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Topological insulators in which the Fermi level is in the bulk gap and intersects only a topological surface state (the Dirac cone) are of special interest in the current research. In the last decades, a fine-tuning of the chemical composition of topological insulators has been carefully explored in order to control the Fermi level position with respect to the Dirac surface state. Taking the SnBi 2 Te 4 crystal as a case study, we provide a characterization of its electronic structure by means of angle-resolved photoemission spectroscopy and first-principles calculations. We show that, going away from the Brillouin zone center, bulk band states energetically overlap with the Dirac cone at the Fermi level, thus providing an unwanted as well as hidden contribution to the transport properties of the material. In addition, the comparison between experimental results of the band structure with state-of-the-art simulations, implemented taking into account the number of defects, leads to useful insights on the existing limits in the description of this material., This research was supported in part by the Proget to STAR2(PIR01-00008) of the Italian Ministry of Education, University, and Research.We acknowledge EUROFEL-ROADMAP ESFRI of the Italian Ministry of Education, University, and Research. The density functional theory calculations were supported by the government research assignment for ISPMS SB RAS, Project No. FWRW-20220001. E.V.C. acknowledges support from Saint Petersburg State University (ProjectIDNo.94031444).

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 effective attenuation length (EAL) of electrons in MgO films has been measured in the (5.5–28) eV energy range by the over-layer method and correlated with the band structure of the material. As expected, the EAL is found to increase when the electron energy is decreased, but the obtained values are smaller than those predicted by the universal curve. The comparison of the experimental results with calculated optical properties available in the literature suggests that, for energies lower than 20 eV, the relevant scattering mechanisms are described by the imaginary part of the dielectric function, accounting in particular for the steep increase of the EAL for energies smaller than the insulator band gap.

Angle-resolved photoemission spectroscopy reveals the presence of a two-dimensional electron gas at the surface of In(2)O(3)(111). Quantized subband states arise within a confining potential well associated with surface electron accumulation. Coupled Poisson-Schrödinger calculations suggest that downward band bending for the conduction band must be much bigger than band bending in the valence band. Surface oxygen vacancies acting as doubly ionized shallow donors are shown to provide the free electrons within this accumulation layer. Identification of the origin of electron accumulation in transparent conducting oxides has significant implications in the realization of devices based on these compounds.

The LUMO-derived band of the compound was investigated by means of x-ray and UV photoemission and photoelectric yield spectroscopies. A double-peak structure is found, with peak maxima at 1.1 eV and 0.35 eV. The relative magnitude of the two peaks is strongly temperature dependent: a large transfer of spectral weight from the lower- to the higher-binding-energy peak takes place upon cooling to 110 K. The peaks are tentatively attributed to the dimer and the polymeric phase respectively. However the behaviour as a function of temperature does not agree with the metastability of the dimer phase at room temperature. The position of the Fermi level is compatible with a correlated system on the border of the metal - insulating transition.

The α phase of Sn/Ge(111) has been investigated from 120 K up to 500 K, using synchrotron radiation core-level photoemission. By means of photoelectron diffraction experiments, we verified that the rippled structure of the low-temperature (3×3) phase is preserved in the (√3×√3)R30° phase at room temperature, thus confirming the order-disorder character of the phase transition. We also found that at least two components are present in the Sn 4d core-level spectra up to 500 K, i.e., about 300 K above the onset of the transition from the low-temperature (3×3) phase to the (√3×√3)R30° phase, thus excluding the occurrence of any displacive transition.

Angle-resolved photoemission spectroscopy reveals the presence of a two-dimensional electron gas at the surface of In(2)O(3)(111). Quantized subband states arise within a confining potential well associated with surface electron accumulation. Coupled Poisson-Schrödinger calculations suggest that downward band bending for the conduction band must be much bigger than band bending in the valence band. Surface oxygen vacancies acting as doubly ionized shallow donors are shown to provide the free electrons within this accumulation layer. Identification of the origin of electron accumulation in transparent conducting oxides has significant implications in the realization of devices based on these compounds.

The LUMO-derived band of the [Formula: see text] compound was investigated by means of x-ray and UV photoemission and photoelectric yield spectroscopies. A double-peak structure is found, with peak maxima at 1.1 eV and 0.35 eV. The relative magnitude of the two peaks is strongly temperature dependent: a large transfer of spectral weight from the lower- to the higher-binding-energy peak takes place upon cooling to 110 K. The peaks are tentatively attributed to the dimer and the polymeric phase respectively. However the behaviour as a function of temperature does not agree with the metastability of the dimer phase at room temperature. The position of the Fermi level is compatible with a correlated system on the border of the metal - insulating transition.

The photoemission spectra of the Sn 4d electrons from the (3×3)-Sn/Ge(111) surface present two components which are attributed to inequivalent Sn atoms in T4 bonding sites. This structure has been explored by photoelectron diffraction experiments performed at the ALOISA beamline of the Elettra storage ring in Trieste (Italy). The modulation of the intensities of the two Sn components, caused by the backscattering of the underneath Ge atoms, has been measured as a function of the emission angle at fixed kinetic energies and vice versa. The bond angle between Sn and its nearest neighbor atoms in the first Ge layer ( Sn–Ge 1) has been measured by taking polar scans along the main symmetry directions and it was found almost equivalent for the two components. The corresponding bond lengths are also quite similar, as obtained by studying the dependence on the photoelectron kinetic energy with the photon polarization and the collection direction parallel to the Sn–Ge 1 bond orientation (bond emission). A clear difference between the two bonding sites is observed when studying the energy dependence at normal emission, where the sensitivity to the Sn height above the Ge atom in the second layer is enhanced. The (3×3)-Sn/Ge(111) is thus characterized by a structure where the Sn atom and its three nearest neighbor Ge atoms form a rather rigid unit that presents a strong vertical distortion with respect to the underneath atom of the second Ge layer.

We present an angle resolved photoemission study of V2O3, a prototype system for the observation of Mott transitions in correlated materials. We show that the spectral features corresponding to the quasiparticle peak in the metallic phase present a marked wave vector dependence, with a stronger intensity along the GammaZ direction. The analysis of their intensity for different probing depths shows the existence of a characteristic length scale for the attenuation of coherent electronic excitations at the surface. This length scale, which is larger than the thickness of the surface region as normally defined for noncorrelated electronic states, is found to increase when approaching the Mott transition. These results are in agreement with the behavior of quasiparticles at surfaces as predicted by Borghi et al.

The Sn/Si(111)-(square root 3 x square root 3)R30 degrees surface was so far believed to be metallic according to the electron counting argument. We show, by using tunneling spectroscopy, scanning tunneling microscopy, photoemission, and photoelectron diffraction, that below 70 K this surface has a very low density of states at the Fermi level and is not appreciably distorted. The experimental results are compatible with the insulating Mott-Hubbard ground state predicted by LSDA+U calculations [G. Profeta and E. Tosatti, Phys. Rev. Lett. 98, 086401 (2007)].

The interaction of single-wall carbon nanotubes (SWCNTs) with various gas molecules has been studied by synchrotron radiation photoemission spectroscopy. Using also x-ray emission microscopy, we found that chemical elements other than C are present in commercial purified SWCNT bucky paper. Removal of these residual contaminants made the electronic spectra insensitive to O2, H2O, N2, and CO, while a strong and reversible effect to NO2, SO2, NO and NH3 was observed. © 2003 American Institute of Physics

Synchrotron radiation high resolution core level spectroscopy has been used to investigate the 1/3 monolayer (ML) Sn1-x/Si-x(111) (root3 x vroot3)R30degrees solid solution at well-defined values of the mutual surface concentration of the Sn and Si adatoms (x = 0.24 and x = 0.40) as a function of temperature (100-300 K). For 24% Si adatoms at the surface there is low energy electron diffraction (LEED) evidence of a nascent 3 x 3 lowering of the surface symmetry at 100 K. Despite this evidence core level spectroscopy data do not show any signature of the nascent phase transition. Our evidence is in agreement with the analogous observation of no significant spectral changes at reduced temperature of the core level spectra of the isoelectronic and isostructural 1/3 ML Sn/Ge(111) surface.

Photoelectron emission spectra in a photon energy range between 7.5 and 21 eV are measured for in situ grown polycrystalline Yb films. By comparing bulk and surface core level shifted 4f components we give an estimation of the effective attenuation length (EAL) for low energy (6-20 eV) electrons in Yb, establishing a moderate increase of the EAL upon electron energy decrease. The experimental EAL data are found to be a factor of four smaller than those predicted from the so-called 'universal curve'.

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The peculiar electrochemical and photophysical properties of porphyrin and fullerene molecules make them promising candidates for the construction of two- and three-dimensional organic-based materials. An important question is how pristine fullerene and porphyrin will organize when deposited on surfaces via in vacuum molecular beam evaporation. Here we show that codeposition of C(70) and Zn-tetraphenyl-porphyrin (ZnTPP) induces the self-assembly of electron-rich flat aromatic molecules at the curved surface of C(70), thus enhancing the chromophore interaction and forming a supramolecular multilayer donor-acceptor structure. While the ground-state electronic spectra almost reflect a simple summation of ZnTPP and C(70) components, the excited-state electrons at the porphyrin macrocycle can rapidly delocalize to the fullerene. The excited charge transfer time scale is faster than 1-2 fs, as shown by resonant photoemission for the core-excited charges.

Here we demonstrate that a molecular dye film composed by a single layer Of C-70 and a single layer of Zn-tetraphenyl-porphyrin (ZnTPP) adsorbed atop can be grown on large substrates using molecular beam epitaxy. The electrostatic interactions between the constituent molecules drive the formation of this ordered donor-acceptor layer. The overlapping between the pi* orbitals of ZnTPP and C-70 allows the delocalisation of the excited electrons from the excited molecule on a timescale of similar to 6 fs. Generally, the ZnTPP/C-70 film, acting as a p-n junction, delocalise more efficiently the excited electrons as compared to pure ZnTPP films on a timescale that competes effectively against loss processes, suggesting its application for a more efficient solar energy conversion in organic-based devices.

CO dissociation on rhomboidal faceted nanopyramids, produced on Rh(110) by fine-tuning of ion irradiation conditions, has been studied by high resolution core-level spectroscopy. We find that this morphology presents a large efficiency towards CO dissociation, a process which is inhibited on flat (110) terraces. We also measured the reactivity of nanostructures bound by different artificial step distributions identifying the sites responsible for the molecular bond disruption in the undercoordinated (n=6) edges running along the [1 (1) over bar2] equivalent directions, with CO sitting in on-top configuration.

The electronic properties and the molecular orientation of Zn-tetraphenyl-porphyrin films deposited on Si(111) have been investigated using synchrotron radiation. For the first time we have revealed and assigned the fine structures in the electronic spectra related to the HOMOs and LUMOs states. This is particularly important in order to understand the orbital interactions, the bond formation and the evolution of the electronic properties with oxidation or reduction of the porphyrins in supramolecular donor-acceptor complexes used in photovoltaic devices., text 11 pages, 4 figures submitted for publication

The Ge(110) surface has been investigated by core-level and valence band (VB) photoemission spectroscopy as a function of the temperature starting from the room temperature c(8×10) reconstructed surface up to 1196 K. Evidence of a phase transition is found at about 750±50 K from core-level and VB data analysis. VB photoemission shows that above 750 K the Ge(110) surface acquires an increasing metallic character up to 1110 K where a sudden, intense jump of the emission intensity at the Fermi level is observed. At higher temperatures up to 1196 K, a finite and constant density of states is observed, indicating the presence of a metallic surface layer.

Resonant inelastic electron scattering is a new method that allows to detect image resonances with an energy resolution that makes line-shape analysis possible at an energy far above the vacuum level. For a better understanding of image states on Pb/Ge(111) and of the scattering mechanism, the two symmetry directions of the surface Γ̄M̄ and Γ̄K̄, and the dependence of the image resonances on the surface phases, the sample doping and the Pb coverage have been investigated. The maximum intensity of the image resonances is found for coverages ≥1 ML.

The Ge(110) surface has been investigated by core-level and valence band (VB) photoemission spectroscopy as a function of temperature starting from the room temperature c (8×10) reconstructed surface up to 1196 K. Evidence of a phase transition is found at about 750±50 K from core-level and valence band data analysis. VB photoemission shows that above 750 K the Ge(110) surface acquires increasing metallic character up to 1110 K, where a sudden, intense jump of the emission intensity at the Fermi level is observed. At higher temperatures up to 1196 K, a finite and constant density of states is observed, indicating the presence of a metallic surface layer.