1,668 research outputs found
Unusual Coulomb excitations in ABC-stacked trilayer graphene
The layer-based random-phase approximation is further developed to
investigate electronic excitations in tri-layer ABC-stacked graphene. All the
layer-dependent atomic interactions and Coulomb interactions are included in
the dynamic charge screening. There exist rich and unique (momentum,
frequency)-excitation phase diagrams, in which the complex single-particle
excitations and five kinds of plasmon modes, are dominated by the unusual
energy bands and doping carrier densities. The latter frequently experience the
significant Landau damping due to the former, leading to the
coexistence/destruction in the energy loss spectra. Specifically, the
dispersion of the only acoustic plasmon in pristine case is dramatically
changed from linear into quadratic even at very low doping.Comment: 17 pages and 4 figure
Feature-Rich Electronic Properties in Graphene Ripples
Graphene ripples possess peculiar essential properties owing to the strong
chemical bonds, as an investigation using first principle calculations clearly
revealed. Various charge distributions, bond lengths, energy bands, and
densities of states strongly depend on the corrugation structures, ripple
curvatures and periods. Armchair ripples belonging to a zero-gap semiconductor
display split middle-energy states, while the zigzag ripples exhibit highly
anisotropic energy bands, semi-metallic behavior implicated by the destruction
of the Dirac cone, and the newly created critical points. Their density of
states exhibit many low-lying prominent peaks and can explain the experimental
measurements. There exist certain important similarities and differences
between graphene ripples and carbon nanotubes.Comment: 16 pages, 7 figure
Combined Effect of Stacking and Magnetic Field on Plasmon Excitations in Bilayer Graphene
The electronic excitations of bilayer graphene (BLG) under a magnetic field
are investigated with the use of the Peierls tight-binding model in conjunction
with random-phase approximation (RPA). The interlayer atomic interactions,
interlayer Coulomb interactions, and magnetic field effects are simultaneously
included in the dielectric-function matrix. That enables us to derive the
magneto-Coulomb-excitation spectrum of different stacking structures. The two
typical arrangements of BLGs, AB and AA, are considered in this article. AB-BLG
exhibits many discrete energy-loss peaks, which correspond to the quantization
of electron energies. On the other hand, the AA-BLG spectra possess a unique
and pronounced peak at low frequency. This peak represents the collective
excitation of the entire low-frequency Landau states. The dependence of the
energy-loss peaks on the momentum transfer and the magnetic field strength is
presented. Accordingly, two kinds of plasmon modes produced by the layer
stacking are clearly distinguished
Configuration- and concentration-dependent electronic properties of hydrogenated graphene
The electronic properties of hydrogenated graphenes are investigated with the
first-principles calculations. Geometric structures, energy bands, charge
distributions, and density of states (DOS) strongly depend on the different
configurations and concentrations of hydrogen adatoms. Among three types of
optimized periodical configurations, only in the zigzag systems the band gaps
can be remarkably modulated by H-concentrations. There exist middle-gap
semiconductors, narrow-gap semiconductors, and gapless systems. The band
structures exhibit the rich features, including the destruction or recovery of
the Dirac-cone structure, newly formed critical points, weakly dispersive
bands, and (C,H)-related partially flat bands. The orbital-projected DOS are
evidenced by the low-energy prominent peaks, delta-function-like peaks,
discontinuous shoulders, and logarithmically divergent peaks. The DOS and
spatial charge distributions clearly indicate that the critical bondings in C-C
and C-H is responsible for the diversified properties
Geometry-diversified Coulomb excitations in trilayer AAB stacking graphene
The lower-symmetry trilayer AAB-stacked graphene exhibits rich electronic
properties and thus diverse Coulomb excitations. Three pairs of unusual valence
and conduction bands create nine available interband excitations for the
undoped case, in which the imaginary (real) part of the polarizability shows 1D
square root asymmetric peaks and 2D shoulder structures (pairs of antisymmetric
peaks and logarithm type symmetric peaks). Moreover, the low frequency acoustic
plasmon, being revealed as a prominent peak in the energy loss spectrum, can
survive in a narrow gap system with the large-density-of-states from the
valence band. This type of plasmon mode is similar to that in a narrow gap
carbon nanotube. However, the decisive mechanism governing this plasmon is the
intraband conduction state excitations. Its frequency, intensity and critical
momentum exhibit a non-monotonic dependence on the Fermi energy. The
well-defined electron-hole excitation boundaries and the higher frequency
optical plasmons are transformed by varying the Fermi energy. There remain
substantial differences between the electronic properties of trilayer AAB, ABC,
AAA and ABA graphene stackings.Comment: 20 pages, 8 figures. arXiv admin note: text overlap with
arXiv:1601.00223 by other author
Chemical Bondings Induced Rich Electronic Properties of Oxygen Absorbed Few-layer Graphenes
Electronic properties of graphene oxides enriched by the strong chemical
bondings are investigated using first-principle calculations. They are very
sensitive to the changes in the number of graphene layer, stacking
configuration, and distribution of oxygen. The feature-rich electronic
structures exhibit the destruction or distortion of Dirac cone, opening of band
gap, anisotropic energy dispersions, O- and (C,O)-dominated energy dispersions,
and extra critical points. All the few-layer graphene oxides are semi-metals
except for the semiconducting monolayer ones. For the former, the distorted
Dirac-cone structures and the O-dominated energy bands near the Fermi level are
revealed simultaneously. The orbital-projected density of states (DOS) have
many special structures mainly coming from a composite energy band, the
parabolic and partially flat ones. The DOS and spatial charge distributions
clearly indicate the critical bondings in O-O, C-O and C-C bonds, being
responsible for the diversified properties
The effect of perpendicular electric field on Temperature-induced plasmon excitations for intrinsic silicene
We use the tight-binding model and the random-phase approximation to
investigate the intrinsic plasmon in silicene. At finite temperatures, an
undamped plasmon is generated from the interplay between the intraband and the
interband-gap transitions. The extent of the plasmon existence range in terms
of momentum and temperature, which is dependent on the size of
single-particle-excitation gap, is further tuned by applying a perpendicular
electric field. The plasmon becomes damped in the interband-excitation region.
A low damped zone is created by the field-induced spin split. The
field-dependent plasmon spectrum shows a strong tunability in plasmon intensity
and spectral bandwidth. This could make silicene a very suitable candidate for
plasmonic applications
Chiral symmetry classes and Dirac nodal lines in three-dimensional layered systems
We study the existence and stability of Dirac nodal lines in
three-dimensional layered systems, whose layers individually have Dirac nodal
points protected by chiral (sublattice) symmetry. The model system we consider
is the rhombohedral stack of graphene layers with each layer subjected to a
uniform external potential that respects either AIII or BDI classes. From the
Hamiltonians in either classes, a pair of nontrivial spiraling Dirac nodal
lines can be derived. The results are reasonable in accord to the topological
classification of gapless phases for codimension . The nodal lines approach
each other as the magnitude of the potential increases, revealing their
annihilation due to the fact that regarding the full system their topological
invariants are cancelled out.Comment: 5 pages and 2 figure
Novel Magnetic Quantization of Bismuthene
The generalized tight-binding model, being based on the spin-dependent
sublattices, is developed to explore the magnetic quantization of monolayer
bismuthene. The sp orbital hybridizations, site energies, nearest and
next-nearest hopping integrals, spin-orbital interactions and magnetic field
( ) are taken into account simultaneously. There exist
three groups of low-lying Landau levels (LLs), in which they are mainly from
the (6p,6p,6p) orbitals, and only the first group belongs to
the unoccupied conduction states. Furthermore, each group is further split into
the spin-up- and spin-down-dominated subgroups. The six subgroups present the
rich and unique -dependent LL energy spectra, covering the specific or
arc-shaped -dependences, the normal/irregular spin-split energies, and
the non-crossing/crossing/anti-crossing behaviors. Specially, the second group
of valence LLs near the Fermi level can create the frequent inter-subgroup LL
anti-crossings since the main and side modes are comparable. The main features
of energy spectra can create the special structures in density of states.Comment: 13 pages, 7 figure
Electronic and optical properties of graphite-related systems
A systematic review is made for the AA-, AB- and ABC-stacked graphites. The
generalized tight-binding model, accompanied with the effective-mass
approximation and the Kubo formula, is developed to investigate electronic and
optical properties in the presence/absence of a uniform magnetic field. The
unusual electronic properties cover the stacking-dependent Dirac-cone
structures, the significant energy widths along the stacking direction, the
Landau subbands (LSs) crossing the Fermi level, the -dependent LS energy
spectra with crossings and anti-crossings, and the monolayer- or bilayer-like
Landau wavefunctions. There exist the configuration-created special structures
in density of states and optical spectra. Three kinds of graphites quite differ
from one another in the available inter-LS excitation channels, including the
number, frequency, intensity and structures of absorption peaks. The
dimensional crossover presents the main similarities and differences between
graphites and graphenes; furthermore, the quantum confinement enriches the
magnetic quantization phenomena in carbon nanotubes and graphene nanoribbons.
The cooperative/competitive relations among the interlayer atomic interactions,
dimensions and magnetic quantization are responsible for the diversified
essential properties. Part of theoretical predictions are consistent with the
experimental measurements.Comment: 124 pages, 49 figure
- …