23 research outputs found
Partial charge transfer and absence of induced magnetization in EuS(111)/Bi2Se3 heterostructures
Heterostructures made from topological and magnetic insulators promise to form excellent platforms for new electronic and spintronic functionalities mediated by interfacial effects. We report the results of a first-principles density functional theory study of the geometric, electronic structure, and magnetic properties of the EuS(111)/Bi2Se3 interface, including van der Waals and relativistic spin-orbit effects. In contrast to previous theoretical studies, we find no appreciable magnetic anisotropy in such a heterostructure. We also do not see additional induced magnetization at the interface or the magnetic proximity effect on the topological states. This is due to the localized nature of Eu moments and because of a partial charge transfer of âŒ0.5 electron from Eu to Se. The formation of the surface dipole shifts the Dirac cone about 0.4 eV below the chemical potential, and the associated electrostatic screening moves the topological state from the first to the second quintuple layer of Bi2Se3
Observation of sixfold degenerate fermions in PdSb
Three types of fermions have been extensively studied in topological quantum
materials: Dirac, Weyl, and Majorana fermions. Beyond the fundamental fermions
in high energy physics, exotic fermions are allowed in condensed matter systems
residing in three-, six- or eightfold degenerate band crossings. Here, we use
angle-resolved photoemission spectroscopy to directly visualize
three-doubly-degenerate bands in PdSb. The ultrahigh energy resolution we
are able to achieve allows for the confirmation of all the sixfold degenerate
bands at the R point, in remarkable consistency with first-principles
calculations. Moreover, we find that this sixfold degenerate crossing has
quadratic dispersion as predicted by theory. Finally, we compare sixfold
degenerate fermions with previously confirmed fermions to demonstrate the
importance of this work: our study indicates a topological fermion beyond the
constraints of high energy physics
Charged iodide in chains behind the highly efficient iodine doping in carbon nanotubes
The origin of highly efficient iodine doping of carbon nanotubes is not well understood. Relying on firstprinciples calculations, we found that iodine molecules (I2) in contact with a carbon nanotube interact to form monoiodide or/and polyiodide from two and three I2 as a result of removing electrons from the carbon nanotube (p-type doping). Charge per iodine atom for monoiodide ion or iodine atom at end of iodine chain is significantly higher than that for I2. This atomic analysis extends previous studies showing that polyiodide ions are the dominant dopants. Moreover, we observed isolated I atoms in atomically resolved transmission electron microscopy, which proves the production of monoiodide. Finally, using Raman spectroscopy, we quantitatively determined the doping level and estimated the number of conducting channels in high electrical conductivity fibers composed of iodine-doped double-wall carbon nanotubes
Dendara métropole
Le chantier « Dendara mĂ©tropole » vise Ă Ă©tudier les divers aspects du temple dâHathor dans son environnement, en portant les investigations sur lâĂ©tude architecturale des monuments ainsi que sur lâexploration archĂ©ologique des quartiers dâhabitations et des cimetiĂšres. Outre la poursuite des travaux sur lâarchitecture monumentale, sur les secteurs associĂ©s aux fondations de Montouhotep II et sur la nĂ©cropole de lâAncien Empire, la campagne 2019 a ouvert de nouvelles perspectives de recherche..
Analyse des améliorations des propriétés électroniques des matériaux carbonés par interaction d'espÚces chimiques : Approche numérique combinée à la spectroscopie Raman
To analyze the improvements in electronic properties of carbon-based materials, an approach based on the density functional theory supported by Raman spectroscopy was used. The heart of this work is the study of doping in order to open up new paths for the design of innovative materials from nanodevices. These new structures are fibers whose the main component is a carbon nanotube or nanocarbon loaded polymers with molecules, optimizing electrical conduction. A brief introduction is presented on non-covalent species, leading to the best results reported in the literature, namely potassium, iodine and super acids. The graphite intercalation compounds by potassium atoms are analyzed first. The large charge transfer of the alkali directly influences the optical properties of graphene, resulting in a unique Raman signature with a shape change when the excitation energy is twice the shift of the Fermi level due to doping. Then, an exhaustive theoretical study of iodine doping is performed on a monolayer of graphene. Analysis of thermodynamic properties shows that a gradual increase in the recovery rate of the molecules, initially generates a phase transition of iodine adsorption mode, and ends with the formation of polyiodide complexes. These complexes, via a strong electron transfer, lead to the increase of the density of states at the Fermi level. This study is extended to carbon nanotubes, where a very large charge transfer is obtained after interacting either with chlorosulfonic acid molecules by redox reaction, or with iodine molecules. When there is a flow of a large electric current in these fibers, the Joule effect produces a desorption of dopants and reduces the electrical conductivity. This phenomenon is explained by the available number of conduction channels deducted from combined Raman signatures and electronic transport calculations. The local and average temperatures are extracted from Raman and transport data respectively. This work constitutes a coherent set of results as a basis for improving the transport properties.Pour analyser les amĂ©liorations des propriĂ©tĂ©s Ă©lectroniques des matĂ©riaux carbonĂ©s, une approche par la thĂ©orie de la fonctionnelle de la densitĂ© appuyĂ©e par la spectroscopie Raman a Ă©tĂ© utilisĂ©e. Le cĆur de ce travail est lâĂ©tude du dopage dans le but dâouvrir de nouvelles voies pour la conception de matĂ©riaux Ă nanocomposants innovants. Ces nouvelles structures sont des fibres dont la brique Ă©lĂ©mentaire est un nanotube de carbone ou des polymĂšres chargĂ©s en nanocarbone avec des molĂ©cules optimisant la conduction Ă©lectrique. Une brĂšve introduction est prĂ©sentĂ©e sur les espĂšces non-covalentes, conduisant aux meilleurs rĂ©sultats reportĂ©s dans la littĂ©rature, Ă savoir : le potassium, lâiode et les super acides. Les composĂ©s dâintercalation du graphite par des atomes de potassium sont analysĂ©s en premier. Le fort transfert de charge de lâalcalin influence directement les propriĂ©tĂ©s optiques du graphĂšne conduisant Ă une signature Raman singuliĂšre avec un changement de forme lorsque lâĂ©nergie dâexcitation est le double du dĂ©placement du niveau de Fermi dĂ» au dopage. Ensuite, une Ă©tude thĂ©orique exhaustive du dopage Ă lâiode est rĂ©alisĂ©e sur une monocouche de graphĂšne. Lâanalyse des propriĂ©tĂ©s thermodynamiques montre quâune augmentation progressive du taux de recouvrement des molĂ©cules engendre dâabord une transition de phase du mode dâadsorption de lâiode et se termine par la formation de complexes polyiodure. Ces complexes, via un fort transfert dâĂ©lectrons, conduisent Ă lâaugmentation de la densitĂ© dâĂ©tats Ă©lectronique au niveau de Fermi. Cette Ă©tude est Ă©tendue aux nanotubes de carbone, oĂč un transfert de charge trĂšs important est obtenu aprĂšs interaction soit avec des molĂ©cules dâacide chlorosulfonique par rĂ©action dâoxydo-rĂ©duction, soit avec des molĂ©cules dâiode. Lors de la circulation dâun fort courant Ă©lectrique dans ces fibres, lâeffet Joule produit une dĂ©sorption des dopants et rĂ©duit la conductivitĂ© Ă©lectrique. Ce phĂ©nomĂšne sâexplique par le nombre de canaux de conduction disponibles dĂ©duit des signatures Raman combinĂ©e Ă des calculs de transport Ă©lectronique. Les tempĂ©ratures locale et moyenne sont extraites des donnĂ©es Raman et de transport respectivement. Ce travail constitue un ensemble cohĂ©rent de rĂ©sultats pouvant servir de base pour amĂ©liorer les propriĂ©tĂ©s de transport
Analysis of the improvements in the electronic properties of carbon materials by interaction with chemical species : Computational approach combined with Raman spectroscopy
Pour analyser les amĂ©liorations des propriĂ©tĂ©s Ă©lectroniques des matĂ©riaux carbonĂ©s, une approche par la thĂ©orie de la fonctionnelle de la densitĂ© appuyĂ©e par la spectroscopie Raman a Ă©tĂ© utilisĂ©e. Le cĆur de ce travail est lâĂ©tude du dopage dans le but dâouvrir de nouvelles voies pour la conception de matĂ©riaux Ă nanocomposants innovants. Ces nouvelles structures sont des fibres dont la brique Ă©lĂ©mentaire est un nanotube de carbone ou des polymĂšres chargĂ©s en nanocarbone avec des molĂ©cules optimisant la conduction Ă©lectrique. Une brĂšve introduction est prĂ©sentĂ©e sur les espĂšces non-covalentes, conduisant aux meilleurs rĂ©sultats reportĂ©s dans la littĂ©rature, Ă savoir : le potassium, lâiode et les super acides. Les composĂ©s dâintercalation du graphite par des atomes de potassium sont analysĂ©s en premier. Le fort transfert de charge de lâalcalin influence directement les propriĂ©tĂ©s optiques du graphĂšne conduisant Ă une signature Raman singuliĂšre avec un changement de forme lorsque lâĂ©nergie dâexcitation est le double du dĂ©placement du niveau de Fermi dĂ» au dopage. Ensuite, une Ă©tude thĂ©orique exhaustive du dopage Ă lâiode est rĂ©alisĂ©e sur une monocouche de graphĂšne. Lâanalyse des propriĂ©tĂ©s thermodynamiques montre quâune augmentation progressive du taux de recouvrement des molĂ©cules engendre dâabord une transition de phase du mode dâadsorption de lâiode et se termine par la formation de complexes polyiodure. Ces complexes, via un fort transfert dâĂ©lectrons, conduisent Ă lâaugmentation de la densitĂ© dâĂ©tats Ă©lectronique au niveau de Fermi. Cette Ă©tude est Ă©tendue aux nanotubes de carbone, oĂč un transfert de charge trĂšs important est obtenu aprĂšs interaction soit avec des molĂ©cules dâacide chlorosulfonique par rĂ©action dâoxydo-rĂ©duction, soit avec des molĂ©cules dâiode. Lors de la circulation dâun fort courant Ă©lectrique dans ces fibres, lâeffet Joule produit une dĂ©sorption des dopants et rĂ©duit la conductivitĂ© Ă©lectrique. Ce phĂ©nomĂšne sâexplique par le nombre de canaux de conduction disponibles dĂ©duit des signatures Raman combinĂ©e Ă des calculs de transport Ă©lectronique. Les tempĂ©ratures locale et moyenne sont extraites des donnĂ©es Raman et de transport respectivement. Ce travail constitue un ensemble cohĂ©rent de rĂ©sultats pouvant servir de base pour amĂ©liorer les propriĂ©tĂ©s de transport.To analyze the improvements in electronic properties of carbon-based materials, an approach based on the density functional theory supported by Raman spectroscopy was used. The heart of this work is the study of doping in order to open up new paths for the design of innovative materials from nanodevices. These new structures are fibers whose the main component is a carbon nanotube or nanocarbon loaded polymers with molecules, optimizing electrical conduction. A brief introduction is presented on non-covalent species, leading to the best results reported in the literature, namely potassium, iodine and super acids. The graphite intercalation compounds by potassium atoms are analyzed first. The large charge transfer of the alkali directly influences the optical properties of graphene, resulting in a unique Raman signature with a shape change when the excitation energy is twice the shift of the Fermi level due to doping. Then, an exhaustive theoretical study of iodine doping is performed on a monolayer of graphene. Analysis of thermodynamic properties shows that a gradual increase in the recovery rate of the molecules, initially generates a phase transition of iodine adsorption mode, and ends with the formation of polyiodide complexes. These complexes, via a strong electron transfer, lead to the increase of the density of states at the Fermi level. This study is extended to carbon nanotubes, where a very large charge transfer is obtained after interacting either with chlorosulfonic acid molecules by redox reaction, or with iodine molecules. When there is a flow of a large electric current in these fibers, the Joule effect produces a desorption of dopants and reduces the electrical conductivity. This phenomenon is explained by the available number of conduction channels deducted from combined Raman signatures and electronic transport calculations. The local and average temperatures are extracted from Raman and transport data respectively. This work constitutes a coherent set of results as a basis for improving the transport properties
Theoretical study of polyiodide formation and stability on monolayer and bilayer graphene
cited By 6International audienceThe presence of polyiodide complexes have been reported several times when carbon-based materials were doped by iodine molecules, but their formation mechanism remains unclear. By using first-principles calculations that include nonlocal correlation effects by means of a van der Waals density functional approach, we propose that the formation of triiodide (I3 -) and pentaiodide (I5 -) is due to a large density of iodine molecules (I2) in interaction with a carbonaceous substrate. As soon as the concentration of surface iodine reaches a threshold value of 12.5% for a graphene monolayer and 6.25% for a bilayer, these complexes spontaneously appear. The corresponding structural and energetic aspects, electronic structures and vibrational frequencies support this statement. An upshift of the Dirac point from the Fermi level with values of 0.45 and 0.52 eV is observed for adsorbed complexes on graphene and intercalated complexes between two layers, respectively. For doped-graphene, it corresponds to a graphene hole density of around 1.1 Ă 1013 cm-2, in quantitative agreement with experiments. Additionally, we have studied the thermal stability at room temperature of these adsorbed ions on graphene by means of ab initio molecular dynamics, which also shows successful p-doping with polyiodide complexes
Theoretical study of graphene doping mechanism by iodine molecules
cited By 10International audienceThe adsorption of iodine atoms and molecules on graphene is studied in detail, using first-principles calculations that include nonlocal correlation effects by means of van der Waals density functional approach. Structural, energetic, and electronic structure properties of these systems are reported. We demonstrate that graphene surface can be doped by atomic and molecular iodine. An upward shift of the Dirac point from the Fermi level with values of 0.45 and 0.08 eV is observed for adsorbed atoms and adsorbed I2, respectively. It corresponds to graphene hole densities to be around 1.2 Ă 1013-3.9 Ă 1011 cm-2. We also show that the iodine molecule does not dissociate in contact with pure graphene monolayer. Calculation of the surface free energy reveals that the orientation of the adsorbed iodine molecules crucially depends on its concentration and the system temperature. The corresponding phase diagram indicates that the in-plan orientation of molecules is more stable when the iodine concentration decreases for temperatures above approximately 200 K; when beyond 500 K, iodine molecules are completely desorbed
Filling mechanisms, composition, structure, and properties of halide-filled DWCNTs
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