| 1. |
- Bentmann, H., et al.
(författare)
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Strong Linear Dichroism in Spin-Polarized Photoemission from Spin-Orbit-Coupled Surface States
- 2017
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Ingår i: Physical Review Letters. - 0031-9007. ; 119:10
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Tidskriftsartikel (refereegranskat)abstract
- A comprehensive understanding of spin-polarized photoemission is crucial for accessing the electronic structure of spin-orbit coupled materials. Yet, the impact of the final state in the photoemission process on the photoelectron spin has been difficult to assess in these systems. We present experiments for the spin-orbit split states in a Bi-Ag surface alloy showing that the alteration of the final state with energy may cause a complete reversal of the photoelectron spin polarization. We explain the effect on the basis of ab initio one-step photoemission theory and describe how it originates from linear dichroism in the angular distribution of photoelectrons. Our analysis shows that the modulated photoelectron spin polarization reflects the intrinsic spin density of the surface state being sampled differently depending on the final state, and it indicates linear dichroism as a natural probe of spin-orbit coupling at surfaces.
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| 2. |
- Bianchi, M., et al.
(författare)
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One-dimensional spin texture of Bi(441): Quantum spin Hall properties without a topological insulator
- 2015
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 91:16
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Tidskriftsartikel (refereegranskat)abstract
- The high index (441) surface of bismuth has been studied using scanning tunneling microscopy (STM), angle resolved photoemission spectroscopy (APRES), and spin-resolved ARPES. The surface is strongly corrugated, exposing a regular array of (110)-like terraces. Two surface localized states are observed, both of which are linearly dispersing in one in-plane direction (k(x)), and dispersionless in the orthogonal in-plane direction (k(y)), and both of which have a Dirac-like crossing at k(x) = 0. Spin ARPES reveals a strong in-plane polarization, consistent with Rashba-like spin-orbit coupling. One state has a strong out-of-plane spin component, which matches with the miscut angle, suggesting its possible origin as an edge state. The electronic structure of Bi(441) has significant similarities with topological insulator surface states and is expected to support one-dimensional quantum spin Hall-like coupled spin-charge transport properties with inhibited backscattering, without requiring a topological insulator bulk.
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| 3. |
- Hellsing, Bo, 1952, et al.
(författare)
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Phonon-induced linewidths of graphene electronic bands
- 2018
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Ingår i: Physical Review B Condensed Matter. - 0163-1829. ; 98:20
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Tidskriftsartikel (refereegranskat)abstract
- The linewidths of the π and σ bands originating from the electron-phonon coupling in graphene are analyzed based on model calculations and experimental angle-resolved photoemission spectroscopy (ARPES) data. We find evidence for crucial contributions to the lifetime broadening from interband scattering π → σ and σ → π , respectively, driven by the out-of-plane ZA acoustic phonons.
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| 4. |
- Kawasaki, Jason K., et al.
(författare)
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A simple electron counting model for half-Heusler surfaces
- 2018
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- Heusler compounds are a ripe platform for discovery and manipulation of emergent properties in topological and magnetic heterostructures. In these applications, the surfaces and interfaces are critical to performance; however, little is known about the atomic-scale structure of Heusler surfaces and interfaces or why they reconstruct. Using a combination of molecular beam epitaxy, core-level and angle-resolved photoemission, scanning tunneling microscopy, and density functional theory, we map the phase diagram and determine the atomic and electronic structures for several surface reconstructions of CoTiSb (001), a prototypical semiconducting half-Heusler. At low Sb coverage, the surface is characterized by Sb-Sb dimers and Ti vacancies, while, at high Sb coverage, an adlayer of Sb forms. The driving forces for reconstruction are charge neutrality and minimizing the number of Sb dangling bonds, which form metallic surface states within the bulk bandgap. We develop a simple electron counting model that explains the atomic and electronic structure, as benchmarked against experiments and first-principles calculations. We then apply the model to explain previous experimental observations at otherhalf-Heusler surfaces, including the topological semimetal PtLuSb and the half-metallic ferromagnet NiMnSb. The model provides a simple framework for understanding and predicting the surface structure and propertiesof these novel quantum materials.
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| 5. |
- Mazzola, Federico, et al.
(författare)
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Strong electron-phonon coupling in the sigma band of graphene
- 2016
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Ingår i: arXiv. ; , s. 1-11
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- First-principles studies of the electron-phonon coupling in graphene predict a high coupling strength for the ? band with values of the dimensionless electron-phonon coupling constant ? up to 0.9. Near the top of the ? band ? is found to be ≈ 0.7. This value is consistent with the observed kink in the ? band dispersion near the Γ-point in the Brillouin zone [1]. The calculations show that the electron-phonon coupling is driven primarily by the optical LO and TO phonon modes in graphene. The photoemission intensity from the ? band is strongly suppressed near the Γ-point due to sub-lattice interference effects. These effects are removed by taking data in the neighbouring Brillouin zone. By this we have been able to disentangle the influence of sub-lattice interference and electron-phonon coupling. Whilst superconductivity has been absent from graphene's list of exceptional properties, despite considerable experimental attempts, we speculate that if the strong EPC in the ? band survives a modification that shifts it to the Fermi level, e.g. by means of atomic substitution, a superconducting transition temperatures of around 70 K could be reached.
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| 6. |
- Mazzola, Frederico, et al.
(författare)
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Strong electron-phonon coupling in the σ band of graphene : Strong electron-phonon coupling in the sigma band of graphene
- 2017
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 2469-9969. ; 95:7
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Tidskriftsartikel (refereegranskat)abstract
- First-principles studies of the electron-phonon coupling in graphene predict a high coupling strength for the σ band with λ values of up to 0.9. Near the top of the σ band, λ is found to be ≈ 0.7. This value is consistent with the recently observed kinks in the σ band dispersion by angle-resolved photoemission. While the photoemission intensity from the σ band is strongly influenced by matrix elements due to sublattice interference, these effects differ significantly for data taken in the first and neighboring Brillouin zones. This can be exploited to disentangle the influence of matrix elements and electron-phonon coupling. A rigorous analysis of the experimentally determined complex self-energy using Kramers-Kronig transformations further supports the assignment of the observed kinks to strong electron-phonon coupling and yields a coupling constant of 0.6(1), in excellent agreement with the calculations.
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| 7. |
- Ndiaye, W., et al.
(författare)
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k dependence of the spin polarization in Mn5Ge3/Ge(111) thin films
- 2015
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 91:12
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Tidskriftsartikel (refereegranskat)abstract
- Mn5Ge3(001) thin films grown on Ge(111) were studied by angle-and spin-resolved photoemission using synchrotron radiation in the 17-40 eV photon energy range. The photoelectron spectra were simulated starting from a first-principles band-structure calculation for the ground state, using the free-electron approximation for the final states, taking into account photohole lifetime effects and k(perpendicular to) broadening plus correlation effects, but ignoring transition matrix elements. The measured spin polarizations for the various k points investigated in the Gamma MLA plane of the Brillouin zone are found to be in fair enough agreement with the simulated ones, providing a strong support to the ground-state band-structure calculations. Possible origins for the departures between either simulations and experiments or previous and present experiments are discussed.
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| 8. |
- Polley, Craig M., et al.
(författare)
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Fragility of the Dirac Cone Splitting in Topological Crystalline Insulator Heterostructures
- 2018
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Ingår i: ACS Nano. - : AMER CHEMICAL SOC. - 1936-0851 .- 1936-086X. ; 12:1, s. 617-626
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Tidskriftsartikel (refereegranskat)abstract
- The "double Dirac cone" 2D topological interface states found on the (001) faces of topological crystalline insulators such as Pb1-xSnxSe feature degeneracies located away from time reversal invariant momenta and are a manifestation of both mirror symmetry protection and valley interactions. Similar shifted degeneracies in 1D interface states have been highlighted as a potential basis for a topological transistor, but realizing such a device will require a detailed understanding of the intervalley physics involved. In addition, the operation of this or similar devices outside of ultrahigh vacuum will require encapsulation, and the consequences of this for the topological interface state must be understood. Here we address both topics for the case of 2D surface states using angle-resolved photoemission spectroscopy. We examine bulk Pb1-xSnxSe(001) crystals overgrown with PbSe, realizing trivial/topological heterostructures. We demonstrate that the valley interaction that splits the two Dirac cones at each (X) over bar is extremely sensitive to atomic-scale details of the surface, exhibiting non-monotonic changes as PbSe deposition proceeds. This includes an apparent total collapse of the splitting for sub-monolayer coverage, eliminating the Lifshitz transition. For a large overlayer thickness we observe quantized PbSe states, possibly reflecting a symmetry confinement mechanism at the buried topological interface.
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| 9. |
- Polley, C. M., et al.
(författare)
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Observation of surface states on heavily indium-doped SnTe(111), a superconducting topological crystalline insulator
- 2016
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 93:7
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Tidskriftsartikel (refereegranskat)abstract
- The topological crystalline insulator tin telluride is known to host superconductivity when doped with indium (Sn1-xInxTe), and for low indium content (x=0.04) it is known that the topological surface states are preserved. Here we present the growth, characterization, and angle resolved photoemission spectroscopy analysis of samples with much heavier In doping (up to x≈0.4), a regime where the superconducting temperature is increased nearly fourfold. We demonstrate that despite strong p-type doping, Dirac-like surface states persist.
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| 10. |
- Thiagarajan, Balasubramanian, et al.
(författare)
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Observation of a topologically non-trivial surface state in half-Heusler PtLuSb(001) thin films
- 2016
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- The discovery of topological insulators, materials with bulk band gaps and protected cross-gap surface states in compounds such as Bi2Se3, has generated much interest in identifying topological surface states (TSSs) in other classes of materials. In particular, recent theoretical calculations suggest that TSSs may be found in half-Heusler ternary compounds. If experimentally realizable, this would provide a materials platform for entirely new heterostructure spintronic devices that make use of the structurally identical but electronically varied nature of Heusler compounds. Here we show the presence of a TSS in epitaxially grown thin films of the half-Heusler compound PtLuSb. Spin- and angle-resolved photoemission spectroscopy, complemented by theoretical calculations, reveals a surface state with linear dispersion and a helical tangential spin texture consistent with previous predictions. This experimental verification of topological behaviour is a significant step forward in establishing half-Heusler compounds as a viable material system for future spintronic devices.
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