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The Dirac point of a topological surface state (TSS) is protected against gapping by time-reversal symmetry. Conventional wisdom stipulates, therefore, that only through magnetisation may a TSS become gapped. However, non-magnetic gaps have now been demonstrated in Bi2Se3 systems doped with Mn or In, explained by hybridisation of the Dirac cone with induced impurity resonances. Recent photoemission experiments suggest that an analogous mechanism applies even when Bi2Se3 is surface dosed with Au. Here, we perform a systematic spin- and angle-resolved photoemission study of Au-dosed Bi2Se3. Although there are experimental conditions wherein the TSS appears gapped due to unfavourable photoemission matrix elements, our photon-energy-dependent spectra unambiguously demonstrate the robustness of the Dirac cone against high Au coverage. We further show how the spin textures of the TSS and its accompanying surface resonances remain qualitatively unchanged following Au deposition, and discuss the mechanism underlying the suppression of the spectral weight. [ABSTRACT FROM AUTHOR]

Transmission of low-energetic electrons through two-dimensional materials leads to unique scattering resonances. These resonances contribute to photoemission from occupied bands where they appear as strongly dispersive features of suppressed photoelectron intensity. Using angle-resolved photoemission, we have systematically studied scattering resonances in epitaxial graphene grown on the chemically differing substrates Ir(111), Bi/Ir, and Ni(111) as well as in graphene/Ir(111) nanopatterned with a superlattice of uniform Ir quantum dots. While the strength of the chemical interaction with the substrate has almost no effect on the dispersion of the scattering resonances, their energy can be controlled by the magnitude of charge transfer from/to graphene. At the same time, a superlattice of small quantum dots deposited on graphene eliminates the resonances completely. We ascribe this effect to a nanodot-induced buckling of graphene and its local rehybridization from sp2 to sp3 towards a three-dimensional structure. Our results suggest nanopatterning as a prospective tool for tuning optoelectronic properties of two-dimensional materials with a graphene-like structure. [ABSTRACT FROM AUTHOR]

By angle-resolved photoemission the electronic structure of quantum films of Mg, Ag, and Au has been compared on W(110) and Mo(110) substrates which are structurally and electronically very similar but differ in atomic number. In all cases, substrate-induced states with characteristic dispersions are observed in the region of a bulk band gap of the substrate. Based on the comparison between Mo and W, we can exclude that previously observed Mg states are spin-orbit split but observe a spin-orbit splitting in Ag and Au monolayers. This splitting is mainly caused by the substrate because it does not differ much between Ag and Au overlayers despite the large difference in atomic number. [ABSTRACT FROM AUTHOR]

We present angle-resolved photoemission spectra of the γ phase of manganese as well as a theoretical analysis using a recently developed approach that combines density functional and dynamical mean-field methods (LDA + DMFT). The comparison of experimental data and theoretical predictions allows us to identify effects of the Coulomb correlations, namely, the presence of broad and nondispersive Hubbard bands in this system. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

Gd/W(110) represents an important system for understanding magnetic coupling phenomena and the magnetic structure of rare earths. In connection with our study of quantum-well states in this system, there is a remarkable behavior of photoemission spectra at lowest coverages (0-3 monolayers). After the disappearance of W surface-derived features, a set of intense, sharp features appears in the energy region of the Gd bulk band gap. Their origin is tested using resonant photoemission at the Gd 4d-4f excitation threshold. [ABSTRACT FROM AUTHOR]

Using angle-resolved photoemission, we observe quantum-well-state formation as pronounced peaks up to large thicknesses in epitaxial Ag and Au films on W(110). These systems serve as a test case for the assignment of sp and d orbital character to quantum-well states since overlap of sp with d states is strong for both systems but differs in detail. Starting out from peak assignments on the basis of the extended phase accumulation model, the angle dispersion of several features is discussed in detail and related to the orbital character. [ABSTRACT FROM AUTHOR]

The topological surface state of Bi2Te3 leads to high photoemisson intensity from its Dirac point. This allows us to investigate the effect of larger amounts of deposited Fe than previously achieved for Bi2Se3. The Dirac point is shown to stay intact up to at least a monolayer showing the robustness of the topological state towards disordered magnetic moments. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) Because magnetic moments lift time reversal symmetry and topological protection, the reaction of topological insulators to deposited magnetic moments is a matter of current debate. The authors contribute to this discussion with the system Fe/Bi2Te3 where spectroscopically favorable conditions allow them to observe the Dirac point even after a full monolayer is deposited. [ABSTRACT FROM AUTHOR]

It has recently been demonstrated that pentacene can serve as protection layer for graphene on SiC preserving the unique E(k) band dispersion after exposure to atmosphere and subsequent annealing in vacuum. We confirm the stability of the ideal graphene band dispersion but without any protection layer. We demonstrate this by angle-resolved photoemission for graphene on Ni(111) intercalated by a Au monolayer. Exposure to air does not carbidize or oxidize the Ni substrate or open an apparent gap in the graphene. Its doping state is not affected and the Rashba-type spin-orbit effect on the graphene π state is preserved. [ABSTRACT FROM AUTHOR]

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 (~-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. [ABSTRACT FROM AUTHOR]

We analyze the strong hexagonal warping of the Dirac cone of Bi2Te3 by angle-resolved photoemission. Along ΓM, the dispersion deviates from a linear behavior meaning that the Dirac cone is warped outwards and not inwards. We show that this introduces an anisotropy in the lifetime broadening of the topological surface state which is larger along ΓK. The result is not consistent with an explanation by nesting properties. Based on the theoretically predicted modifications of the ground-state spin texture of a strongly warped Dirac cone, we propose spin-dependent scattering processes as explanation for the anisotropic scattering rates. These results could help paving the way for optimizing future spintronic devices using topological insulators and controlling surface-scattering processes via external gate voltages. [ABSTRACT FROM AUTHOR]

The spin-orbit splitting of graphene p states can be strongly enhanced by external influences such as corrugation or proximity to heavier atoms. Here we investigate experimentally and theoretically whether such strong enhancement is possible for graphene on SiC(0001). By spin- and angle-resolved photoemission we found for two independently grown samples no resolvable spin-orbit splitting with an upper limit of our analysis of 20 meV. Our ab initio calculations predict a low spin-orbit splitting of 0.05 meV with small anisotropy but a local tenfold enhancement where hybridization with SiC substrate bands occurs. [ABSTRACT FROM AUTHOR]

Graphene grown catalytically on Ir(111) shows a large substrate-induced spin-orbit splitting of Δso ~ 50 meV in spin- and angle-resolved photoemission. Its origin is hybridization as revealed by large gaps in the π band. Reducing the growth temperature results in a very large splitting of the Dirac cone, which is found to be of geometric origin and allows for the structural identification of a rotated graphene phase. The spin-orbit splitting on this phase is considerably smaller, which implies that the magnitude of the extrinsic spin-orbit splitting in supported graphene can be controlled by its rotational displacement to the substrate. [ABSTRACT FROM AUTHOR]

The spin polarization of W(llO) and Al/W(l 10) surfaces is studied by spin- and angle-resolved photoemission. On both surfaces distinct E(k∥) dispersions are identified with an unusual topology: A single spectral branch is spin polarized antisymmetrically relative to the Ѓ point, and two spin-polarized branches cross at Ѓ. The crossing branches disperse linearly but this similarity to a Dirac cone is lost after deposition of the Al, where they acquire a parabolic dispersion. Based on ab initio one-step photoemission theory, we show that the measured spin polarization is a property of the ground state and identify the effect as the counterpart of the recently discovered Rashba polarization of bulk states at the surface, but with a distinct non-Rashba topology. [ABSTRACT FROM AUTHOR]

We reveal a giant Rashba effect (&agr;R ≈ 1.3 eV Å) on a surface state of Ir(111) by angle-resolved photoemission and by density functional theory. It is demonstrated that the existence of the surface state, its spin polarization, and the size of its Rashba-type spin-orbit splitting remain unaffected when Ir is covered with graphene. The graphene protection is, in turn, sufficient for the spin-split surface state to survive in ambient atmosphere. We discuss this result along with indications for a topological protection of the surface state. [ABSTRACT FROM AUTHOR]

Despite their low atomic number, Al films show large spin-orbit splittings when grown on W( 110). Our spin- and angle-resolved photoemission experiment reveals two types of spin-orbit split states: quantum-well slates (QWSs) with small Rashba splitting proportional to the electron wave vector in the film plane k1 [Rashba parameter αR ∼ 7 x 10-12 eV m for a 10-monolayer (ML) film] and substrate-derived interlace slates with large (∼0.5-eV) splitting. The E(k||) dispersion of this pair of interface states changes only slightly up to 3 ML Al. At higher Al coverages, the QWSs and interlace states show a remarkable avoided-crossing effect in their band dispersions. This avoided-crossing effect obeys symmetry as well as spin and. therefore, leads to a strongly enhanced spin-orbit splitting of Al QWSs. This is shown by E(k||) band dispersions and by spin- and angle-resolved spectra for several thicknesses up to 15 ML Al. [ABSTRACT FROM AUTHOR]

Abstract: The behavior of quantum-well states (QWS) in ultrathin Ag-films grown on W(100) with thickness from 1 to 12monolayers (ML) is probed by angle-resolved photoemission for the first time. Unlike the related and established systems Ag/Fe(100) and Ag/V(100), the QWS energies are with increasing thickness seen to evolve symmetrically about the QWS of 1ML Ag (−2.5eV). The QWSs are well described by the phase accumulation model down to lowest thicknesses only if the nearly-free-electron description of the parent Ag band established for Ag/Fe(100) is replaced by the simplest tight-binding approximation (linear chain approximation) with the band centered around the energy of the QWS of 1ML Ag. The total phase shift across the bulk band gap is found to be of the order of 2π, i.e., larger than reported for Ag/Fe(100). [Copyright &y& Elsevier]

Abstract: Strong superlattice effects are reported on () band dispersions measured by angle-resolved photoemission on W(3 3 1) and W(5 5 1), and graphene nanostripes on Ni(7 7 1). A splitting of the dispersions with reciprocal superlattice vector / is observed for a surface resonance on W(3 3 1) and W(5 5 1) and for -states of graphene/Ni(7 7 1). Photon-energy-dependent measurements and comparison to LEED show that electrons are confined to the one-dimensional terraces and stripes, respectively, and that the observed superlattice effects are due to diffraction in the final state of the photoemission transitions. [Copyright &y& Elsevier]

In this paper we review a variety of spectra for Ce compounds, i.e. valence photoemission and inverse valence photoemission, core-level and resonant photoemission as well as X-ray absorption, X-ray emission and X-ray magnetic circular dichroism (MCD). Those spectra are theoretically analysed by using the single-site Anderson model with various final state interactions. In the first part of the present article we are considering intermetallic Ce compounds, especially several Ce–Pd ordered phases, as well as CeRh3, CeSn3 and CeFe2. In the second part we focus on insulating Ce compounds like CeF3 as well as related systems like Ce oxides. Also we compare Ce hydroxides to other rare-earth hydroxides. Discussions are given on the physical information derived from the analysis, especially through a few key parameters of an extended version of the previously considered Anderson model. Those parameters (i.e. the Coulomb interaction Ufd between 5d and 4f electrons as well as the core hole potential -Udc acting on the 5d electron) have necessarily to be taken into account in order to interpret the experimental L3 X-ray absorption spectra. Another important point stressed in the present paper is the surface/bulk sensitivities of the various spectra to be investigated and theoretically analysed. Also similarities and differences in spectral features between metallic and insulating systems are pointed out. [ABSTRACT FROM AUTHOR]

Abstract: Au and Ag on the stepped W(1 4 5) surface with step width form quantum stripes up to a thickness of 2 and 3 monolayers (ML), respectively. The 2 ML stripes show a one-dimensional E () dispersion which perpendicular to the stripes is determined by a repetition with the reciprocal superlattice vector . [Copyright &y& Elsevier]

We perform ultrahigh-resolution angle-resolved photoemission experiments at a temperature T=0.8 K on the type-II Weyl semimetal candidate WTe2. 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 WTe2 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. [ABSTRACT FROM AUTHOR]