8,031 research outputs found
Energy-Momentum dispersion relation of plasmarons in bilayer graphene
The relation between the energy and momentum of plasmarons in bilayer
graphene is investigated within the Overhauser approach, where the
electron-plasmon interaction is described as a field theoretical problem. We
find that the Dirac-like spectrum is shifted by depending on the electron concentration and
electron momentum. The shift increases with electron concentration as the
energy of plasmons becomes larger. The dispersion of plasmarons is more
pronounced than in the case of single layer graphene, which is explained by the
fact that the energy dispersion of electrons is quadratic and not linear. We
expect that these predictions can be verified using angle-resolved
photoemission spectroscopy (ARPES).Comment: 4 pages, 3 figure
Single-layer and bilayer graphene superlattices: collimation, additional Dirac points and Dirac lines
We review the energy spectrum and transport properties of several types of
one- dimensional superlattices (SLs) on single-layer and bilayer graphene. In
single-layer graphene, for certain SL parameters an electron beam incident on a
SL is highly collimated. On the other hand there are extra Dirac points
generated for other SL parameters. Using rectangular barriers allows us to find
analytic expressions for the location of new Dirac points in the spectrum and
for the renormalization of the electron velocities. The influence of these
extra Dirac points on the conductivity is investigated. In the limit of
{\delta}-function barriers, the transmission T through, conductance G of a
finite number of barriers as well as the energy spectra of SLs are periodic
functions of the dimensionless strength P of the barriers, P{\delta}(x) ~ V
(x). For a Kronig-Penney SL with alternating sign of the height of the barriers
the Dirac point becomes a Dirac line for P = {\pi}/2 + n{\pi} with n an
integer. In bilayer graphene, with an appropriate bias applied to the barriers
and wells, we show that several new types of SLs are produced and two of them
are similar to type I and type II semiconductor SLs. Similar as in single-layer
graphene extra "Dirac" points are found. Non-ballistic transport is also
considered.Comment: 26 pages, 17 figure
Magneto-optical transport properties of monolayer phosphorene
The electronic properties of monolayer phosphorene are exotic due to its
puckered structure and large intrinsic direct band gap. We derive and discuss
its band structure in the presence of a perpendicular magnetic field. Further,
we evaluate the magneto-optical Hall and longitudinal optical conductivities,
as functions of temperature, magnetic field, and Fermi energy, and show that
they are strongly influenced by the magnetic field. The imaginary part of the
former and the real part of the latter exhibit regular {\it interband}
oscillations as functions of the frequency in the range
eV. Strong {\it intraband} responses in the latter
and week ones in the former occur at much lower frequencies. The
magneto-optical response can be tuned in the microwave-to-terahertz and visible
frequency ranges in contrast with a conventional two-dimensional electron gas
or graphene in which the response is limited to the terahertz regime. This
ability to isolate carriers in an anisotropic structure may make phosphorene a
promising candidate for new optical devices.Comment: 7 pages and 8 figure
Extra Dirac points in the energy spectrum for superlattices on single-layer graphene
We investigate the emergence of extra Dirac points in the electronic
structure of a periodically spaced barrier system, i.e., a superlattice, on
single-layer graphene, using a Dirac-type Hamiltonian. Using square barriers
allows us to find analytic expressions for the occurrence and location of these
new Dirac points in k-space and for the renormalization of the electron
velocity near them in the low-energy range. In the general case of unequal
barrier and well widths the new Dirac points move away from the Fermi level and
for given heights of the potential barriers there is a minimum and maximum
barrier width outside of which the new Dirac points disappear. The effect of
these extra Dirac points on the density of states and on the conductivity is
investigated.Comment: 7 pages, 8 figures, accepted for publication in Phys. Rev.
Characterization of the size and position of electron-hole puddles at a graphene p-n junction
The effect of an electron-hole puddle on the electrical transport when
governed by snake states in a bipolar graphene structure is investigated. Using
numerical simulations we show that information on the size and position of the
electron-hole puddle can be obtained using the dependence of the conductance on
magnetic field and electron density of the gated region. The presence of the
scatterer disrupts snake state transport which alters the conduction pattern.
We obtain a simple analytical formula that connects the position of the
electron-hole puddle with features observed in the conductance. Size of the
electron-hole puddle is estimated from the magnetic field and gate potential
that maximizes the effect of the puddle on the electrical transport.Comment: This is an author-created, un-copyedited version of an article
published in Nanotechnology. IOP Publishing Ltd is not responsible for any
errors or omissions in this version of the manuscript or any version derived
from it. The Version of Record is available online at
doi:10.1088/0957-4484/27/10/10520
Strain controlled valley filtering in multi-terminal graphene structures
Valley-polarized currents can be generated by local straining of
multi-terminal graphene devices. The pseudo-magnetic field created by the
deformation allows electrons from only one valley to transmit and a current of
electrons from a single valley is generated at the opposite side of the locally
strained region. We show that valley filtering is most effective with bumps of
a certain height and width. Despite the fact that the highest contribution to
the polarized current comes from electrons from the lowest sub-band,
contributions of other sub-bands are not negligible and can significantly
enhance the output current.Comment: 4 pages, 4 figure
Tactile spatial attention enhances gamma-band activity in somatosensory cortex and reduces low-frequency activity in parieto-occipital areas.
We investigated the effects of spatial-selective attention on oscillatory neuronal dynamics in a tactile delayed-match-to-sample task.
Whole-head magnetoencephalography was recorded in healthy subjects while dot patterns were presented to their index fingers using Braille stimulators. The subjects’ task was to report the reoccurrence of an initially presented sample pattern in a series of up to eight test stimuli that were presented unpredictably to their right or left index finger. Attention was cued to one side (finger) at the beginning of each trial, and subjects performed the task at the attended side, ignoring the unattended side.
After stimulation, high-frequency gamma-band activity (60 –95 Hz) in presumed primary somatosensory cortex (S1) was enhanced, whereas alpha- and beta-band activity were suppressed in somatosensory and occipital areas and then rebounded. Interestingly, despite the absence of any visual stimulation, we also found time-locked activation of medial occipital, presumably visual, cortex. Most relevant,
spatial tactile attention enhanced stimulus-induced gamma-band activity in brain regions consistent with contralateral S1 and deepened and prolonged the stimulus induced suppression of beta- and alpha-band activity, maximal in parieto-occipital cortex. Additionally, the
beta rebound over contralateral sensorimotor areas was suppressed.
Wehypothesize that spatial-selective attention enhances the saliency of sensory representations by synchronizing neuronal responses in early somatosensory cortex and thereby enhancing their impact on downstream areas and facilitating interareal processing. Furthermore, processing of tactile patterns also seems to recruit visual cortex and this even more so for attended compared with unattended
stimuli
Spectroscopy of snake states using a graphene Hall bar
An approach to observe snake states in a graphene Hall bar containing a
pn-junction is proposed. The magnetic field dependence of the bend resistance
in a ballistic graphene Hall bar structure containing a tilted pn-junction
oscillates as a function of applied magnetic field. We show that each
oscillation is due to a specific snake state that moves along the pn-interface.
Furthermore depending on the value of the magnetic field and applied potential
we can control the lead in which the electrons will end up and hence control
the response of the system
Graphene Hall bar with an asymmetric pn-junction
We investigated the magnetic field dependence of the Hall and the bend
resistances in the ballistic regime for a single layer graphene Hall bar
structure containing a pn-junction. When both regions are n-type the Hall
resistance dominates and Hall type of plateaus are formed. These plateaus occur
as a consequence of the restriction on the angle imposed by Snell's law
allowing only electrons with a certain initial angles to transmit though the
potential step. The size of the plateau and its position is determined by the
position of the potential interface as well as the value of the applied
potential. When the second region is p-type the bend resistance dominates which
is asymmetric in field due to the presence of snake states. Changing the
position of the pn-interface in the Hall bar strongly affects these states and
therefore the bend resistance is also changed. Changing the applied potential
we observe that the bend resistance exhibits a peak around the
charge-neutrality point (CNP) which is independent of the position of the
pn-interface, while the Hall resistance shows a sign reversal when the CNP is
crossed, which is in very good agreement with a recent experiment [J. R.
Williams et al., Phys. Rev. Lett. 107, 046602(2011)]
Plasmons and their interaction with electrons in trilayer graphene
The interaction between electrons and plasmons in trilayer graphene is
investigated within the Overhauser approach resulting in the 'plasmaron'
quasi-particle. This interaction is cast into a field theoretical problem, nd
its effect on the energy spectrum is calculated using improved Wigner-Brillouin
perturbation theory. The plasmaron spectrum is shifted with respect to the bare
electron spectrum by for ABC
stacked trilayer graphene and for ABA trilayer graphene by () for the hyperbolic linear) part of the spectrum. The shift in general
increases with the electron concentration and electron momentum. The
dispersion of plasmarons is more pronounced in \textit{ABC} stacked than in ABA
tacked trilayer graphene, because of the different energy band structure and
their different plasmon dispersion.Comment: arXiv admin note: substantial text overlap with arXiv:1310.623
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