11,086 research outputs found
Gate-controlled conductance through bilayer graphene ribbons
We study the conductance of a biased bilayer graphene flake with monolayer
nanoribbon contacts. We find that the transmission through the bilayer ribbon
strongly depends on the applied bias between the two layers and on the relative
position of the monolayer contacts. Besides the opening of an energy gap on the
bilayer, the bias allows to tune the electronic density on the bilayer flake,
making possible the control of the electronic transmission by an external
parameter.Comment: 5 pages, 5 figures include
Electronic transport through bilayer graphene flakes
We investigate the electronic transport properties of a bilayer graphene
flake contacted by two monolayer nanoribbons. Such a finite-size bilayer flake
can be built by overlapping two semiinfinite ribbons or by depositing a
monolayer flake onto an infinite nanoribbon. These two structures have a
complementary behavior, that we study and analyze by means of a tight-binding
method and a continuum Dirac model. We have found that for certain energy
ranges and geometries, the conductance of these systems oscillates markedly
between zero and the maximum value of the conductance, allowing for the design
of electromechanical switches. Our understanding of the electronic transmission
through bilayer flakes may provide a way to measure the interlayer hopping in
bilayer graphene.Comment: 11 pages, 8 figure
Phonon runaway in nanotube quantum dots
We explore electronic transport in a nanotube quantum dot strongly coupled
with vibrations and weakly with leads and the thermal environment. We show that
the recent observation of anomalous conductance signatures in single-walled
carbon nanotube (SWCNT) quantum dots can be understood quantitatively in terms
of current driven `hot phonons' that are strongly correlated with electrons.
Using rate equations in the many-body configuration space for the joint
electron-phonon distribution, we argue that the variations are indicative of
strong electron-phonon coupling requiring an analysis beyond the traditional
uncorrelated phonon-assisted transport (Tien-Gordon) approach.Comment: 8 pages, 6 figure
Relevance of Induced Gauge Interactions in Decoherence
Decoherence in quantum cosmology is shown to occur naturally in the presence
of induced geometric gauge interactions associated with particle production.A
new 'gauge '-variant form of the semiclassical Einstein equations is also
presented which makes the non-gravitating character of the vacuum polarisation
energy explicit.Comment: 10 pages, LATEX, IC/94/16
Effect of gas flow on electronic transport in a DNA-decorated carbon nanotube
We calculate the two-time current correlation function using the experimental
data of the current-time characteristics of the Gas-DNA-decorated carbon
nanotube field effect transistor. The pattern of the correlation function is a
measure of the sensitivity and selectivity of the sensors and suggest that
these gas flow sensors may also be used as DNA sequence detectors. The system
is modelled by a one-dimensional tight-binding Hamiltonian and we present
analytical calculations of quantum electronic transport for the system using
the time-dependent nonequilibrium Green's function formalism and the adiabatic
expansion. The zeroth and first order contributions to the current
and are calculated, where is the Landauer formula. The formula for the time-dependent current
is then used to compare the theoretical results with the experiment.Comment: 14 pages, 5 figures and 2 table
Scanning tunneling microscopy simulations of poly(3-dodecylthiophene) chains adsorbed on highly oriented pyrolytic graphite
We report on a novel scheme to perform efficient simulations of Scanning
Tunneling Microscopy (STM) of molecules weakly bonded to surfaces. Calculations
are based on a tight binding (TB) technique including self-consistency for the
molecule to predict STM imaging and spectroscopy. To palliate the lack of
self-consistency in the tunneling current calculation, we performed first
principles density-functional calculations to extract the geometrical and
electronic properties of the system. In this way, we can include, in the TB
scheme, the effects of structural relaxation upon adsorption on the electronic
structure of the molecule. This approach is applied to the study of
regioregular poly(3-dodecylthiophene) (P3DDT) polymer chains adsorbed on highly
oriented pyrolytic graphite (HOPG). Results of spectroscopic calculations are
discussed and compared with recently obtained experimental datComment: 15 pages plus 5 figures in a tar fil
Transport properties of graphene quantum dots
In this work we present a theoretical study of transport properties of a
double crossbar junction composed by segments of graphene ribbons with
different widths forming a graphene quantum dot structure. The systems are
described by a single-band tight binding Hamiltonian and the Green's function
formalism using real space renormalization techniques. We show calculations of
the local density of states, linear conductance and I-V characteristics. Our
results depict a resonant behavior of the conductance in the quantum dot
structures which can be controlled by changing geometrical parameters such as
the nanoribbon segments widths and relative distance between them. By applying
a gate voltage on determined regions of the structure, it is possible to
modulate the transport response of the systems. We show that negative
differential resistance can be obtained for low values of gate and bias
voltages applied.Comment: 9 pages, 9 figures, accepted to Phys. Rev.
M-atom conductance oscillations of a metallic quantum wire
The electron transport through a monoatomic metallic wire connected to leads
is investigated using the tight-binding Hamiltonian and Green's function
technique. Analytical formulas for the transmittance are derived and M-atom
oscillations of the conductance versus the length of the wire are found. Maxima
of the transmittance function versus the energy, for the wire consisted of N
atoms, determine the (N+1) period of the conductance. The periods of
conductance oscillations are discussed and the local and average quantum wire
charges are presented. The average charge of the wire is linked with the period
of the conductance oscillations and it tends to the constant value as the
length of the wire increases. For M-atom periodicity there are possible (M-1)
average occupations of the wire states.Comment: 8 pages, 5 figures. J.Phys.: Condens. matter (2005) accepte
Spin-Hall effect in two-dimensional mesoscopic hole systems
The spin Hall effect in two dimensional hole systems is studied by using the
four-terminal Landauer-B\"{u}ttiker formula with the help of Green functions.
The spin Hall effect exists even when there are {\em not} any correlations
between the spin-up and -down heavy holes (light holes) and when the
-point degeneracy of the heavy hole and light hole bands is lifted by
the confinement or recovered by the strain. When only a heavy hole charge
current without any spin polarization is injected through one lead, under right
choice of lead voltages, one can get a pure heavy (light) hole spin current,
combined with a possible impure light (heavy) hole spin current from another
two leads. The spin Hall coefficients of both heavy and light holes depend on
the Fermi energy, devise size and the disorder strength. It is also shown that
the spin Hall effect of two dimensional hole systems is much more robust than
that of electron systems with the Rashba spin-orbit coupling and the spin Hall
coefficients do not decrease with the system size but tend to some nonzero
values when the disorder strength is smaller than some critical value.Comment: 5 pages, 4 figure
Husimi Maps in Lattices
We build upon previous work that used coherent states as a measurement of the
local phase space and extended the flux operator by adapting the Husimi
projection to produce a vector field called the Husimi map. In this article, we
extend its definition from continuous systems to lattices. This requires making
several adjustments to incorporate effects such as group velocity and multiple
bands. Several phenomena which uniquely occur in lattice systems, like
group-velocity warping and internal Bragg diffraction, are explained and
demonstrated using Husimi maps. We also show that scattering points between
bands and valleys can be identified in the divergence of the Husimi map
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