thin limit. We identify the kink as induced by the coupling between the Au(111) surface state and emergent quantum-well states which probe directly the buried gold-tungsten interface. The signatures of the coupling are further revealed by our time-resolved measurements which show that surface state and quantum-well states thermalize together behaving as dynamically locked electron populations. In particular, relaxation of hot carriers following laser excitation is similar for both surface state and quantum-well states and much slower than expected for a bulk metallic system. The influence of quantum confinement on the interplay between elementary scattering processes of the electrons at the surface and ultrafast carrier transport in the direction perpendicular to the surface is shown to be the reason for the slow electron dynamics. Nanyang Technological University Published version Financial support from the Impuls-und Vernetzungsfonds der Helmholtz-Gemeinschaft under Grant No. HRSF-0067 (Helmholtz-Russia Joint Research Group) is gratefully acknowledged. M.B. gratefully acknowledges financial support from the Nanyang Technological University, NAP-SUG.

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

Magneticky dopované topologické izolátory umožňují kvantově anomální Hallův efekt (QAHE), která poskytuje kvantizované okrajové stavy pro bezeztrátové aplikace pro transport náboje. Hraniční stavy jsou v Diracově bodě vedeny mezerou magnetické energie, ale dosud všechny pokusy o přímé pozorování této mezery byly neúspěšné. Pozorování mezery je považováno za zásadní pro překonání omezení QAHE, k níž dosud dochází pouze při teplotách, které jsou o jeden až dva řády nižší než teplota feromagnetické Curie, TC. Zde používáme nízkoteplotní fotoelektronovou spektroskopii k jednoznačnému odhalení magnetické mezery Bi2Te3 dopovaného Mn, která zobrazuje feromagnetickou mimosměrnou spinovou strukturu a otevírá se pouze pod TC. Naše analýza překvapivě odhaluje velké mezery při 1 kelvinu až 90 milielektronvoltů, což je ive krát větší, než bylo teoreticky předpovězeno. Použitím analýzy ve více stupních ukazujeme, že toto vylepšení je způsobeno pozoruhodným modifikace struktury vyvolaná dopingem Mn: místo neuspořádaného systému nečistot se vytvoří samoorganizovaná střídavá sekvence septuplů MnBi2Te4 a kvintuplních vrstev Bi2Te3. To zvyšuje překrytí vlnové funkce a velikost magnetické mezery. Mn-dotovaný Bi2Se3 a Mn-dotovaný Sb2Te3 tvoří podobné heterostruktury, ale pro Bi2Se3 se vytvoří pouze nemagnetická mezera a magnetizace je v povrchové rovině. To je vysvětleno menší interakcí spin-orbita ve srovnání s Bi2Te3 dotovaným Mn. Naše ukazatele poskytují poznatky, které budou klíčové při prosazování bezeztrátového transportu v topologických izolátorech směrem k aplikacím s teplotou místnosti. Magnetically doped topological insulators enable the quantum anomalous Hall efect (QAHE), which provides quantized edge states for lossless charge-transport applications. The edge states are hosted by a magnetic energy gap at the Dirac point, but hitherto all attempts to observe this gap directly have been unsuccessful. Observing the gap is considered to be essential to overcoming the limitations of the QAHE, which so far occurs only at temperatures that are one to two orders of magnitude below the ferromagnetic Curie temperature, TC. Here we use lowtemperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi2Te3, which displays ferromagnetic out-of-plane spin texture and opens up only below TC. Surprisingly, our analysis reveals large gap sizes at 1 kelvin of up to 90 millielectronvolts, which is ive times larger than theoretically predicted. Using multiscale analysis we show that this enhancement is due to a remarkable structure modiication induced by Mn doping: instead of a disordered impurity system, a self-organized alternating sequence of MnBi2Te4 septuple and Bi2Te3 quintuple layers is formed. This enhances the wavefunction overlap and size of the magnetic gap. Mn-doped Bi2Se3 and Mn-doped Sb2Te3 form similar heterostructures, but for Bi2Se3 only a nonmagnetic gap is formed and the magnetization is in the surface plane. This is explained by the smaller spin–orbit interaction by comparison with Mn-doped Bi2Te3. Our indings provide insights that will be crucial in pushing lossless transport in topological insulators towards roomtemperature applications.

The authors investigate the electronic and spin structure of topological surface states across a topological quantum phase transition. They find a surface band gap opening at the Dirac point of the topological surface states, giving rise to massive fermions with nonzero spin polarization on both sides of the phase transition. A mechanism of bulk-mediated scattering processes that increase with decreasing spin-orbit coupling strength is proposed as an explanation for the problem of the unconventional transformation between spin Dirac phases.

Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE), which provides quantized edge states for lossless charge-transport applications

Ultrafast dynamics in three-dimensional topological insulators (TIs) opens new routes for increasing the speed of information transport up to frequencies thousand times faster than in modern electronics. However, up to date, disentangling the exact contributions from bulk and surface transport to the subpicosecond dynamics of these materials remains a difficult challenge. Here, using time- and angle-resolved photoemission, we demonstrate that driving a TI from the bulk-conducting into the bulk-insulating transport regime allows to selectively switch on and off the emergent channels of ultrafast transport between the surface and the bulk. We thus establish that ultrafast transport is one of the main driving mechanisms responsible for the decay of excited electrons in prototypical TIs following laser excitation. We further show how ultrafast transport strongly affects the thermalization and scattering dynamics of the excited states up to high energies above the Fermi level. In particular, we observe how inhibiting the transport channels leads to a thermalization bottleneck that substantially slows down electron-hole recombination via electron-electron scatterings. Our results pave the way for exploiting ultrafast transport to control thermalization time scales in TI-based optoelectronic applications, and expand the capabilities of TIs as intrinsic solar cells., Comment: 10 pages, 5 figures