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Register a new userOrigin of the flat band in heavily Cs doped graphene
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A flat energy dispersion of electrons at the Fermi level of a material leads to instabilities in the electronic system and can drive phase transitions. Here we introduce a method to induce a flat band in two-dimensional (2D) materials. We show that the flat band can be achieved by sandwiching the 2D material by two cesium (Cs) layers. We apply this method to monolayer graphene and investigate the flat band by a combination of angle-resolved photoemission spectroscopy experiment and the calculation. Our work highlights that charge transfer, zone folding of graphene bands and the covalent bonding between C and Cs atoms are at the origin of the flat energy band formation. The presented approach is an alternative route for obtaining flat band materials to twisting bilayer graphene which yields thermodynamically stable flat band materials in large areas.
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We present an emph{ab-initio} study of the graphene quasi-particle band structure as function of the doping in G_0 W_0 approximation. We show that the LDA Fermi velocity is substantially renormalized and this renormalization rapidly decreases as function of the doping. We found, in agreement with previous papers, that close to the Dirac point the linear dispersion of the bands is broken but this behaviour disappears with a small doping. We discuss our results in the light of recent experiments on graphene and intercalate graphite.
Coupling of plasmons in graphene at terahert (THz) frequencies with surface plasmons in a heavily-doped substrate is studied theoretically. We reveal that a huge scattering rate may completely damp out the plasmons, so that proper choices of material and geometrical parameters are essential to suppress the coupling effect and to obtain the minimum damping rate in graphene. Even with the doping concentration 10^{19} - 10^{20} cm^{-3} and the thickness of the dielectric layer between graphene and the substrate 100 nm, which are typical values in real graphene samples with a heavily-doped substrate, the increase in the damping rate is not negligible in comparison with the acoustic-phonon-limited damping rate. Dependence of the damping rate on wavenumber, thicknesses of graphene-to-substrate and gate-to-graphene separation, substrate doping concentration, and dielectric constants of surrounding materials are investigated. It is shown that the damping rate can be much reduced by the gate screening, which suppresses the field spread of the graphene plasmons into the substrate.
Magic angle twisted bilayer graphene has emerged as a powerful platform for studying strongly correlated electron physics, owing to its almost dispersionless low-energy bands and the ability to tune the band filling by electrostatic gating. Techniques to control the twist angle between graphene layers have led to rapid experimental progress but improving sample quality is essential for separating the delicate correlated-electron physics from disorder effects. Owing to the 2D nature of the system and the relatively low carrier density, the samples are highly susceptible to small doping inhomogeneity which can drastically modify the local potential landscape. This potential disorder is distinct from the twist-angle variation which has been studied elsewhere. Here, by using low temperature scanning tunneling spectroscopy and planar tunneling junction measurements, we demonstrate that flat bands in twisted bilayer graphene can amplify small doping inhomogeneity that surprisingly leads to carrier confinement, which in graphene could previously only be realized in the presence of a strong magnetic field.
In Raman spectroscopy of graphite and graphene, the $D$ band at $sim 1355$cm$^{-1}$ is used as the indication of the dirtiness of a sample. However, our analysis suggests that the physics behind the $D$ band is closely related to a very clear idea for describing a molecule, namely bonding and antibonding orbitals in graphene. In this paper, we review our recent work on the mechanism for activating the $D$ band at a graphene edge.
While graphene shows a characteristic conical dispersion with a vanishing density of states (DOS) near the Fermi energy E$_F$, it has been suggested that under extremely-high doping ($sim$ 1/4), the extended flat band can be shifted to near E$_F$, resulting in a diverging DOS with strong many-body interactions and electronic instabilities. Although such highly-doped graphene has attracted tremendous research interests, so far the experimental demonstration of doping-induced flat band as well as its associated intriguing phenomena remains rather limited. Here, we report the observation of an extended flat band around the M point in a Li-intercalated graphene, in which the Li ions not only dope graphene with a high electron concentration, but also induce a Kekule order which breaks the chiral symmetry. At such high electron doping, pronounced electron-phonon and electron-electron interactions are clearly identified by the notable kinks in the band dispersion and a strong reduction of the band width. Moreover, by following the evolution of the band structure upon Li intercalation, we find that the flat band and the Kekule order, with the characteristic flat band near M and folded Dirac cones near $Gamma$ respectively, emerge simultaneously, which indicates that they are strongly coupled. Our work identifies Li-intercalated graphene as a fertile platform for investigating the unique physics of the extended flat band, strong many-body interactions as well as the Kekule order.
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