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Occupancy of different structural positions in a crystal lattice often seems to play a key role in material prop-erties. Several experimental techniques have been developed to uncover this issue, all of them being mostly bulk sensitive. However, many materials including topological insulators (TIs), which are among the most intriguing modern materials, are intended to be used in devices as thin films, for which the sublattice occupancy may differ from the bulk. One of the possible approaches to occupancy analysis is X-ray Photoelectron Diffraction (XPD), a structural method in surface science with chemical sensitivity. We applied this method in a case study of Sb2(Te1-xSex)3 mixed crystals, which belong to prototypical TIs. We used high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) as a reference method to verify our analysis. We revealed that the XPD data for vacuum cleaved bulk crystals are in excellent agreement with the reference ones. Also, we demonstrate that the anion occupancy near a naturally formed surface can be rather different from that of the bulk. The present results are relevant for a wide range of compositions where the system remains a topological phase, as we ultimately show by probing the transiently occupied topological surface state above the Fermi level by ultrafast photoemission.

V-doped (Bi,Sb)$_2$Te$_3$ has a ten times higher magnetic coercivity than its Cr-doped counterpart and therefore is believed to be a superior system for the quantum anomalous Hall effect (QAHE). The QAHE requires the opening of a magnetic band gap at the Dirac point. We do not find this gap by angle-resolved photoelectron spectroscopy down to 1 K. By x-ray magnetic circular dichroism (XMCD) we directly probe the magnetism at the V site and in zerofield. Hysteresis curves of the XMCD signal show a strong dependence of the coercivity on the ramping velocity of the magnetic field. The XMCD signal decays on a time scale of minutes which we conclude contributes to the absence of a detectable magnetic gap at the Dirac point., 12 pages, 3 figures

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.

Proximity to heavy sp-elements is considered promising for reaching a band gap in graphene that could host quantum spin Hall states. The recent report of an induced spin-orbit gap of 0.2 eV in Pb-intercalated graphene detectable by spin-resolved photoemission has spurred renewed interest in such systems (Klimovskikh et al 2017 ACS Nano 11, 368). In the case of Bi intercalation an even larger band gap of 0.4 eV has been observed but was assigned to the influence of a dislocation network (Warmuth et al 2016 Phys. Rev. B 93, 165 437). Here, we study Bi intercalation under graphene on Ir(111) and report a nearly ideal graphene dispersion without band replicas and no indication of hybridization with the substrate. The band gap is small (0.19 eV) and can be tuned by ±25 meV through the Bi coverage. The Bi atomic density is higher than in the recent report. By spin-resolved photoemission we exclude induced spin-orbit interaction as origin of the gap. Quantitative agreement of a photoemission intensity analysis with the measured band gap suggests sublattice symmetry breaking as one of the possible band gap opening mechanisms. We test several Bi structures by density functional theory. Our results indicate the possibility that Bi intercalates in the phase of bismuthene forming a graphene-bismuthene van der Waals heterostructure.

Three-dimensional topological insulators are exotic materials with unique properties. Tetradymite type binary chalcogenides of bismuth and antimony, as well as their mixed crystals, belong to prototypical TIs. Potential device applications of these materials require in-depth knowledge of their stability in the ambient atmosphere and other media maintained during their processing. Here we investigated the reactivity of mixed crystals with anion substitution, Bi2(Se1-xTex)3 and Sb2(Se1-xTex)3, towards molecular oxygen using both in situ and ex situ X-ray photoelectron spectroscopy. The results indicate that, in contrast to cation substitution, partial substitution of tellurium by selenium atoms leads to anomalously high surface reactivity, which even exceeds that of the most reactive binary constituent. We attribute this effect to anion ordering that essentially modifies the bond geometry, especially the respective bond angles as modeled by DFT.

For hybrid organic-inorganic as well as all-inorganic lead halide perovskites a Rashba effect has been invoked to explain the high efficiency in energy conversion by prohibiting direct recombination. Both a bulk and surface Rashba effect have been predicted. In the valence band of methylammonium (MA) lead bromide a Rashba effect has been reported by angle-resolved photoemission and circular dichroism with giant values of 7-11 eV angstrom. We present band dispersion measurements of MAPbBr(3) and spin-resolved photoemission of CsPbBr3 to show that a large Rashba effect detectable by photoemission or circular dichroism does not exist and cannot be the origin of the high effciency.

Vanadium‐doped Bi2–xTe2.4Se0.6 single crystals, with x = 0.015 and 0.03, are grown by the Bridgman method. Bandstructure characterization by angle‐resolved photoemission spectroscopy (ARPES) measurements shows gapless topological surface states for both vanadium concentrations. The Van‐der‐Pauw resistivity, the Hall charge carrier density, and the mobility in the temperature range from 0.3 to 300 K are strongly dependent on vanadium concentration, with carrier densities as low as 1.5 × 1016 cm−3 and mobilities as high as 570 cm2 V−1s−1. As expected for transport in gapless topological surface states, the resistivity, carrier density, and mobility are constant below 10 K. The magnetoresistance shows weak antilocalization for both vanadium concentrations in the same temperature range. The weak antilocalization is analyzed with the Hikami–Larkin–Nagaoka model, which yields phase‐coherence lengths of up to 250 nm for x = 0.015. Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659 Helmholtz-Gemeinschaft http://dx.doi.org/10.13039/501100001656

Spin-orbit interaction and structure inversion asymmetry in combination with magnetic ordering is a promising route to novel materials with highly mobile spin-polarized carriers at the surface. Spin-resolved measurements of the photoemission current from the Si-terminated surface of the antiferromagnet TbRh2Si2 and their analysis within an ab initio one-step theory unveil an unusual triple winding of the electron spin along the fourfold-symmetric constant energy contours of the surface states. A two-band k⋅p model is presented that yields the triple winding as a cubic Rashba effect. The curious in-plane spin-momentum locking is remarkably robust and remains intact across a paramagnetic-antiferromagnetic transition in spite of spin-orbit interaction on Rh atoms being considerably weaker than the out-of-plane exchange field due to the Tb 4f moments., This work was supported by the German Research Foundation (Grants No. KR-3831/5-1, No. LA655/20-1, GRK1621, Fermi-NESt No. ANR-16-CE92-0018, and SFB1143, project-id 247310070) and the Spanish Ministry of Science, Innovation, and Universities (Grant Nos. FIS2016-76617-P and MAT-2017-88374-P). We also acknowledge funding from the Department of Education of the Basque government (Grant No. IT1164-19), St. Petersburg State University (Project ID 51126254), and the Russian Foundation for Basic Research (Grant No. 20-32-70127). The SR-ARPES experiments at HiSOR were performed with the approval of the Proposal Assessing Committee of the Hiroshima Synchrotron Radiation Center (Proposal No. 18BG023). We also acknowledge the Impuls-und Vernetzungsfonds der Helmholtz Gemeinschaft (Grant No. HRSF-0067)

Renewed interest in the ferroelectric semi-conductor germanium telluride was recently triggered by the direct observation of a giant Rashba effect and a 30-year-old dream about a functional spin field-effect transistor. In this respect, all-electrical control of the spin texture in this material in combination with ferroelectric properties at the nanoscale would create advanced functionalities in spintronics and data information processing. Here, we investigate the atomic and electronic properties of GeTe bulk single crystals and their (111) surfaces. We succeeded in growing crystals possessing solely inversion domains of similar to 10 nm thickness parallel to each other. Using HAADF-TEM we observe two types of domain boundaries, one of them being similar in structure to the van der Waals gap in layered materials. This structure is responsible for the formation of surface domains with preferential Te-termination (similar to 68%) as we determined using photoelectron diffraction and XPS. The lateral dimensions of the surface domains are in the range of similar to 10-100 nm, and both Ge- and Te-terminations reveal no reconstruction. Using spin-ARPES we establish an intrinsic quantitative relationship between the spin polarization of pure bulk states and the relative contribution of different terminations, a result that is consistent with a reversal of the spin texture of the bulk Rashba bands for opposite configurations of the ferroelectric polarization within individual nanodomains. Our findings are important for potential applications of ferroelectric Rashba semiconductors in nonvolatile spintronic devices with advanced memory and computing capabilities at the nanoscale.

By measuring the spin polarization of GeTe films as a function of light polarization we observed that the bulk states are fully spin polarized in the initial state, in strong contrast with observations for other systems with a strong spin orbit interaction and the surface derived states in the same system. In agreement with state of the art theory, our experimental results show that fully spin polarized bulk states are an intrinsic property of the ferroelectric Rashba semiconductor amp; 945; GeTe 111 . The fact that the measured spin polarization vector does not change with light polarization can be explained by the absence of a mixing of states with a different total angular momentum J

Topological insulators constitute a new phase of matter protected by symmetries. Time-reversal symmetry protects strong topological insulators of the Z2 class, which possess an odd number of metallic surface states with dispersion of a Dirac cone. Topological crystalline insulators are merely protected by individual crystal symmetries and exist for an even number of Dirac cones. Here, we demonstrate that Bi-doping of Pb1−xSnxSe (111) epilayers induces a quantum phase transition from a topological crystalline insulator to a Z2 topological insulator. This occurs because Bi-doping lifts the fourfold valley degeneracy and induces a gap at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar \Gamma $$\end{document}Γ¯, while the three Dirac cones at the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bar{\rm M}}$$\end{document}M¯ points of the surface Brillouin zone remain intact. We interpret this new phase transition as caused by a lattice distortion. Our findings extend the topological phase diagram enormously and make strong topological insulators switchable by distortions or electric fields., Transitions between topological phases of matter protected by different symmetries remain rare. Here, Mandal et al. report a quantum phase transition from a topological crystalline insulator to a Z2 topological insulator by doping Bi into Pb1-xSnxSe (111) thin films.

To realize spintronic devices based on topological insulators (TIs), well-defined interfaces between magnetic metals and TIs are required. Here, we characterize atomically precisely the interface between the 3d transition metal Fe and the TI Bi2Te3 at different stages of its formation. Using photoelectron diffraction and holography, we show that after deposition of up to 3 monolayers Fe on Bi2Te3 at room temperature, the Fe atoms are ordered at the interface despite the surface disorder revealed by our scanning-tunneling microscopy images. We find that Fe occupies two different sites: a hollow adatom deeply relaxed into the Bi2Te3 quintuple layers and an interstitial atom between the third (Te) and fourth (Bi) atomic layers. For both sites, our core-level photoemission spectra and density-functional theory calculations demonstrate simultaneous chemical bonding of Fe to both Te and Bi atoms. We further show that upon deposition of Fe up to a thickness of 20 nm, the Fe atoms penetrate deeper into the bulk forming a 2-5 nm interface layer containing FeTe. In addition, excessive Bi is pushed down into the bulk of Bi2Te3 leading to the formation of septuple layers of Bi3Te4 within a distance of similar to 25 nm from the interface. Controlling the magnetic properties of the complex interface structures revealed by our work will be of critical importance when optimizing the efficiency of spin injection in TI-based devices.

SmB6 is predicted to be the first member of the intersection of topological insulators and Kondo insulators, strongly correlated materials in which the Fermi level lies in the gap of a many-body resonance that forms by hybridization between localized and itinerant states. While robust, surface-only conductivity at low temperature and the observation of surface states at the expected high symmetry points appear to confirm this prediction, we find both surface states at the (100) surface to be topologically trivial. We find the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bar{\varGamma }}$$\end{document}Γ¯ state to appear Rashba split and explain the prominent \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar X$$\end{document}X¯ state by a surface shift of the many-body resonance. We propose that the latter mechanism, which applies to several crystal terminations, can explain the unusual surface conductivity. While additional, as yet unobserved topological surface states cannot be excluded, our results show that a firm connection between the two material classes is still outstanding., Samarium hexahoride is argued to be a topological Kondo insulator, but this claim remains under debate. Here, Hlawenka et al. provide a topologically trivial explanation for the conducting states at the (100) surface of samarium hexaboride; an explanation based on Rashba splitting and a surface shift of the Kondo resonance.

Structure inversion asymmetry is an inherent feature of quantum confined heterostructures with non equivalent interfaces. It leads to a spin splitting of the electron states and strongly affects the electronic band structure. The effect is particularly large in topological insulators because the topological surface states are extremely sensitive to the interfaces. Here, the first experimental observation and theoretical explication of this effect are reported for topological crystalline insulator quantum wells made of Pb1 amp; 8722;xSnxSe confined by Pb1 amp; 8722;yEuySe barriers on one side and by vacuum on the other. This provides a well defined structure asymmetry controlled by the surface condition. The electronic structure is mapped out by angle resolved photoemission spectroscopy and tight binding calculations, evidencing that the spin splitting decisively depends on hybridization and, thus, quantum well width. Most importantly, the topological boundary states are not only split in energy but also separated in space unlike conventional Rashba bands that are splitted only in momentum. The splitting can be strongly enhanced to very large values by control of the surface termination due to the charge imbalance at the polar quantum well surface. The findings thus, open up a wide parameter space for tuning of such systems for device applications

Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. Epitaxial (Bi$_{1-x}$Mn$_{x}$)$_{2}$Se$_{3}$ is a prototypical magnetic topological insulator with a pronounced surface band gap of $\sim100$ meV. We show that this gap is neither due to ferromagnetic order in the bulk or at the surface nor to the local magnetic moment of the Mn, making the system unsuitable for realizing the novel phases. We further show that Mn doping does not affect the inverted bulk band gap and the system remains topologically nontrivial. We suggest that strong resonant scattering processes cause the gap at the Dirac point and support this by the observation of in-gap states using resonant photoemission. Our findings establish a novel mechanism for gap opening in topological surface states which challenges the currently known conditions for topological protection., 26 pages, 7 figures

Topological materials promise new functionalities, which are revealed with the help of angle resolved photoemission. Herein, the search for the magnetic bandgap at the Dirac point as a precondition for the quantum anomalous Hall effect is reviewed and its opening for the topological insulator heterostructure MnBi2Te4 Bi2Te3 is demonstrated. Essential preconditions are explained and the reasons why nonmagnetic gaps occur when Se replaces Te. Angle resolved photoelectron spectroscopy ARPES probes the quantum mechanical final state, and this allows investigation of spin manipulation by light using spin resolved ARPES and the dependence of the charge carrier lifetime on the peculiar spin texture of topological states. It is shown that ARPES data do not support SmB6 as the first strongly correlated topological insulator and an alternative, trivial explanation for the results of ARPES and electrical transport experiments is formulated. Epitaxially grown topological crystalline insulators are, due to their dependence on crystal symmetries, more versatile in the control of individual bulk band inversions. It is shown that this leads to topological quantum phase transitions and associated novel functionalities. Finally, the surface and bulk band connectivity of a type II 3D Weyl semimetal is investigated and an outlook is given for the scientific field

SmB6 has recently attracted considerable interest as a candidate for the first strongly correlated topological insulator. Such materials promise entirely new properties such as correlation-enhanced bulk bandgaps or a Fermi surface from spin excitations. Whether SmB6 and its surface states are topological or trivial is still heavily disputed however, and a solution is hindered by major disagreement between angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM) results. Here, a combined ARPES and STM experiment is conducted. It is discovered that the STM contrast strongly depends on the bias voltage and reverses its sign beyond 1 V. It is shown that the understanding of this contrast reversal is the clue to resolving the discrepancy between ARPES and STM results. In particular, the scanning tunneling spectra reflect a low-energy electronic structure at the surface, which supports a trivial origin of the surface states and the surface metallicity of SmB6 .

Using high-resolution angle-resolved photoemission, we observe a strong spin-dependent renormalization and lifetime broadening of the quasiparticle excitations in the electronic band structure of Ni(111) in an energy window of $\sim$0.3 eV below the Fermi level. We derive a quantitative result for the spin-dependent lifetime broadening by comparing the scattering rates of majority and minority $d$ states, and further show that spin-dependent electron correlations are instead negligible for $sp$ states. From our analysis we experimentally determine the effective on-site Coulomb interaction $U$ caused by Stoner-like interband transitions between majority and minority $d$ states. The present results unambiguously demonstrate the remarkable impact of spin-dependent electron correlation effects originating from single-particle excitations in a prototypical 3$d$ transition metal, paving the way for further refinement of current many-body theoretical approaches., 5 pages, 4 figures, submitted on 24.05.2018

Most recently, theoretical calculations predicted the stability of a novel two-dimensional phosphorus honeycomb lattice named blue phosphorus. Here, we report on the growth of blue phosphorus on Au(111) and unravel its structural details using diffraction, microscopy and theoretical calculations. Most importantly, by utilizing angle-resolved photoemission spectroscopy we identify its momentum-resolved electronic structure. We find that Au(111) breaks the sublattice symmetry of blue phosphorus leading to an orbital-dependent band renormalization upon the formation of a (4 × 4) superstructure. Notably, the semiconducting two-dimensional phosphorus realizes its valence band maximum at 0.9 eV binding energy, however, shifted in momentum space due to the substrate-induced band renormalization.

By angle- and spin-resolved photoemission we have studied electronic structure and spin–orbit splitting in graphene/Ni(1 1 1) intercalated with alloy of high spin–orbit materials Bi and Au. Results are compared to ultimate cases of (i) graphene/Ni intercalated only with Au which shows giant Rashba splitting ( ∼ 100 meV) and (ii) graphene/Ni intercalated only with Bi for which spin splitting of the Dirac cone is nearly zero ( ⩽ 10 meV). Our results demonstrate that partial substitution of intercalated Au with Bi reduces spin splitting of the Dirac cone in graphene and even allows for its systematic adjustment through concentration of Bi. Observed effects are ascribed to peculiar electronic structure of Bi which has in the valence band no d electronic states responsible for the onset of giant Rashba effect through electronic hybridization with π band of graphene. We also report in details electronic properties of graphene/Ni(1 1 1) intercalated only with Bi in various concentrations. Such graphene reveals undistorted band structure characteristic to quasifreestanding graphene with small n-doping and a minor band gap at the Dirac point ( ∼ 200 meV) which depends weakly on the concentration of Bi. It has been shown that this gap is related to the symmetry breaking rather than to the topological phase formation.

Negative longitudinal magnetoresistance (NLMR) is shown to occur in topological materials in the extreme quantum limit, when a magnetic field is applied parallel to the excitation current. We perform pulsed and DC field measurements on Pb1-xSnxSe epilayers where the topological state can be chemically tuned. The NLMR is observed in the topological state, but is suppressed and becomes positive when the system becomes trivial. In a topological material, the lowest N=0 conduction Landau level disperses down in energy as a function of increasing magnetic field, while the N=0 valence Landau level disperses upwards. This anomalous behavior is shown to be responsible for the observed NLMR. Our work provides an explanation of the outstanding question of NLMR in topological insulators and establishes this effect as a possible hallmark of bulk conduction in topological matter., Comment: Accepted in Physical Review Letters

Helically spin-polarized Dirac fermions (HSDF) in protected topological surface states (TSS) are of high interest as a new state of quantum matter. In three-dimensional (3D) materials with TSS, electronic bulk states often mask the transport properties of HSDF. Recently, the high-field Hall resistance and low-field magnetoresistance indicate that the TSS may coexist with a layered two-dimensional electronic system (2DES). Here, we demonstrate quantum oscillations of the Hall resistance at temperatures up to 50 K in nominally undoped bulk Bi2Se3 with a high electron density n of about 2·1019 cm−3. From the angular and temperature dependence of the Hall resistance and the Shubnikov-de Haas oscillations we identify 3D and 2D contributions to transport. Angular resolved photoemission spectroscopy proves the existence of TSS. We present a model for Bi2Se3 and suggest that the coexistence of TSS and 2D layered transport stabilizes the quantum oscillations of the Hall resistance.

Quantum-phase transitions between trivial insulators and topological insulators differ from ordinary metal-insulator transitions in that they arise from the inversion of the bulk band structure due to strong spin–orbit coupling. Such topological phase transitions are unique in nature as they lead to the emergence of topological surface states which are characterized by a peculiar spin texture that is believed to play a central role in the generation and manipulation of dissipationless surface spin currents on ultrafast timescales. Here, we provide a generalized G W +Boltzmann approach for the description of ultrafast dynamics in topological insulators driven by electron–electron and electron–phonon scatterings. Taking the prototypical insulator Bi 2 Te 3 as an example, we test the robustness of our approach by comparing the theoretical prediction to results of time- and angle-resolved photoemission experiments. From this comparison, we are able to demonstrate the crucial role of the excited spin texture in the subpicosecond relaxation of transient electrons, as well as to accurately obtain the magnitude and strength of electron–electron and electron–phonon couplings. Our approach could be used as a generalized theory for three-dimensional topological insulators in the bulk-conducting transport regime, paving the way for the realization of a unified theory of ultrafast dynamics in topological materials.

Phase change alloys are used for non-volatile random access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. However, this contrast has never been directly related to the electronic band structure. Here, we employ photoelectron spectroscopy to map the relevant bands for metastable, epitaxial GeSbTe films. The constant energy surfaces of the valence band close to the Fermi level are hexagonal tubes with little dispersion perpendicular to the (111) surface. The electron density responsible for transport belongs to the tails of this bulk valence band, which is broadened by disorder, i.e., the Fermi level is 100 meV above the valence band maximum. This result is consistent with transport data of such films in terms of charge carrier density and scattering time. In addition, we find a state in the bulk band gap with linear dispersion, which might be of topological origin., Comment: 14 pages, 6 figures, 5 tables

The existence of ferromagnetic ordering in graphene on cobalt is demonstrated by means of resonant magnetic reflection spectroscopy exploiting the transversal magneto-optical Kerr-effect (T-MOKE). Using linearly polarized synchrotron radiation in the soft x-ray range with energies spanning the carbon 1s edge, the π- and σ- bonds of graphene were excited individually, showing that magnetism in graphene is carried by the π – orbitals. Magnetic signals were detected over a wide energy range from 257 – 340 eV with a T-MOKE peak value of 1.1 % at the π – resonance energy near 285 eV. By comparison with corresponding spectra measured at the 2p edges of the Co substrate, a large induced magnetic moment of 0.14 μB was derived for graphene. Individual hysteresis curves monitored at the Co 2p and C 1s edges show that the carbon magnetism is induced by the Co substrate.

One of the most promising platforms for spintronics and topological quantum computation is the two-dimensional electron gas (2DEG) with strong spin-orbit interaction and out-of-plane ferromagnetism. In proximity to an s-wave superconductor, such 2DEG may be driven into a topologically non-Trivial superconducting phase, predicted to support zero-energy Majorana fermion modes. Using angle-resolved photoemission spectroscopy and ab initio calculations, we study the 2DEG at the surface of the vanadium-doped polar semiconductor with a giant Rashba-Type splitting, BiTeI. We show that the vanadium-induced magnetization in the 2DEG breaks time-reversal symmetry, lifting Kramers degeneracy of the Rashba-split surface state at the Brillouin zone center via formation of a huge gap of about 90 meV. As a result, the constant energy contour inside the gap consists of only one circle with spin-momentum locking. These findings reveal a great potential of the magnetically-doped semiconductors with a giant Rashba-Type splitting for realization of novel states of matter., The work was partially supported by grant of Saint Petersburg State University for scientific investigations (N. 15.61.202.2015). This study was supported by the Russian Science Foundation (project N. 17-12-01047, in part of crystal growth, structural characterization and ARPES measurements (Figs 1, 2)). The funding by the University of the Basque Country (Grant Nos GIC07IT36607 and IT-756-13), the Spanish Ministry of Science and Innovation (Grant Nos FIS2013-48286-C02-02-P, FIS2013-48286-C02-01-P, and FIS2016-75862-P) and Tomsk State University Academic D.I. Mendeleev Fund Program in 2015 (research grant N 8.1.05.2015) are also gratefully acknowledged. The authors also acknowledge support from the Russian-German laboratory at BESSY II, the “German-Russian Interdisciplinary Science Center”(G-RISC) program and the Impuls- und Vernetzungsfonds der Helmholtz-Gemeinschaft (Grant No. HRJRG-408).

Using time-, spin- and angle-resolved photoemission, we investigate the ultrafast spin dynamics of hot electrons on the surface of the topological insulator Bi$_2$Te$_3$ following optical excitation by fs-infrared pulses. We observe two surface-resonance states above the Fermi level coexisting with a transient population of Dirac fermions that relax in about $\sim$2 ps. One state is located below $\sim$0.4 eV just above the bulk continuum, the other one at $\sim$0.8 eV inside a projected bulk band gap. At the onset of the excitation, both states exhibit a reversed spin texture with respect to that of the transient Dirac bands, in agreement with our one-step photoemission calculations. Our data reveal that the high-energy state undergoes spin relaxation within $\sim$0.5 ps, a process that triggers the subsequent spin dynamics of both the Dirac cone and the low-energy state, which behave as two dynamically-locked electron populations. We discuss the origin of this behavior by comparing the relaxation times observed for electrons with opposite spins to the ones obtained from a microscopic Boltzmann model of ultrafast band cooling introduced into the photoemission calculations. Our results demonstrate that the nonequilibrium surface dynamics is governed by electron-electron rather than electron-phonon scattering, with a characteristic time scale unambiguously determined by the complex spin texture of excited states above the Fermi level. Our findings reveal the critical importance of detecting momentum and energy-resolved spin textures with fs resolution to fully understand the sub-ps dynamics of transient electrons on the surface of topological insulators., 10 pages, 5 figures

Nano research 6(8), 562 - 570 (2013). doi:10.1007/s12274-013-0331-9, The atomic and electronic structure of graphene synthesized on commercially available cubic-SiC(001)/Si(001) wafers have been studied by low energy electron microscopy (LEEM), scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and angle resolved photoelectron spectroscopy (ARPES). LEEM and STM data prove the wafer-scale continuity and uniform thickness of the graphene overlayer on SiC(001). LEEM, STM and ARPES studies reveal that the graphene overlayer on SiC(001) consists of only a few monolayers with physical properties of quasi-freestanding graphene. Atomically resolved STM and micro-LEED data show that the top graphene layer consists of nanometersized domains with four different lattice orientations connected through the 〈110〉-directed boundaries. ARPES studies reveal the typical electron spectrum of graphene with the Dirac points close to the Fermi level. Thus, the use of technologically relevant SiC(001)/Si(001) wafers for graphene fabrication represents a realistic way of bridging the gap between the outstanding properties of graphene and their applications., Published by Tsinghua Press, [S.l.]

In this Rapid Communication we investigate the coupling between excited electrons and phonons in the highly anisotropic electronic structure of the prototypical topological insulator Bi$_2$Te$_3$. Using time- and angle-resolved photoemission spectroscopy we are able to identify the emergence and ultrafast temporal evolution of the longitudinal-optical A$_{1g}$ coherent-phonon mode in Bi$_2$Te$_3$. We observe an antiphase behavior in the onset of the coherent-phonon oscillations between the $\overline{\Gamma K}$ and the $\overline{\Gamma M}$ high-symmetry directions that is consistent with warping. The qualitative agreement between our density-functional theory calculations and the experimental results reveals the critical role of the anisotropic coupling between Dirac fermions and phonon modes in the topological insulator Bi$_2$Te$_3$.

We reinvestigate the putative giant spin splitting at the surface of SrTiO3 reported by Santander-Syro et al. [Nature Mat. 13, 1085 (2014)]. Our spin- and angle-resolved photoemission experiments on fractured (001) oriented surfaces supporting a two-dimensional electron liquid with high carrier density show no detectable spin polarization in the photocurrent. We demonstrate that this result excludes a giant spin splitting while it is consistent with the unconventional Rashba-like splitting seen in band structure calculations that reproduce the experimentally observed ladder of quantum confined subbands. Postprint

Heat-assisted magnetic recording (HAMR) uses the tiny hot spot of a laser to flip bit states, and suitable materials are needed to enable this technology. The authors show that antidots of a ferrimagnetic alloy arrayed atop nonmagnetic holes present an effective bit size of 45 nm, and four magnetic states at room temperature. Key here is the antidots' magnetic-anisotropy reorientation at just 350 K, whereas a typical HAMR material must be heated above its Curie temperature (~750 K, or higher) to work. These $f\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ nanostructures do not suffer from a superparamagnetic limit, and seem promising for ultrahigh-density data storage.

Three-dimensional topological insulators (TIs) are characterized by spin-polarized Dirac-cone surface states that are protected from backscattering by time-reversal symmetry. Control of the spin polarization of topological surface states (TSSs) using femtosecond light pulses opens novel perspectives for the generation and manipulation of dissipationless surface spin currents on ultrafast timescales. Using time-, spin-, and angle-resolved spectroscopy, we directly monitor for the first time the ultrafast response of the spin polarization of photoexcited TSSs to circularly-polarized femtosecond pulses of infrared light. We achieve all-optical switching of the transient out-of-plane spin polarization, which relaxes in about 1.2 ps. Our observations establish the feasibility of ultrafast optical control of spin-polarized Dirac fermions in TIs and pave the way for novel optospintronic applications at ultimate speeds., Comment: 9 pages, 4 figures

We perform ultrahigh-resolution angle-resolved photoemission experiments at a temperature $T=0.8$ K on the type-II Weyl semimetal candidate ${\mathrm{WTe}}_{2}$. We find a surface Fermi arc connecting the bulk electron and hole pockets on the (001) surface. Our results show that the surface Fermi arc connectivity to the bulk bands is strongly mediated by distinct surface resonances dispersing near the border of the surface-projected bulk band gap. By comparing the experimental results to first-principles calculations, we argue that the coupling to these surface resonances, which are topologically trivial, is compatible with the classification of ${\mathrm{WTe}}_{2}$ as a type-II Weyl semimetal hosting topological Fermi arcs. We further support our conclusion by a systematic characterization of the bulk and surface character of the different bands and discuss the similarity of our findings to the case of topological insulators.

Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides1–4. Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO3(001) surface5–7 to reveal a remarkably complex evolution of electron–phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO3/SrTiO3 interface8–13, our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron–phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO3-based 2DELs. Postprint

Photoelectron spectroscopy in combination with piezoforce microscopy reveals that the helicity of Rashba bands is coupled to the nonvolatile ferroelectric polarization of GeTe(111). A novel surface Rashba band is found and fingerprints of a bulk Rashba band are identified by comparison with density functional theory calculations.

We show in detail monitoring by photoelectron spectroscopy how graphene can be grown by chemical vapour deposition on the transition-metal surfaces Ni(111) and Co(0001) and intercalated by a monoatomic layer of Au. For both systems, a linear E(k) dispersion of massless Dirac fermions appears in the graphene π-band in the vicinity of the Fermi energy. In order to study ferromagnetism and spin-orbit effects by spin- and angle- resolved photoelectron spectroscopy, the sample must be magnetized in remanence. To this end, a W(110) substrate is prepared, its cleanliness verified by photoemission from W(110) surface states and surface core levels, and epitaxial Ni(111) and Co(0001) thin films are grown on top. Spin-resolved photoemission from the π-band shows that the ferromagnetic polarization of graphene/Ni(111) and graphene/Co(0001) is negligible and that graphene on Ni(111) is after intercalation of Au spin-orbit split by the Rashba effect.

Arrays of Cobalt nanowires with a controlled length of 6 μ m have been fabricated by electrochemical deposition into the pores of track-etched polycarbonate membranes with a nominal pore diameter of 30 nm. The magnetic properties of Co-deposited nanowires and the effects of a magnetic field applied during electrodeposition of the arrays have been studied. An enhancement of the mass deposition rate due to the presence of a 50 Oe magnetic field along the nanowire axis has been observed by measuring the experimental development of the current in the electrochemical cell during the fabrication process. X-ray diffraction measurements reveal a different polycrystalline degree for each deposition configuration, indicating that the crystalline structure of the deposited material has been substantially modified. Magnetic measurements show a clear dependence of the anisotropy directions on the orientation of the magnetic field applied during the electrodeposition.

Rozbíjení inverzní symetrie ve fereeleRashba efekt; Fotoemisse; DFTktrickém polovodiči způsobuje děleni stavů, tzv Rashba efekt. V tomto článku ukazujeme kompletně mapování spinové polarizace těchto Rashba stavů za pomoci spinovo rozlišené fotoemisse. The breaking of bulk inversion symmetry in ferroelectric semiconductors causes a Rashba-type spin splitting of electronic bulk bands. This is shown by a comprehensive mapping of the spin polarization of the electronic bands in ferroelectric α- GeTe(111) films using a time-of-flight momentum microscope equipped with an imaging spin filter that enables a simultaneous measurement of more than 10 000 data points. The experiment reveals an opposite spin helicity of the inner and outer Rashba bands with a different spin polarization in agreement with theoretical predictions, confirming a complex spin texture of bulk Rashba states. The outer band has about twice larger spin polarization than the inner one, giving evidence of a spin-orbit effect being related to the orbital composition of the band states. The switchable inner electric field of GeTe implies new functionalities for spintronic devices.

Spin and pseudospin in graphene are known to interact under enhanced spin–orbit interaction giving rise to an in-plane Rashba spin texture. Here we show that Au-intercalated graphene on Fe(110) displays a large (∼230 meV) bandgap with out-of-plane hedgehog-type spin reorientation around the gapped Dirac point. We identify two causes responsible. First, a giant Rashba effect (∼70 meV splitting) away from the Dirac point and, second, the breaking of the six-fold graphene symmetry at the interface. This is demonstrated by a strong one-dimensional anisotropy of the graphene dispersion imposed by the two-fold-symmetric (110) substrate. Surprisingly, the graphene Fermi level is systematically tuned by the Au concentration and can be moved into the bandgap. We conclude that the out-of-plane spin texture is not only of fundamental interest but can be tuned at the Fermi level as a model for electrical gating of spin in a spintronic device., Potential electronic applications of graphene rely on controlling its spin-dependent properties. Here, the authors use spin-resolved photoemission spectroscopy to demonstrate how Au-intercalation produces gapped one-dimensional quasi-freestanding graphene on Fe(110) with tunable Fermi surface spin texture.

Low-field magnetotransport measurements of topological insulators such as Bi$_2$Se$_3$ are important for revealing the nature of topological surface states by quantum corrections to the conductivity, such as weak-antilocalization. Recently, a rich variety of high-field magnetotransport properties in the regime of high electron densities ($\sim10^{19}$ cm$^{-3}$) were reported, which can be related to additional two-dimensional layered conductivity, hampering the identification of the topological surface states. Here, we report that quantum corrections to the electronic conduction are dominated by the surface states for a semiconducting case, which can be analyzed by the Hikami-Larkin-Nagaoka model for two coupled surfaces in the case of strong spin-orbit interaction. However, in the metallic-like case this analysis fails and additional two-dimensional contributions need to be accounted for. Shubnikov-de Haas oscillations and quantized Hall resistance prove as strong indications for the two-dimensional layered metallic behavior. Temperature-dependent magnetotransport properties of high-quality Bi$_2$Se$_3$ single crystalline exfoliated macro and micro flakes are combined with high resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy, confirming the structure and stoichiometry. Angle-resolved photoemission spectroscopy proves a single-Dirac-cone surface state and a well-defined bulk band gap in topological insulating state. Spatially resolved core-level photoelectron microscopy demonstrates the surface stability., Comment: Sci. Rep. (2016)

The interface of graphene with ferromagnets is highly relevant for spintronics, because graphene on Co(0001) shows a largely intact Dirac cone and strong hybridization with Co $3d$ states breaking the sublattice symmetry that had been considered mutually exclusive. Here we show by spin- and angle-resolved photoemission that the Dirac cone and Dirac point are also highly spin polarized $(\ensuremath{\sim}\ensuremath{-}25%)$, which reinforces the puzzling issue of a strong graphene-substrate interaction. The problem is solved by our ab initio calculations which show that (i) the upper and lower halves of the Dirac cone belong to different sublattices and (ii) one half is spin polarized by spin-dependent hybridization because it is situated at the edge of a minority-spin band gap of the Co substrate.

Topological insulators are characterized by Dirac-cone surface states with electron spins locked perpendicular to their linear momenta. Recent theoretical and experimental work implied that this specific spin texture should enable control of photoelectron spins by circularly polarized light. However, these reports questioned the so far accepted interpretation of spin-resolved photoelectron spectroscopy. We solve this puzzle and show that vacuum ultraviolet photons (50-70 eV) with linear or circular polarization indeed probe the initial-state spin texture of Bi2Se3 while circularly polarized 6-eV low-energy photons flip the electron spins out of plane and reverse their spin polarization, with its sign determined by the light helicity. Our photoemission calculations, taking into account the interplay between the varying probing depth, dipole-selection rules, and spin-dependent scattering effects involving initial and final states, explain these findings and reveal proper conditions for light-induced spin manipulation. Our results pave the way for future applications of topological insulators in optospintronic devices.

High-resolution spin- and angle-resolved photoemission spectroscopy measurements were performed on the three-dimensional topological insulator Bi${}_{2}$Te${}_{2.4}$Se${}_{0.6}$, which is characterized by enhanced thermoelectric properties. The Fermi level position is found to be located in the bulk energy gap independent of temperature and it is stable over a long time. Spin textures in the Dirac-cone state at energies above and below the Dirac point as well as in the Rashba-type valence band surface state are observed in agreement with theoretical prediction. The calculations of the surface electronic structure demonstrate that the fractional stoichiometry induced disorder within the Te/Se sublattice does not influence the Dirac-cone state dispersion. In spite of relatively high resistivity, temperature dependence of conductivity shows a weak metallic behavior that could explain the effective thermoelectric properties of the Bi${}_{2}$Te${}_{2.4}$Se${}_{0.6}$ compound with the in-plane Seebeck coefficient reaching $\ensuremath{-}330$ $\ensuremath{\mu}$V/K at room temperature.

The atomic structure of the cubic-SiC(001) surface during ultra-high vacuum graphene synthesis has been studied using scanning tunneling microscopy (STM) and low-energy electron diffraction. Atomically resolved STM studies prove the synthesis of a uniform, millimeter-scale graphene overlayer consisting of nanodomains rotated by ±13.5° relative to the left angle bracket 110 right angle bracket-directed boundaries. The preferential directions of the domain boundaries coincide with the directions of carbon atomic chains on the SiC(001)-c(2 × 2) reconstruction, fabricated prior to graphene synthesis. The presented data show the correlation between the atomic structures of the SiC(001)-c(2 × 2) surface and the graphene/SiC(001) rotated domain network and pave the way for optimizing large-area graphene synthesis on low-cost cubic-SiC(001)/Si(001) wafers.

Understanding the spin-texture behavior of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nano-devices. Here by using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we report tunneling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We observe strongly binding energy- and wavevector-dependent spin polarization for the topological surface electrons in the ultra-thin gapped-Dirac-cone limit. The polarization decreases significantly with enhanced tunneling realized systematically in thin insulating films, whereas magnitude of the polarization saturates to the bulk limit faster at larger wavevectors in thicker metallic films. We present a theoretical model which captures this delicate relationship between quantum tunneling and Fermi surface spin polarization. Our high-resolution spin-based spectroscopic results suggest that the polarization current can be tuned to zero in thin insulating films forming the basis for a future spin-switch nano-device., To appear in Nature Communications (2014); Expanded version of http://arxiv.org/abs/1307.5485