37 research outputs found
Interacting electrons in graphene nanoribbons in the lowest Landau level
We study the effect of electron-electron interaction and spin on electronic
and transport properties of gated graphene nanoribbons (GNRs) in a
perpendicular magnetic field in the regime of the lowest Landau level (LL). The
electron-electron interaction is taken into account using the Hartree and
Hubbard approximations, and the conductance of GNRs is calculated on the basis
of the recursive Greens function technique within the Landauer formalism. We
demonstrate that, in comparison to the one-electron picture, electron-electron
interaction leads to the drastic changes in the dispersion relation and
structure of propagating states in the regime of the lowest LL showing a
formation of the compressible strip and opening of additional conductive
channels in the middle of the ribbon. We show that the latter are very
sensitive to disorder and get scattered even if the concentration of disorder
is moderate. In contrast, the edge states transport is very robust and can not
be suppressed even in the presence of a strong spin-flipping.Comment: 6 pages, 3 figure
Plasmons in dimensionally mismatched Coulomb coupled graphene systems
We calculate the plasmon dispersion relation for Coulomb coupled metallic
armchair graphene nanoribbons and doped monolayer graphene. The crossing of the
plasmon curves, which occurs for uncoupled 1D and 2D systems, is split by the
interlayer Coulomb coupling into a lower and an upper plasmon branch. The upper
branch exhibits a highly unusual behavior with endpoints at finite .
Accordingly, the structure factor shows either a single or a double peak
behavior, depending on the plasmon wavelength. The new plasmon structure is
relevant to recent experiments, its properties can be controlled by varying the
system parameters, and be used in plasmonic applications.Comment: 5 pages, 3 figures; in press in Phys. Rev. Let
Conductivity of epitaxial and CVD graphene with correlated line defects
Transport properties of single-layer graphene with correlated one-dimensional
defects are studied using the time-dependent real-space Kubo-Greenwood
formalism. Such defects are present in epitaxial graphene, comprising atomic
terraces and steps due to the substrate morphology, and in polycrystalline
chemically-vapor-deposited (CVD) graphene due to the grain boundaries, composed
of a periodic array of dislocations, or quasi-periodic nanoripples originated
from the metal substrate. The extended line defects are described by the
long-range Lorentzian-type scattering potential. The dc conductivity is
calculated numerically for different cases of distribution of line defects.
This includes a random (uncorrelated) and a correlated distribution with a
prevailing direction in the orientation of lines. The anisotropy of the
conductivity along and across the line defects is revealed, which agrees with
experimental measurements for epitaxial graphene grown on SiC. We performed a
detailed study of the conductivity for different defect correlations,
introducing the correlation angle alpha_max (i.e. the maximum possible angle
between any two lines). We find that for a given electron density, the relative
enhancement of the conductivity for the case of fully correlated line defects
in comparison to the case of uncorrelated ones is larger for a higher defect
density. Finally, we study the conductivity of realistic samples where both
extended line defects as well as point-like scatterers such as adatoms and
charged impurities are presented.Comment: 8 pages, 7 figure
Interactions and screening in gated bilayer graphene nanoribbons
The effects of Coulomb interactions on the electronic properties of bilayer
graphene nanoribbons (BGNs) covered by a gate electrode are studied
theoretically. The electron density distribution and the potential profile are
calculated self-consistently within the Hartree approximation. A comparison to
their single-particle counterparts reveals the effects of interactions and
screening. Due to the finite width of the nanoribbon in combination with
electronic repulsion, the gate-induced electrons tend to accumulate along the
BGN edges where the potential assumes a sharp triangular shape. This has a
profound effect on the energy gap between electron and hole bands, which
depends nonmonotonously on the gate voltage and collapses at intermediate
electric fields. We interpret this behavior in terms of interaction-induced
warping of the energy dispersion.Comment: 6 pages, 4 figure
Influence of correlated impurities on conductivity of graphene sheets: Time-dependent real-space Kubo approach
Exact numerical calculations of the conductivity of graphene sheets with
random and correlated distributions of disorders have been performed using the
time-dependent real-space Kubo formalism. The disorder was modeled by the
long-range Gaussian potential describing screened charged impurities and by the
short-range potential describing neutral adatoms both in the weak and strong
scattering regime. Our central result is that correlation in the spatial
distribution for the strong short-range scatterers and for the long-range
Gaussian potential do not lead to any enhancement of the conductivity in
comparison to the uncorrelated case. Our results strongly indicate that the
temperature enhancement of the conductivity reported in the recent study (Yan
and Fuhrer, Phys. Rev. Lett. 107, 206601 (2011)) and attributed to the effect
of dopant correlations was most likely caused by other factors not related to
the correlations in the scattering potential.Comment: 14 pages, 10 figure
Plasmon-mediated Coulomb drag between graphene waveguides
We analyze theoretically charge transport in Coulomb coupled graphene
waveguides (GWGs). The GWGs are defined using antidot lattices, and the lateral
geometry bypasses many technological challenges of earlier designs. The drag
resistivity , which is a measure of the many-particle interactions
between the GWGs, is computed for a range of temperatures and waveguide
separations. It is demonstrated that for the drag is significantly
enhanced due to plasmons, and that in the low-temperature regime a complicated
behavior may occur. In the weak coupling regime the dependence of drag on the
interwaveguide separation follows , where .Comment: 6 pages, 4 figure
Electron polarization function and plasmons in metallic armchair graphene nanoribbons
We calculate the polarization function of Dirac fermions in metallic armchair
graphene nanoribbons for an arbitrary temperature and doping. We find that at
finite temperatures due to the phase space redistribution among inter-band and
intra-band electronic transitions in the conduction and valence bands, the full
polarization function becomes independent of the temperature and the position
of the chemical potential. As a result, for a given width of nanoribbons there
exists a single plasmon mode, with the energy dispersion determined by the
graphene's fine structure constant. In Coulomb-coupled nanoribbons, this
plasmon splits into the basic in-phase and out-of-phase plasmon modes, with the
splitting energy determined additionally by the inter-ribbon spacing.Comment: 7 pages, 4 figures; in press in Phys. Rev.
Generic suppression of conductance quantization of interacting electrons in graphene nanoribbons in a perpendicular magnetic field
The effects of electron interaction on the magnetoconductance of graphene
nanoribbons (GNRs) are studied within the Hartree approximation. We find that a
perpendicular magnetic field leads to a suppression instead of an expected
improvement of the quantization. This suppression is traced back to
interaction-induced modifications of the band structure leading to the
formation of compressible strips in the middle of GNRs. It is also shown that
the hard wall confinement combined with electron interaction generates overlaps
between forward and backward propagating states, which may significantly
enhance backscattering in realistic GNRs. The relation to available experiments
is discussed.Comment: 4 pages, 3 figure