31 research outputs found
Unimpeded tunneling in graphene nanoribbons
We studied the Klein paradox in zigzag (ZNR) and anti-zigzag (AZNR) graphene
nanoribbons. Due to the fact that ZNR (the number of lattice sites across the
nanoribbon (N is even) and AZNR (N is odd) configurations are indistinguishable
when treated by the Dirac equation, we supplemented the model with a
pseudo-parity operator whose eigenvalues correctly depend on the sublattice
wavefunctions for the number of carbon atoms across the ribbon, in agreement
with the tight-binding model. We have shown that the Klein tunneling in zigzag
nanoribbons is related to conservation of the pseudo-parity rather than
pseudo-spin in infinite graphene. The perfect transmission in the case of
head-on incidence is replaced by perfect transmission at the center of the
ribbon and the chirality is interpreted as the projection of the pseudo-parity
on momentum at different corners of the Brillouin zone
Field enhanced electron mobility by nonlinear phonon scattering of Dirac electrons in semiconducting graphene nanoribbons
The calculated electron mobility for a graphene nanoribbon as a function of
applied electric field has been found to have a large threshold field for
entering a nonlinear transport regime. This field depends on the lattice
temperature, electron density, impurity scattering strength, nanoribbon width
and correlation length for the line-edge roughness. An enhanced electron
mobility beyond this threshold has been observed, which is related to the
initially-heated electrons in high energy states with a larger group velocity.
However, this mobility enhancement quickly reaches a maximum due to the Fermi
velocity in graphene and the dramatically increased phonon scattering.
Super-linear and sub-linear temperature dependence of mobility seen in the
linear and nonlinear transport regimes. By analyzing the calculated
non-equilibrium electron distribution function, this difference is attributed
separately to the results of sweeping electrons from the right Fermi edge to
the left one through the elastic scattering and moving electrons from
low-energy states to high-energy ones through field-induced electron heating.
The threshold field is pushed up by a decreased correlation length in the high
field regime, and is further accompanied by a reduced magnitude in the mobility
enhancement. This implies an anomalous high-field increase of the line-edge
roughness scattering with decreasing correlation length due to the occupation
of high-energy states by field-induced electron heating.Comment: 20 pages and 6 figure
Photonic band mixing in linear chains of optically coupled micro-spheres
The paper deals with optical excitations arising in a one-dimensional chain
of identical spheres due optical coupling of whispering gallery modes (WGM).
The band structure of these excitations depends significantly on the
inter-mixing between WGMs characterized by different values of angular quantum
number, . We develop a general theory of the photonic band structure of
these excitations taking these effects into account and applied it to several
cases of recent experimental interest. In the case of bands originating from
WQMs with the angular quantum number of the same parity, the calculated
dispersion laws are in good qualitative agreement with recent experiment
results. Bands resulting from hybridization of excitations resulting from
whispering gallery modes with different parity of exhibits anomalous
dispersion properties characterized by a gap in the allowed values of
\emph{wave numbers} and divergence of group velocity.Comment: RevTex, 28 pages, 7 Figure
Plasmons in single- and double-component helical liquids: Application to two-dimensional topological insulators
The plasmon excitations in proposed single- and double-component helical
liquid (HL) models are investigated within the random-phase approximation, by
calculating the density-density, spin-density and spin-spin waves. The effect
due to broken time-reversal symmetry on intraband-plasmon dispersion relation
in the single-component HL system is analyzed and compared to those of
well-known cases, such as conventional quasi-one-dimensional electron gases and
armchair graphene nanoribbons. The equivalence between the density-density wave
in the single-component HL to the coupled spin-density and density-density
waves in the double-component HL is shown here and explained, in addition to
the difference between intraband and interband-plasmon excitations in these two
systems. Since the two-component HL can physically be thought of as a Kramers
pair in two-dimensional topological insulators, our proposed single-component
HL model with broken time-reversal symmetry, which is an artificial construct,
can be viewed as an "effective" model in this sense and its prediction may be
verified in realistic systems in future experiments
Spectroscopic Characterization of Gapped Graphene in the Presence of Circularly Polarized Light
We present a description of the energy loss of a charged particle moving
parallel to a graphene layer and graphene double layers. Specifically, we
compare the stopping power of the plasma oscillations for these two
configurations in the absence as well as the presence of circularly polarized
light whose frequency and intensity can be varied to yield an energy gap of
several hundred between the valence and conduction bands. The
dressed states of the Dirac electrons by the photons yield collective plasma
excitations whose characteristics are qualitatively and quantitatively
different from those produced by Dirac fermions in gapless graphene, due in
part to the finite effective mass of the dressed electrons. For example, the
range of wave numbers for undamped self-sustaining plasmons is increased as the
gap is increased, thereby increasing the stopping power of graphene for some
range of charged particle velocity when graphene is radiated by circularly
polarized light