| 1. |
- Dahl, J., et al.
(författare)
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Formation of polar InN with surface Fermi level near the valence band maximum by means of ammonia nitridation
- 2012
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 86:24
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Tidskriftsartikel (refereegranskat)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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| 2. |
- Dziawa, P., et al.
(författare)
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Topological crystalline insulator states in Pb1-xSnxSe
- 2012
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Ingår i: Nature Materials. - 1476-1122 .- 1476-4660. ; 11:12, s. 1023-1027
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Tidskriftsartikel (refereegranskat)abstract
- Topological insulators are a class of quantum materials in which time-reversal symmetry, relativistic effects and an inverted band structure result in the occurrence of electronic metallic states on the surfaces of insulating bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical results have suggested the existence of topological crystalline insulators (TCIs), a class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection(1,2). In this study we show that the narrow-gap semiconductor Pb1-xSnxSe is a TCI for x = 0.23. Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a TCI. These experimental findings add a new class to the family of topological insulators, and we anticipate that they will lead to a considerable body of further research as well as detailed studies of topological phase transitions.
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| 3. |
- Krishnakumar, S. K. Mahatha, et al.
(författare)
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Unoccupied electronic structure of graphite probed by angle-resolved photoemission spectroscopy
- 2011
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 84:11
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Tidskriftsartikel (refereegranskat)abstract
- We report the observation of anomalous bands in graphite valence band structure in angle-resolved photoemission spectroscopy (ARPES) experiments. The photon energy dependence of these bands shows a constant kinetic energy nature. Our results are supported by the very low energy electron diffraction data reported on graphite surfaces which essentially map the unoccupied states representing the photoemission final states. This suggests that the ARPES technique is capable of probing the unoccupied electronic states governed by the secondary electron emission process, along with the occupied bands of solids.
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| 4. |
- Kuzmin, M., et al.
(författare)
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Atomic structure of Yb/Si(100)(2X6) : Interrelation between the silicon dimer arrangement and Si 2p photoemission line shape
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:11, s. 113302-
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Tidskriftsartikel (refereegranskat)abstract
- Combining photoelectron spectroscopy and density-functional theory calculations, we have studied the atomic geometry of Yb/Si(100)(2 X 6) reconstruction and the mechanisms responsible for its stabilization as well as the influence of this reconstruction on Si 2p core-level photoemission. The analysis of measured and calculated surface core-level shifts supports the recently proposed model of the Yb/Si(100)(2 X 6). It involves, in agreement with valence-band measurements, unbuckled (symmetrical) silicon dimers, leading to unusually narrowed Si 2p line shape as compared to those of related systems. The origin of the symmetrical dimers in the (2 X 6) structure is discussed in the context of previous results in literature.
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| 5. |
- Kuzmin, M., et al.
(författare)
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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
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:24, s. 245322-
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Tidskriftsartikel (refereegranskat)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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| 6. |
- Kuzmin, M., et al.
(författare)
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Surface core-level shifts on Ge(111)c(2 x 8) : Experiment and theory
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:24, s. 245319-
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Tidskriftsartikel (refereegranskat)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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| 7. |
- Laverock, J., et al.
(författare)
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k-resolved susceptibility function of 2H-TaSe2 from angle-resolved photoemission
- 2013
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 88:3
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Tidskriftsartikel (refereegranskat)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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| 8. |
- Laverock, J., et al.
(författare)
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Photoemission evidence for crossover from Peierls-like to Mott-like transition in highly strained VO2
- 2012
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 86:19
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Tidskriftsartikel (refereegranskat)abstract
- We present a spectroscopic study that reveals that the metal-insulator transition of strained VO2 thin films may be driven towards a purely electronic transition, which does not rely on the Peierls dimerization, by the application of mechanical strain. Comparison with a moderately strained system, which does involve the lattice, demonstrates the crossover from Peierls- to Mott-like transitions.
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| 9. |
- Mazzola, Federico, et al.
(författare)
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Kinks in the σ Band of Graphene Induced by Electron-Phonon Coupling
- 2013
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 111:21, s. 216806-
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Tidskriftsartikel (refereegranskat)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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| 10. |
- Ndiaye, W., et al.
(författare)
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Bulk electronic structure of Mn5Ge3/Ge(111) films by angle-resolved photoemission spectroscopy
- 2013
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 87:16
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Tidskriftsartikel (refereegranskat)abstract
- Mn5Ge3(001) thin films grown on Ge(111)-c(2 x 8) reconstructed surfaces were studied by angle-resolved photoemission using synchrotron radiation in the 14-94 eV photon energy range. The results obtained in the Gamma ALM plane and in the Gamma AHK plane are in agreement with simulations starting with band structure calculations based on the density functional theory. This provides a unique validation of band structure calculations for a proper description of the electronic properties of Mn5Ge3. Only the spectral feature very close to the Fermi level cannot be well explained by the simulation. This departure is discussed in terms of the three-dimensional nature of the sample and of correlation effects.
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