| 1. |
- Bahramy, M. S., et al.
(författare)
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Ubiquitous formation of bulk Dirac cones and topological surface states from a single orbital manifold in transition-metal dichalcogenides
- 2018
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Ingår i: Nature Materials. - 1476-1122. ; 17:1, s. 21-27
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Tidskriftsartikel (refereegranskat)abstract
- Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied bulk properties, while their single-layer variants have become one of the most prominent examples of two-dimensional materials beyond graphene. Their disparate ground states largely depend on transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date. Here, we focus on the chalcogen-derived states. From density-functional theory calculations together with spin- and angle-resolved photoemission, we find that these generically host a co-existence of type-I and type-II three-dimensional bulk Dirac fermions as well as ladders of topological surface states and surface resonances. We demonstrate how these naturally arise within a single p-orbital manifold as a general consequence of a trigonal crystal field, and as such can be expected across a large number of compounds. Already, we demonstrate their existence in six separate TMDs, opening routes to tune, and ultimately exploit, their topological physics.
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| 2. |
- Reed, B. P., et al.
(författare)
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Diamond (111) surface reconstruction and epitaxial graphene interface
- 2022
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Ingår i: Physical Review B. - 2469-9950. ; 105:20
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Tidskriftsartikel (refereegranskat)abstract
- The evolution of the diamond (111) surface as it undergoes reconstruction and subsequent graphene formation is investigated with angle-resolved photoemission spectroscopy, low energy electron diffraction, and complementary density functional theory calculations. The process is examined starting at the C(111)-(2×1) surface reconstruction that occurs following detachment of the surface adatoms at 920 ∘C, and continues through to the liberation of the reconstructed surface atoms into a freestanding monolayer of epitaxial graphene at temperatures above 1000 ∘C. Our results show that the C(111)-(2×1) surface is metallic as it has electronic states that intersect the Fermi level. This is in strong agreement with a symmetrically π-bonded chain model and should contribute to resolving the controversies that exist in the literature surrounding the electronic nature of this surface. The graphene formed at higher temperatures exists above a newly formed C(111)-(2×1) surface and appears to have little substrate interaction as the Dirac point is observed at the Fermi level. Finally, we demonstrate that it is possible to hydrogen-terminate the underlying diamond surface by means of plasma processing without removing the graphene layer, forming a graphene-semiconductor interface. This could have particular relevance for doping the graphene formed on the diamond (111) surface via tuneable substrate interactions as a result of changing the terminating species at the diamond-graphene interface by plasma processing.
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| 3. |
- Thiagarajan, Balasubramanian, et al.
(författare)
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Spin-valley locking in the normal state of a transition-metal dichacogenide superconductor
- 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
- Metallic transition-metal dichalcogenides (TMDCs) are benchmark systems for studying and controlling intertwined electronic orders in solids, with superconductivity developing from a charge-density wave state. The interplay between such phases is thought to play a critical role in the unconventional superconductivity of cuprates, Fe-based and heavy-fermion systems, yet even for the more moderately-correlated TMDCs, their nature and origins have proved controversial. Here, we study a prototypical example, 2H-NbSe2, by spin- and angle-resolved photoemission and first-principles theory. We find that the normal state, from which its hallmark collective phases emerge, is characterized by quasiparticles whose spin is locked to their valley pseudospin. This results from a combination of strong spin–orbit interactions and local inversion symmetry breaking, while interlayer coupling further drives a rich three-dimensional momentum dependence of the underlying Fermi-surface spin texture. These findings necessitate a re-investigation of the nature of charge order and superconducting pairing in NbSe2 and related TMDCs.
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| 4. |
- Cooil, S. P., et al.
(författare)
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Iron-mediated growth of epitaxial graphene on SiC and diamond
- 2012
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223. ; 50:14, s. 5099-5105
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Tidskriftsartikel (refereegranskat)abstract
- Ordered graphene films have been fabricated on Fe-treated SiC and diamond surfaces using the catalytic conversion of sp(3) to sp(2) carbon. In comparison with the bare SiC(0 0 0 1) surface, the graphitization temperature is reduced from over 1000 degrees C to 600 degrees C and for diamond (111), this new approach enables epitaxial graphene to be grown on this surface for the first time. For both substrates, a key development is the in situ monitoring of the entire fabrication process using real-time electron spectroscopy that provides the necessary precision for the production of films of controlled thickness. The quality of the graphene/graphite layers has been verified using angle-resolved photoelectron spectroscopy, scanning tunneling microscopy and low energy electron diffraction. Graphene is only formed on treated regions of the surface and so this offers a method for fabricating and patterning graphene structures on SiC and diamond in the solid-state at industrially realistic temperatures. (c) 2012 Elsevier Ltd. All rights reserved.
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| 5. |
- Rost, Hakon, I, et al.
(författare)
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A Simplified Method for Patterning Graphene on Dielectric Layers
- 2021
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:31, s. 37500-37506
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Tidskriftsartikel (refereegranskat)abstract
- The large-scale formation of patterned, quasi-freestanding graphene structures supported on a dielectric has so far been limited by the need to transfer the graphene onto a suitable substrate and contamination from the associated processing steps. We report mu m scale, few-layer graphene structures formed at moderate temperatures (600-700 degrees C) and supported directly on an interfacial dielectric formed by oxidizing Si layers at the graphene/substrate interface. We show that the thickness of this underlying dielectric support can be tailored further by an additional Si intercalation of the graphene prior to oxidation. This produces quasi-freestanding, patterned graphene on dielectric SiO2 with a tunable thickness on demand, thus facilitating a new pathway to integrated graphene microelectronics.
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| 6. |
- Cooil, S. P., et al.
(författare)
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Controlling the growth of epitaxial graphene on metalized diamond (111) surface
- 2015
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 107:18
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Tidskriftsartikel (refereegranskat)abstract
- The 2-dimensional transformation of the diamond (111) surface to graphene has been demonstrated using ultrathin Fe films that catalytically reduce the reaction temperature needed for the conversion of sp(3) to sp(2) carbon. An epitaxial system is formed, which involves the re-crystallization of carbon at the Fe/vacuum interface and that enables the controlled growth of monolayer and multilayer graphene films. In order to study the initial stages of single and multilayer graphene growth, real time monitoring of the system was preformed within a photoemission and low energy electron microscope. It was found that the initial graphene growth occurred at temperatures as low as 500 degrees C, whilst increasing the temperature to 560 degrees C was required to produce multi-layer graphene of high structural quality. Angle resolved photoelectron spectroscopy was used to study the electronic properties of the grown material, where a graphene-like energy momentum dispersion was observed. The Dirac point for the first layer is located at 2.5 eV below the Fermi level, indicating an n-type doping of the graphene due to substrate interactions, while that of the second graphene layer lies close to the Fermi level. (C) 2015 AIP Publishing LLC.
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