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1.
  • Adell, Johan, et al. (author)
  • Formation of epitaxial MnBi layers on (Ga,Mn)As
  • 2009
  • In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121 .- 2469-9950 .- 2469-9969. ; 80:7
  • Journal article (peer-reviewed)abstract
    • The initial growth of MnBi on MnAs-terminated (GaMn)As is studied by means of synchrotron-based photoelectron spectroscopy. From analysis of surface core-level shifts we conclude that a continued epitaxial MnBi layer is formed, in which the MnAs/MnBi interface occurs between As and Bi atomic planes. The well-defined 1×2 surface reconstruction of the MnAs surface is preserved for up to 2 ML of MnBi before clear surface degradation occurs. The MnBi layer appears to be free from intermixed As.
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2.
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3.
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4.
  • Laukkanen, P., et al. (author)
  • A comparative study of clean and Bi-stabilized InP(100)(2 x 4) surfaces by the core-level photoelectron spectroscopy
  • 2007
  • In: Surface Science. - : Elsevier BV. - 0039-6028. ; 601, s. 3395-
  • Journal article (peer-reviewed)abstract
    • The bismuth-stabilized (2 x 4)-reconstructed InP(100) surface [Bi/InP(100)(2 x 4)] has been studied by synchrotron-radiation core-level photoelectron spectroscopy. The spectra are compared with previous core-level data obtained on a clean InP(100)(2 x 4) surface. The findings support that the P 2p surface-core-level shift (SCLS) of the clean InP(100)(2 x 4), which has higher kinetic energy than the bulk emission, arises from the third-layer P atoms and that the second P 2p SCLS, which has lower kinetic energy than the bulk, arises from the top-layer P atoms. Similar In 4d SCLSs are found on the clean and Bi-stabilized InP(100)(2 x 4) surfaces, indicating that these shifts contain contributions of the In atoms that lie in the second and/or fourth layers. In addition to this, the results improve our understanding of the atomic structure of the Bi/InP(100)(2 x 4) surface and lead to refined surface models which include Bi-Bi and Bi-P dimers. (C) 2007 Elsevier B.V. All rights reserved.
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5.
  • Adell, Anna, et al. (author)
  • Open-access portal with hindcast wave data for Skåne and Halland
  • 2021
  • In: Vatten: tidskrift för vattenvård /Journal of Water Management and research. - 0042-2886. ; 77:2, s. 81-90
  • Journal article (other academic/artistic)abstract
    • Wave climate data for the Swedish provinces Skane and Halland, were hindcast using SWAN, a third-generation spectral wave model. The 40-year wave dataset, from 1979 to 2019, is made available through an open-access data portal (https://gis.sgi.se/vagmodell/). The wave data has a three-hour resolution and includes significant wave height, peak wave period, and wave direction. The wave model domain encompasses the Baltic Sea, Öresund, Kattegat, and Skagerrak. Along the coast of Skane and Halland, the spatial resolution of the computational nodes, from which data can be extracted in the portal, is 250 m. In the offshore areas, the resolution of the computational grid is coarser. The simulated significant wave height was validated against observations from 25 wave gauges, operating intermittently during the simulation period. The coefficient of determination, R2, for these comparisons ranged from 0.46 to 0.93 for the different stations. For 15 wave gauges, R2 values for the comparisons exceeded 0.80. The wave model will continuously be updated and developed.
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6.
  • Adell, Johan, 1980, et al. (author)
  • Electron spectroscopic studies of nanowires formed by (GaMn)As growth on GaAs(111)B
  • 2011
  • In: Solid State Communications. - : Elsevier BV. - 0038-1098 .- 1879-2766. ; 151:11, s. 850-854
  • Journal article (peer-reviewed)abstract
    • Valence band photoemission with photon energies around the Mn2p excitation threshold has been used to study the development of nanowires catalyzed by MnAs particles. A gradual change in the spectra with increasing nanowire length is observed, such that the resonant photoemission eventually dominates over the Auger decay channel. The change is ascribed to dilution of Mn, showing that Mn is transferred from the MnAs particles into the nanowires. (C) 2011 Elsevier Ltd. All rights reserved.
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7.
  • Adell, Johan, 1980 (author)
  • Synchrotron light based spectroscopy of MBE-grown (GaMn)As structures
  • 2009
  • Doctoral thesis (other academic/artistic)abstract
    • Structures based on the diluted magnetic semiconductor (GaMn)As have been grown by low temperature molecular beam epitaxy and studied using synchrotron radiation based electron spectroscopy. An investigation of modifications due to in situ low temperature annealing under amorphous As capping has been performed. A clear accumulation of Mn is observed. This is ascribed to a reaction between diffusing Mn interstitials and the As capping. The reacted surface is smooth and well ordered with a 1x2 reconstruction. All data indicate that the annealed (GaMn)As is terminated by a monolayer MnAs in zinc-blende structure. The good surface quality of the MnAs-terminated layer allows further epitaxial growth. As an example, it was possible to deposit a 3 ML thick MnBi film layer-by-layer. MnBi is particularly interesting in this context due to its predicted half-metal characteristics The GaAs/(GaMn)As interface has been studied with focus on the electrostatic properties. No band offset between the two systems was observed. The continuous transition is explained as an effect of electronic smearing of the atomically abrupt interface. Annealing induced diffusion of Mn interstitials through GaAs films grown on (GaMn)As during thermal treatment was also studied. It was found that GaAs layers thicker than 8 monolayers effectively hinder all Mn out diffusion. The effect was ascribed to the electrostatic potential barrier due to band bending in the GaAs film. Nanowires catalyzed by MnAs particles were studied using valence band photoemission with photon energies around the Mn2p threshold. It was found that with increasing nanowire length the resonant photoemission eventually dominated over the Auger decay channel. This was ascribed to dilution of Mn in the wires.
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8.
  • Adell, Johan, 1980, et al. (author)
  • Thermal diffusion of Mn through GaAs overlayers on (Ga, Mn)As
  • 2011
  • In: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 23:8
  • Journal article (peer-reviewed)abstract
    • Thermally stimulated diffusion of Mn through thin layers of GaAs has been studied by x-ray photoemission. (Ga, Mn)As samples with 5 at% Mn were capped with 4, 6 and 8 monolayer (ML) GaAs, and Mn diffusing through the GaAs was trapped on the surface by means of amorphous As. It was found that the out-diffusion is completely suppressed for an 8 ML thick GaAs film. The short diffusion length is attributed to an electrostatic barrier formed at the (Ga, Mn)As/GaAs interface.
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9.
  • Belopolski, Ilya, et al. (author)
  • Observation of a linked-loop quantum state in a topological magnet
  • 2022
  • In: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 604:7907, s. 647-652
  • Journal article (peer-reviewed)abstract
    • Quantum phases can be classified by topological invariants, which take on discrete values capturing global information about the quantum state1–13. Over the past decades, these invariants have come to play a central role in describing matter, providing the foundation for understanding superfluids5, magnets6,7, the quantum Hall effect3,8, topological insulators9,10, Weyl semimetals11–13 and other phenomena. Here we report an unusual linking-number (knot theory) invariant associated with loops of electronic band crossings in a mirror-symmetric ferromagnet14–20. Using state-of-the-art spectroscopic methods, we directly observe three intertwined degeneracy loops in the material’s three-torus, T3, bulk Brillouin zone. We find that each loop links each other loop twice. Through systematic spectroscopic investigation of this linked-loop quantum state, we explicitly draw its link diagram and conclude, in analogy with knot theory, that it exhibits the linking number (2, 2, 2), providing a direct determination of the invariant structure from the experimental data. We further predict and observe, on the surface of our samples, Seifert boundary states protected by the bulk linked loops, suggestive of a remarkable Seifert bulk–boundary correspondence. Our observation of a quantum loop link motivates the application of knot theory to the exploration of magnetic and superconducting quantum matter.
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10.
  • Dahl, J., et al. (author)
  • Formation of polar InN with surface Fermi level near the valence band maximum by means of ammonia nitridation
  • 2012
  • In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 86:24
  • Journal article (peer-reviewed)abstract
    • Development of InN films for devices is hindered due to metallic In clusters, formed readily during growth, and unintentional n-type conductivity of the nominally undoped films, including surface electron-accumulation layers via the Fermi level pinning into the conduction band. Plasma nitridation eliminates even large In clusters from the surface by changing them to two-dimensional InN [Yamaguchi and Nanishi, Appl. Phys. Expr. 2, 051001 (2009)]. Here we utilized a similar approach, that is, nitridation of In-covered surfaces with ammonia (NH3) to grow thin, up to 25 nm thick polar InN films on Si(111) and GaN(0001) substrates. By means of scanning tunneling microscopy and spectroscopy, as well as photoelectron spectroscopy, we show that this simple NH3 nitridation provides the hitherto not reported formation of polar InN(000-1) films with the surface Fermi level close to the valence band maximum, as recent calculations [Belabbes et al., Phys. Rev. B 84, 205304 (2011)] predict. DOI: 10.1103/PhysRevB.86.245304
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11.
  • Eriksson, Johan, 1979-, et al. (author)
  • Origin of a surface state above the Fermi level on Ge(001) and Si(001) studied by temperature-dependent ARPES and LEED
  • 2008
  • In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:L8, s. 085406-1-085406-5
  • Journal article (peer-reviewed)abstract
    • Variable temperature photoemission studies in the literature have revealed the presence of a surface state above the Fermi level on clean Ge(001). We present photoemission and low energy electron diffraction results from Ge(001) obtained between 185 and 760 K. Our measurements show a peak above the Fermi level with a maximum intensity at a sample temperature of around 625 K. At higher temperatures, we observe a gradual decrease in the intensity. Angle resolved spectra show that the surface state has a k̅ ∥ dependence and is therefore not attributed to defects. Very similar results were obtained on both an intrinsic (30 Ω cm) and a 10 m Ω cm n-type sample. The overall appearance of the spectral feature is found to be quite insensitive to sample preparation. Low energy electron diffraction investigations show how the sharp c(4×2) pattern becomes streaky and finally turns into a 2×1 pattern. The onset of the structure above the Fermi level takes place just before all c(4×2) streaks have disappeared which corresponds to a temperature of around 470 K. On Si(001), we also observe photoemission intensity above the Fermi level. It is weaker than on Ge(001) and appears at higher temperature. We find that the emission above the Fermi level can be explained by thermal occupation of the π* band derived from a 2×1 ordering of asymmetric dimers on the surface.
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12.
  • Kowalski, B. J., et al. (author)
  • Angle-resolved photoemission study and pseudopotential calculations of GeTe and Ge1-xMnxTe band structure
  • 2010
  • In: Physics Procedia. - : Elsevier BV. - 1875-3892. ; 3:2, s. 1357-1362
  • Journal article (peer-reviewed)abstract
    • The valence band structure along the Γ-T and T-W-L directions in the Brillouin zone of GeTe is studied by means of angle-resolved photoemission and compared with the results of ab initio pseudopotential calculations. For Ge1-xMnxTe surface alloy, changes in the valence band induced by presence of Mn atoms are revealed.
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13.
  • Kuzmin, M., et al. (author)
  • Dimer-T(3) reconstruction of the Sm/Si(100)(2 x 3) surface studied by high-resolution photoelectron spectroscopy and density functional theory calculations
  • 2011
  • In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:24, s. 245322-
  • Journal article (peer-reviewed)abstract
    • Silicon dimer-containing reconstructions on Si(100) can be induced by submonolayer amounts of rare earth (RE) metals. The tilt of dimer bonds in such reconstructions can be controlled by the coverage and electronic properties of RE adsorbates. In this study, we have utilized improved high-resolution photoelectron spectroscopy with the synchrotron radiation and density functional theory (DFT) calculations to exploit the structural and electronic properties of the Sm/Si(100)(2 x 3) system. A careful analysis of photoelectron spectra, in combination with DFT calculations of surface core-level shifts for silicon atoms in energetically plausible structural models, has allowed us to establish the favorable atomic configuration of Sm/Si(100)(2 x 3) with a buckled Si dimer and to explain characteristic features of Si 2p line shape in detail. It is shown that the dimer buckling leads to a significant core-level binding-energy splitting of the first-layer Si atoms, affecting the lower-binding-energy region of Si 2p spectra drastically. An interpretation of the Si 2p line shape for RE/Si(100)(2 x 3) that is based on combined initial state and complete screening data is suggested. The mechanism underlying the buckling and symmetrization of silicon dimers in RE/Si(100) reconstructions is discussed.
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14.
  • Kuzmin, M., et al. (author)
  • Photoemission and density functional theory study of Ge(100) : Clean surface and Yb-induced (2x4) reconstruction
  • 2013
  • In: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 615, s. 88-96
  • Journal article (peer-reviewed)abstract
    • Clean and metal-adsorbed (100) surfaces of group-IV semiconductors, such as Si and Ge, often exhibit electronically and structurally similar reconstructions. However, the fundamental bulk properties of group-IV materials can have an impact on particular features of such systems, which are related, e.g., to final-state relaxation in photoemission and thus determine their spectral line shape. Here we have studied Yb/Ge(100)(2 x 4) reconstruction as well as clean Ge(100) surface by high-resolution photoelectron spectroscopy and ab initio calculations. An atomic geometry of both surfaces is thoroughly investigated. A detailed analysis of Ge 3d core-level photoemission, atomic origins of surface-shifted components, and final-state screening effects is presented. In particular, it is demonstrated that the core-hole screening plays an essential role in Ge 3d measurements, and that its amount in the complete screening model correlates well with the core-level binding energy of respective Ge atoms in the initial state. The results are discussed in the proper context of related reconstructions on Si(100).
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15.
  • Kuzmin, M., et al. (author)
  • Surface core-level shifts on Ge(111)c(2 x 8) : Experiment and theory
  • 2011
  • In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:24, s. 245319-
  • Journal article (peer-reviewed)abstract
    • Combining high-resolution photoelectron spectroscopy and density functional theory (DFT) calculations, 3d photoemission line shape and surface core-level shifts have been reinvestigated on the Ge(111)c(2 x 8) surface. It is found that 3d spectra include, in addition to the bulk and three surface-shifted components reported in literature, a component that was not identified in earlier measurements with a lower resolution. The detailed interpretation of these spectra and their line shape is made on the basis of DFT calculations. It is shown that the lowest binding energy component is due to the rest atoms. The higher binding energy emission is caused by the adatoms and the third-layer atoms that are below the adatoms. Finally, the two other surface components originate from the first- and second-layer atoms. The screening effects in the Ge(111)c(2 x 8) are discussed.
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16.
  • Lang, J. J. K., et al. (author)
  • Tin-stabilized (1 x 2) and (1 x 4) reconstructions on GaAs(100) and InAs(100) studied by scanning tunneling microscopy, photoelectron spectroscopy, and ab initio calculations
  • 2011
  • In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 605:9-10, s. 883-888
  • Journal article (peer-reviewed)abstract
    • Tin (Sn) induced (1 x 2) reconstructions on GaAs(100) and InAs(100) substrates have been studied by low energy electron diffraction (LEED), photoelectron spectroscopy, scanning tunneling microscopy/spectroscopy (STM/STS) and ab initio calculations. The comparison of measured and calculated STM images and surface core-level shifts shows that these surfaces can be well described with the energetically stable building blocks that consist of Sn-III dimers. Furthermore, a new Sn-induced (1 x 4) reconstruction was found. In this reconstruction the occupied dangling bonds are closer to each other than in the more symmetric (1 x 2) reconstruction, and it is shown that the (1 x 4) reconstruction is stabilized as the adatom size increases.
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17.
  • Laukkanen, P., et al. (author)
  • Anomalous bismuth-stabilized (2x1) reconstructions on GaAs(100) and InP(100) surfaces
  • 2008
  • In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 100, s. 086101-
  • Journal article (peer-reviewed)abstract
    • First-principles phase diagrams of bismuth-stabilized GaAs- and InP(100) surfaces demonstrate for the first time the presence of anomalous (2 x 1) reconstructions, which disobey the common electron counting principle. Combining these theoretical results with our scanning-tunneling-microscopy and photoemission measurements, we identify novel (2 x 1) surface structures, which are composed of symmetric Bi-Bi and asymmetric mixed Bi-As and Bi-P dimers, and find that they are stabilized by stress relief and pseudogap formation.
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18.
  • Laukkanen, P., et al. (author)
  • Bismuth-stabilized c(2X6) reconstruction on a InSb(100) substrate : Violation of the electron counting model
  • 2010
  • In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 81:3
  • Journal article (peer-reviewed)abstract
    • By means of scanning tunneling microscopy/spectroscopy (STM/STS), photoelectron spectroscopy, and first-principles calculations, we have studied the bismuth (Bi) adsorbate-stabilized InSb(100) substrate surface which shows a c(2X6) low-energy electron diffraction pattern [thus labeled Bi/InSb(100)c(2X6) surface] and which includes areas with metallic STS curves as well as areas with semiconducting STS curves. The first-principles phase diagram of the Bi/InSb(100) surface demonstrates the presence of the Bi-stabilized metallic c(2X6) reconstruction and semiconducting (4X3) reconstruction depending on the chemical potentials, in good agreement with STS results. The existence of the metallic c(2X6) phase, which does not obey the electron counting model, is attributed to the partial prohibition of the relaxation in the direction perpendicular to dimer rows in the competing reconstructions and the peculiar stability of the Bi-stabilized dimer rows. Based on (i) first-principles phase diagram, (ii) STS results, and (iii) comparison of the measured and calculated STM and photoemission data, we show that the measured Bi/InSb(100)c(2X6) surface includes metallic areas with the stable c(2X6) atomic structure and semiconducting areas with the stable (4X3) atomic structure.
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19.
  • Laukkanen, P., et al. (author)
  • Core-level shifts of the c(8 x 2)-reconstructed InAs(100) and InSb(100) surfaces
  • 2010
  • In: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 177:1, s. 52-57
  • Journal article (peer-reviewed)abstract
    • We have studied In-stabilized c(8 2)-reconstructed InAs(1 0 0) and InSb(1 0 0) semiconductor surfaces, which play a key role in growing improved III-V interfaces for electronics devices, by core-level photoelectron spectroscopy and first-principles calculations. The calculated surface core-level shifts (SCLSs) for the zeta and zeta a models, which have been previously established to describe the atomic structures of the III-V(1 00)c(8 x 2) surfaces, yield hitherto not reported interpretation for the As 3d, In 4d, and Sb 4d core-level spectra of the III-V(1 00)c(8 x 2) surfaces, concerning the number and origins of SCLSs. The fitting analysis of the measured spectra with the calculated zeta and zeta a SCLS values shows that the InSb spectra are reproduced by the zeta SCLSs better than by the zeta a SCLSs. Interestingly, the zeta a fits agree better with the InAs spectra than the zeta fits do, indicating that the zeta a model describes the InAs surface better than the InSb surface. These results are in agreement with previous X-ray diffraction data. Furthermore, an introduction of the complete-screening model, which includes both the initial and final state effects, does not improve the fitting of the InSb spectra, proposing the suitability of the initial-state model for the SCLSs of the III-V(1 0 0)c(8 x 2) surfaces. The found SCLSs are discussed with the ab initio on-site charges.
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20.
  • Laukkanen, P., et al. (author)
  • Ultrathin (1x2)-Sn layer on GaAs(100) and InAs(100) substrates : A catalyst for removal of amorphous surface oxides
  • 2011
  • In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 98:23, s. 231908-1-231908-3
  • Journal article (peer-reviewed)abstract
    • Amorphous surface oxides of III-V semiconductors are harmful in many contexts of device development. Using low-energy electron diffraction and photoelectron spectroscopy, we demonstrate that surface oxides formed at Sn-capped GaAs(100) and InAs(100) surfaces in air are effectively removed by heating. This Sn-mediated oxide desorption procedure results in the initial well-defined Sn-stabilized (1x2) surface even for samples exposed to air for a prolonged time. Based on ab initio calculations we propose that the phenomenon is due to indirect and direct effects of Sn. The Sn-induced surface composition weakens oxygen adsorption.
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21.
  • Laverock, J., et al. (author)
  • k-resolved susceptibility function of 2H-TaSe2 from angle-resolved photoemission
  • 2013
  • In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 88:3
  • Journal article (peer-reviewed)abstract
    • The connection between the Fermi surface and charge-density-wave (CDW) order is revisited in 2H-TaSe2. Using angle-resolved photoemission spectroscopy, ab initio band-structure calculations, and an accurate tight-binding model, we develop the empirical k-resolved susceptibility function, which we use to highlight states that contribute to the susceptibility for a particular q vector. We show that although the Fermi surface is involved in the peaks in the susceptibility associated with CDW order, it is not through conventional Fermi surface nesting, but rather through finite energy transitions from states located far from the Fermi level. Comparison with monolayer TaSe2 illustrates the different mechanisms that are involved in the absence of bilayer splitting.
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22.
  • Laverock, J., et al. (author)
  • Resonant Soft-X-Ray Emission as a Bulk Probe of Correlated Electron Behavior in Metallic SrxCa1-xVO3
  • 2013
  • In: Physical Review Letters. - 1079-7114. ; 111:4
  • Journal article (peer-reviewed)abstract
    • The evolution of electron correlation in SrxCa1-xVO3 has been studied using a combination of bulk-sensitive resonant soft x-ray emission spectroscopy, surface-sensitive photoemission spectroscopy, and ab initio band structure calculations. We show that the effect of electron correlation is enhanced at the surface. Strong incoherent Hubbard subbands are found to lie similar to 20% closer in energy to the coherent quasiparticle features in surface-sensitive photoemission spectroscopy measurements compared with those from bulk-sensitive resonant soft x-ray emission spectroscopy, and a similar to 10% narrowing of the overall bandwidth at the surface is also observed.
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23.
  • Mazzola, Federico, et al. (author)
  • Kinks in the σ Band of Graphene Induced by Electron-Phonon Coupling
  • 2013
  • In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 111:21, s. 216806-
  • Journal article (peer-reviewed)abstract
    • Angle-resolved photoemission spectroscopy reveals pronounced kinks in the dispersion of the σ band of graphene. Such kinks are usually caused by the combination of a strong electron-boson interaction and the cutoff in the Fermi-Dirac distribution. They are therefore not expected for the σ band of graphene that has a binding energy of more than ≈3.5  eV. We argue that the observed kinks are indeed caused by the electron-phonon interaction, but the role of the Fermi-Dirac distribution cutoff is assumed by a cutoff in the density of σ states. The existence of the effect suggests a very weak coupling of holes in the σ band not only to the π electrons of graphene but also to the substrate electronic states. This is confirmed by the presence of such kinks for graphene on several different substrates that all show a strong coupling constant of λ≈1.
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24.
  • Patel, Sahil J., et al. (author)
  • Surface and electronic structure of epitaxial PtLuSb (001) thin films
  • 2014
  • In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 104:20
  • Journal article (peer-reviewed)abstract
    • The surface and electronic structure of single crystal thin films of PtLuSb (001) grown by molecular beam epitaxy were studied. Scanning tunneling spectroscopy (STS), photoemission spectroscopy, and temperature dependent Hall measurements of PtLuSb thin films are consistent with a zero-gap semiconductor or semi-metal. STS and photoemission measurements show a decrease in density of states approaching the Fermi level for both valence and conduction bands as well as a slight shift of the Fermi level position into the valence band. Temperature dependent Hall measurements also corroborate the Fermi level position by measurement of p-type carriers. (C) 2014 AIP Publishing LLC.
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25.
  • Polley, Craig, et al. (author)
  • Observation of topological crystalline insulator surface states on (111)-oriented Pb1-xSnxSe films
  • 2014
  • In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:7, s. 075317-
  • Journal article (peer-reviewed)abstract
    • We present angle-resolved photoemission spectroscopy measurements of the surface states on in-situ grown (111) oriented films of Pb1-xSnxSe, a three-dimensional topological crystalline insulator. We observe surface states with Dirac-like dispersion at (Gamma) over bar and (M) over bar in the surface Brillouin zone, supporting recent theoretical predictions for this family of materials. We study the parallel dispersion isotropy and Dirac-point binding energy of the surface states, and perform tight-binding calculations to support our findings. The relative simplicity of the growth technique is encouraging, and suggests a clear path for future investigations into the role of strain, vicinality, and alternative surface orientations in (Pb,Sn)Se solid solutions.
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26.
  • Punkkinen, M. P. J., et al. (author)
  • Bismuth-stabilized (2x1) and (2x4) reconstructions on GaAs(100) surfaces : Combined first-principles, photoemission, and scanning tunneling microscopy study
  • 2008
  • In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 78, s. 195304-
  • Journal article (peer-reviewed)abstract
    • Bismuth adsorbate-stabilized (2x1) and (2x4) reconstructions of the GaAs(100) surfaces have been studied by first-principles calculations, valence-band and core-level photoelectron spectroscopies, and scanning tunneling microscopy. It is demonstrated that large Bi atom size leads to the formation of the pseudogap at the Fermi energy and to the lower energy of an adsorbate-derived surface band, which contributes to the stabilization of the exceptional Bi/GaAs(100)(2x1) reconstruction. It is proposed that the Bi/GaAs(100)(2x4) reconstructions include asymmetric mixed Bi-As dimers, in addition to the Bi-Bi dimers. Based on the calculations, we solve the atomic origins of the surface core-level shifts (SCLSs) of the Bi 5d photoemission spectra from the Bi/GaAs(100)(2x4) surfaces. This allows for resolving the puzzle related to the identification of two SCLS components often found in the measurements of the Bi 5d and Sb 4d core-level emissions of the Bi/III-V and Sb/III-V(100)(2x4) surfaces. Finally, the reason for the absence of the common (2x4)-beta 2 structure and additional support for the stability of the (2x1) structure on the Bi/III-V(100) surfaces are discussed in terms of Bi atom size and subsurface stress.
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27.
  • Starowicz, P., et al. (author)
  • Valence Band of Ce2Co0.8Si3.2 and Ce2RhSi3 Studied by Resonant Photoemission Spectroscopy and FPLO Calculations
  • 2014
  • In: Acta Physica Polonica. Series A: General Physics, Physics of Condensed Matter, Optics and Quantum Electronics, Atomic and Molecular Physics, Applied Physics. - 0587-4246. ; 126:4A, s. 144-147
  • Journal article (peer-reviewed)abstract
    • This work presents studies of the valence band of two Kondo lattice systems: Ce2Co0.8Si3.2, which is paramagnetic with the Kondo temperature T-K approximate to 50 K and Ce2RhSi3, which is antiferromagnetic below T-N = 4.5 K and exhibits TK approximate to 9 K. The photoemission spectra, which are obtained with photon energy tuned to Ce - 4d 4f resonance, reveal a Kondo peak at the Fermi energy (E-F), its spin orbit splitting partner at 0.24 eV and a broad maximum related to Ce f(0) final state. The spectra indicate that Kondo peak has a higher intensity for Ce2Co0.8Si3.2. The off-resonance photoemission data reveal that a maximum in the 3d electron density of states is shifted towards EF for Ce2Co0.8Si3.2 as compared to Ce2RhSi3. Full-potential local-orbital calculations were realized with local spin density approach +U approach for 213 stoichiometry. They show that a higher density of states near EF is observed for Ce2CoSi3. The calculations also reveal the existing tendencies for antiferromagnetic and ferromagnetic ground states in a case of Ce2RhSi3 and Ce2CoSi3, respectively.
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28.
  • Thomas, Andrew G., et al. (author)
  • Adsorption Studies of p-Aminobenzoic Acid on the Anatase TiO2(101) Surface
  • 2014
  • In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 30:41, s. 12306-12314
  • Journal article (peer-reviewed)abstract
    • The adsorption of p-aminobenzoic acid (pABA) on the anatase TiO2(101) surface has been investigated using synchrotron radiation photoelectron spectroscopy, near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). Photoelectron spectroscopy indicates that the molecule is adsorbed in a bidentate mode through the carboxyl group following deprotonation. NEXAFS spectroscopy and DFT calculations of the adsorption structures indicate the ordering of a monolayer of the amino acid on the surface with the plane of the ring in an almost upright orientation. The adsorption of pABA on nanoparticulate TiO2 leads to a red shift of the optical absorption relative to bare TiO2 nanoparticles. DFT and valence band photoelectron spectroscopy suggest that the shift is attributed to the presence of the highest occupied molecular orbitals in the TiO2 band gap region and the presence of new molecularly derived states near the foot of the TiO2 conduction band.
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29.
  • Tian, Shangjie, et al. (author)
  • Magnetic topological insulator MnBi6 Te10 with a zero-field ferromagnetic state and gapped Dirac surface states
  • 2020
  • In: Physical Review B. - 2469-9950. ; 102:3
  • Journal article (peer-reviewed)abstract
    • Magnetic topological insulators (TIs) with nontrivial topological electronic structure and broken time-reversal symmetry exhibit various exotic topological quantum phenomena. The realization of such exotic phenomena at high temperature is one of the central topics in this area. We reveal that MnBi6Te10 is a magnetic TI with an antiferromagnetic ground state below 10.8 K whose nontrivial topology is manifested by Dirac-like surface states. The ferromagnetic axion insulator state with Z4=2 emerges once spins are polarized at a field as low as 0.1 T, accompanied with saturated anomalous Hall resistivity up to 10 K. Such a ferromagnetic state is preserved even with an external field down to zero at 2 K. Theoretical calculations indicate that the few-layer ferromagnetic MnBi6Te10 is also topologically nontrivial with a nonzero Chern number. Angle-resolved photoemission spectroscopy experiments further reveal three types of Dirac surface states arising from different terminations on the cleavage surfaces, one of which has insulating behavior with an energy gap of ∼28 meV at the Dirac point. These outstanding features suggest that MnBi6Te10 is a promising system to realize various topological quantum effects at zero field and high temperature.
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30.
  • Ulfat, Intikhab, 1966, et al. (author)
  • As3d core level studies of (GaMn)As annealed under As capping
  • 2010
  • In: Surface Science. - : Elsevier BV. - 0039-6028. ; 604:2, s. 125-128
  • Journal article (peer-reviewed)abstract
    • The surface of a Ga(0.95)Mn(0.05)As layer subjected to low temperature annealing under As capping has been studied by core level photoemission with focus on As3d spectrum. By detailed comparison with the surface of pure GaAs subjected to the same surface treatment, the As spectral component of the reacted surface layer has been identified. The relative intensity of this component is consistent with the notion of an MnAs monolayer terminating the annealed (GaMn)As surface.
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31.
  • Ulfat, Intikhab, 1966, et al. (author)
  • Electron Spectroscopic Studies of Homogenous (GaMn)As layers
  • 2012
  • In: Advanced Materials Research. - 1662-8985 .- 1022-6680. - 9783037853634 ; 463-464, s. 380-384
  • Conference paper (peer-reviewed)abstract
    • By incorporating magnetism into semiconductors, it may possibly be viable to enhance the functionality of materials. An exceptionally important material in this context is GaAs, which can be doped with Mn atoms. (GaMn)As has fascinated research community as a promising candidate for spintronic application. It is quite appealing due to both its compatibility with existing HI-V technology and great progress in improving its magnetic properties. Being fabricated by low temperature molecular beam epitaxy (LT-MBE), due to thermal instability at elevated temperatures, the material contains a high density of various defects compensating Mn acceptors. It is a well-established fact that the ferromagnetic state of (GaMn)As can be stabilized via post growth annealing. Nevertheless, in general, the annealed (GaMn)As layers do not remain useful for further epitaxial overgrowth that might be included in multilayer structure. We present a summary of our investigations regarding the synchrotron-based characterization of (GaMn)As layers grown via molecular beam epitaxy carried out at the Swedish National Facility of Synchrotron Radiation-the MAX-lab aiming at the reduction of the density of Mn interstitial and increase in the content of Mn.
  •  
32.
  • Ulfat, Intikhab, 1966, et al. (author)
  • Post-growth annealing of (Ga,Mn)As under Sb capping
  • 2012
  • In: Applied Mechanics and Materials. - 1660-9336 .- 1662-7482. - 9783037855102 ; 243-246, s. 243-246
  • Conference paper (peer-reviewed)abstract
    • (Ga,Mn)As is a model diluted ferromagnet system in which the atomic spins of Mn ions are ferromagnetically arranged due to the exchange interaction with valence band holes. An important tecchnological concern regarding this system has been approaches that might result in reduction of the density of Mn interstitial and increase in the content of Mn in order to make the system practically feasible. To accomplish the objective we report the results of our recent synchrotron radiation based spectroscopic investigations concening annealing induced modification of as-grown (Ga,Mn)As layers covered with Sb capping.
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33.
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34.
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35.
  • Xu, Su-Yang, et al. (author)
  • Lifshitz transition and Van Hove singularity in a three-dimensional topological Dirac semimetal
  • 2015
  • In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 92:7
  • Journal article (peer-reviewed)abstract
    • A three-dimensional (3D) Dirac semimetal is a novel state of quantum matter which has recently attracted much attention as an apparent 3D version of graphene. In this paper, we report results on the electronic structure of the 3D Dirac semimetal Na3Bi at a surface that reveals its nontrivial ground state. Our studies reveal that the two 3D Dirac cones go through a topological change in the constant energy contour as a function of the binding energy, featuring a Lifshitz point, which is missing in a strict 3D analog of graphene. Our results identify an example of a band saddle-point singularity in 3D Dirac materials. This is in contrast to its two-dimensional analogs such as graphene and the Dirac surface states of a topological insulator. The observation of multiple Dirac nodes in Na3Bi connecting via a Lifshitz point along its crystalline rotational axis away from the Kramers point serves as a decisive signature for the symmetry-protected nature of the Dirac semimetal's topological bulk ground state.
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