44 research outputs found
Stable two-dimensional dumbbell stanene: A quantum spin Hall insulator
Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al.From density functional theory within the generalized gradient approximation we predict a structure of stanene with dumbbell units (DBs), and show that it is a two-dimensional topological insulator with an inverted band gap which can be tuned by compressive strain. Furthermore, we propose that the boron nitride sheet and reconstructed (2x2) InSb(111) surfaces are ideal substrates for the experimental realization of DB stanene, maintaining its nontrivial topology. Combined with standard semiconductor technologies, such as magnetic doping and electrical gating, the quantum anomalous Hall effect, Chern half metallicity, and topological superconductivity can be realized in DB stanene on those substrates. These properties make the two-dimensional supported stanene a good platform for the study of quantum spin Hall insulators as well as other exotic quantum states of matter.We acknowledge support from the Ministry of Science and Technology of China (Grants No. 2011CB606405 and No. 2011CB921901) and the National Natural Science Foundation of China (Grant No. 11334006). A.R., S.C., and L.X. acknowledge financial support from the European Research Council Grant DYNamo (ERC-2010-AdG No. 267374) Spanish Grants (FIS2010-21282-C02-01), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13), and EC project CRONOS (280879-2 CRONOS CP-FP7.Peer Reviewe
Germanene: a novel two-dimensional germanium allotrope akin to graphene and silicene
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.We have grown an atom-thin, ordered, two-dimensional multi-phase film in situ through germanium molecular beam epitaxy using a gold (111) surface as a substrate. Its growth is similar to the formation of silicene layers on silver (111) templates. One of the phases, forming large domains, as observed in scanning tunneling microscopy, shows a clear, nearly flat, honeycomb structure. Thanks to thorough synchrotron radiation core-level spectroscopy measurements and advanced density functional theory calculations we can identify it as a √3 × √3 R(30°) germanene layer in conjunction with a √7 × √7 R(19.1°) Au(111) supercell, presenting compelling evidence of the synthesis of the germanium-based cousin of graphene on gold.Funding from the 2D-NANOLATTICES project within the 7th Framework Programme for Research of the European Commission, under FET-Open grant number 270749 is greatly appreciated. We acknowledge support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG-267374), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-578-13) and European Commission project CRONOS (grant number 280879-2).Peer Reviewe
Engineering Three-Dimensional Moire Flat Bands
Twisting two adjacent layers of van der Waals materials with respect to each other can lead to flat two-dimensional electronic bands which enables a wealth of physical phenomena. Here, we generalize this concept of so-called moire flat bands to engineer flat bands in all three spatial dimensions controlled by the twist angle. The basic concept is to stack the material such that the large spatial moire interference patterns are spatially shifted from one twisted layer to the next. We exemplify the general concept by considering graphitic systems, boron nitride, and WSe2, but the approach is applicable to any two-dimensional van der Waals material. For hexagonal boron nitride, we develop an ab initio fitted tight binding model that captures the corresponding three-dimensional low-energy electronic structure. We outline that interesting three-dimensional correlated phases of matter can be induced and controlled following this route, including quantum magnets and unconventional superconducting states.This work is supported by the European Research Council (ERC-2015-AdG-694097), Grupos Consolidados (IT124919), and SFB925. A.R. is supported by the Flatiron Institute, a division of the Simons Foundation. We acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under RTG 1995 within the Priority Program SPP 2244 2DMP under Germany's Excellence Strategy -Cluster of Excellence and Advanced Imaging of Matter (AIM) EXC 2056-390715994 and RTG 2247. L.X. acknowledges the support from Distinguished Junior Fellowship program by the South Bay Interdisciplinary Science Center in the Songshan Lake Materials Laboratory. J.Z. acknowledges funding received from the European Union Horizon 2020 research and innovation program under Marie Sklodowska-Curie Grant Agreement 886291 (PeSD-NeSL)
Instantaneous band gap collapse in photoexcited monoclinic VO due to photocarrier doping
Using femtosecond time-resolved photoelectron spectroscopy we demonstrate
that photoexcitation transforms monoclinic VO quasi-instantaneously into a
metal. Thereby, we exclude an 80 femtosecond structural bottleneck for the
photoinduced electronic phase transition of VO. First-principles many-body
perturbation theory calculations reveal a high sensitivity of the VO
bandgap to variations of the dynamically screened Coulomb interaction,
supporting a fully electronically driven isostructral insulator-to-metal
transition. We thus conclude that the ultrafast band structure renormalization
is caused by photoexcitation of carriers from localized V 3d valence states,
strongly changing the screening \emph{before} significant hot-carrier
relaxation or ionic motion has occurred
The atomic structure of the phase of silicene on Ag(111)
The growth of the reconstructed silicene on Ag
substrate has been frequently observed in experiments while its atomic
structure and formation mechanism is poorly understood. Here by
first-principles calculations we show that
reconstructed silicene is constituted by dumbbell units of Si atoms arranged in
a honeycomb pattern. Our model shows excellent agreement with the
experimentally reported lattice constant and STM image. We propose a new
mechanism for explaining the spontaneous and consequential formation of
structures from structures on Ag
substrate. We show that the reconstruction is mainly
determined by the interaction between Si atoms and have weak influence from Ag
substrate. The proposed mechanism opens the path to understanding of multilayer
silicon
Link between interlayer hybridization and ultrafast charge transfer in WS2-graphene heterostructures
Ultrafast charge separation after photoexcitation is a common phenomenon in various van-der-Waals (vdW) heterostructures with great relevance for future applications in light harvesting and detection. Theoretical understanding of this phenomenon converges towards a coherent mechanism through charge transfer states accompanied by energy dissipation into strongly coupled phonons. The detailed microscopic pathways are material specific as they sensitively depend on the band structures of the individual layers, the relative band alignment in the heterostructure, the twist angle between the layers, and interlayer interactions resulting in hybridization. We used time- and angle-resolved photoemission spectroscopy combined with tight binding and density functional theory electronic structure calculations to investigate ultrafast charge separation and recombination in WS2-graphene vdW heterostructures. We identify several avoided crossings in the band structure and discuss their relevance for ultrafast charge transfer. We relate our own observations to existing theoretical models and propose a unified picture for ultrafast charge transfer in vdW heterostructures where band alignment and twist angle emerge as the most important control parameters