171 research outputs found
Electrical Nanoprobing of Semiconducting Carbon Nanotubes using an Atomic Force Microscope
We use an Atomic Force Microscope (AFM) tip to locally probe the electronic
properties of semiconducting carbon nanotube transistors. A gold-coated AFM tip
serves as a voltage or current probe in three-probe measurement setup. Using
the tip as a movable current probe, we investigate the scaling of the device
properties with channel length. Using the tip as a voltage probe, we study the
properties of the contacts. We find that Au makes an excellent contact in the
p-region, with no Schottky barrier. In the n-region large contact resistances
were found which dominate the transport properties.Comment: 4 pages, 5 figure
Planar Heterostructure Graphene -- Narrow-Gap Semiconductor -- Graphene
We investigate a planar heterostructure composed of two graphene films
separated by a narrow-gap semiconductor ribbon. We show that there is no the
Klein paradox when the Dirac points of the Brillouin zone of graphene are in a
band gap of a narrow-gap semiconductor. There is the energy range depending on
an angle of incidence, in which the above-barrier damped solution exists.
Therefore, this heterostructure is a "filter" transmitting particles in a
certain range of angles of incidence upon a potential barrier. We discuss the
possibility of an application of this heterostructure as a "switch".Comment: 9 pages, 2 figure
Giant Nonlocality near the Dirac Point in Graphene
Transport measurements have been a powerful tool for uncovering new
electronic phenomena in graphene. We report nonlocal measurements performed in
the Hall bar geometry with voltage probes far away from the classical path of
charge flow. We observe a large nonlocal response near the Dirac point in
fields as low as 0.1T, which persists up to room temperature. The nonlocality
is consistent with the long-range flavor currents induced by lifting of
spin/valley degeneracy. The effect is expected to contribute strongly to all
magnetotransport phenomena near the neutrality point
Energetics of Quantum Antidot States in Quantum Hall Regime
We report experiments on the energy structure of antidot-bound states. By
measuring resonant tunneling line widths as function of temperature, we
determine the coupling to the remote global gate voltage and find that the
effects of interelectron interaction dominate. Within a simple model, we also
determine the energy spacing of the antidot bound states, self consistent edge
electric field, and edge excitation drift velocity.Comment: 4 pages, RevTex, 5 Postscript figure
Stability of an Exciton bound to an Ionized Donor in Quantum Dots
Total energy, binding energy, recombination rate (of the electron hole pair)
for an exciton (X) bound in a parabolic two dimensional quantum dot by a donor
impurity located on the z axis at a distance d from the dot plane, are
calculated by using the Hartree formalism with a recently developed numerical
method (PMM) for the solution of the Schroedinger equation. As our analysis
indicates there is a critical dot radius such that for radius less than the
critical radius the complex is unstable and with an increase of the impurity
distance this critical radius increases. Furthermore, there is a critical value
of the mass ratio such that for mass ratio less than the critical value the
complex is stable. The appearance of this stability condition depends both on
the impurity distance and the dot radius, in a way that with an increase of the
impurity distance we have an increase in the maximum dot radius where this
stability condition appears. For dot radii greater than this maximum dot radius
(for fixed impurity distance) the complex is always stable.Comment: 17 pages, 7 figures Applying a new numerical method which is based on
the adiabatic stability of quantum mechanics, we study the stability of an
exciton (X) bound in a parabolic two dimensional quantum dot by a donor
impurity located on the z axis at a distance d from the dot plan
Analytic results for particles with interaction in two dimensions and an external magnetic field
The -dimensional quantum problem of particles (e.g. electrons) with
interaction in a two-dimensional parabolic potential
(e.g. quantum dot) and magnetic field , reduces exactly to solving a
-dimensional problem which is independent of and . An
exact, infinite set of relative mode excitations are obtained for any . The
problem reduces to that of a ficticious particle in a two-dimensional,
non-linear potential of strength , subject to a ficticious magnetic
field , the relative angular momentum.Comment: To appear in Physical Review Letters (in press). RevTeX file. Two
figures available from [email protected] or
[email protected]
Non-volatile molecular memory elements based on ambipolar nanotube field effect transistors
We have fabricated air-stable n-type, ambipolar carbon nanotube field effect
transistors (CNFETs), and used them in nanoscale memory cells. N-type
transistors are achieved by annealing of nanotubes in hydrogen gas and
contacting them by cobalt electrodes. Scanning gate microscopy reveals that the
bulk response of these devices is similar to gold-contacted p-CNFETs,
confirming that Schottky barrier formation at the contact interface determines
accessibility of electron and hole transport regimes. The transfer
characteristics and Coulomb Blockade (CB) spectroscopy in ambipolar devices
show strongly enhanced gate coupling, most likely due to reduction of defect
density at the silicon/silicon-dioxide interface during hydrogen anneal. The CB
data in the ``on''-state indicates that these CNFETs are nearly ballistic
conductors at high electrostatic doping. Due to their nanoscale capacitance,
CNFETs are extremely sensitive to presence of individual charge around the
channel. We demonstrate that this property can be harnessed to construct data
storage elements that operate at the few-electron level.Comment: 6 pages text, 3 figures and 1 table of content graphic; available as
NanoLetters ASAP article on the we
Magnetization of noncircular quantum dots
We calculate the magnetization of quantum dots deviating from circular
symmetry for noninteracting electrons or electrons interacting according to the
Hartree approximation. For few electrons the magnetization is found to depend
on their number, and the shape of the dot. The magnetization is an ideal probe
into the many-electron state of a quantum dot.Comment: 11 RevTeX pages with 6 included Postscript figure
Percolation-type description of the metal-insulator transition in two dimensions
A simple non-interacting-electron model, combining local quantum tunneling
and global classical percolation (due to a finite dephasing time at low
temperatures), is introduced to describe a metal-insulator transition in two
dimensions. It is shown that many features of the experiments, such as the
exponential dependence of the resistance on temperature on the metallic side,
the linear dependence of the exponent on density, the scale of the
critical resistance, the quenching of the metallic phase by a parallel magnetic
field and the non-monotonic dependence of the critical density on a
perpendicular magnetic field, can be naturally explained by the model.Comment: 4 pages, 4 figure
Spin effects in a confined 2DEG: Enhancement of the g-factor, spin-inversion states and their far-infrared absorption
We investigate several spin-related phenomena in a confined two-dimensional
electron gas (2DEG) using the Hartree-Fock approximation for the mutual Coulomb
interaction of the electrons. The exchange term of the interaction causes a
large splitting of the spin levels whenever the chemical potential lies within
a Landau band (LB). This splitting can be reinterpreted as an enhancement of an
effective g-factor, g*. The increase of g* when a LB is half filled can be
accompanied by a spontaneous formation of a static spin-inversion state (SIS)
whose details depend on the system sision state (SIS) whose details depend on
the system size. The coupling of the states of higher LB's into the lowest band
by the Coulomb interaction of the 2DEG is essential for the SIS to occur. The
far-infrared absorption of the system, relatively insensitive to the spin
splitting, develops clear signs of the SIS.Comment: 7 figure
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