81 research outputs found
Transport in Nanotubes: Effect of Remote Impurity Scattering
Theory of the remote Coulomb impurity scattering in single--wall carbon
nanotubes is developed within one--electron approximation. Boltzmann equation
is solved within drift--diffusion model to obtain the tube conductivity. The
conductivity depends on the type of the nanotube bandstructure (metal or
semiconductor) and on the electron Fermi level. We found exponential dependence
of the conductivity on the Fermi energy due to the Coulomb scattering rate has
a strong dependence on the momentum transfer. We calculate intra-- and
inter--subband scattering rates and present general expressions for the
conductivity. Numerical results, as well as obtained analytical expressions,
show that the degenerately doped semiconductor tubes may have very high
mobility unless the doping level becomes too high and the inter--subband
transitions impede the electron transport.Comment: 13 pages, 4 figure
Spontaneous decay of an emitter's excited state near a finite-length metallic carbon nanotube
The spontaneous decay of an excited state of an emitter placed in the
vicinity of a metallic single-wall carbon nanotube (SWNT) was examined
theoretically. The emitter-SWNT coupling strongly depends on the position of
the emitter relative to the SWNT, the length of the SWNT, the dipole transition
frequency and the orientation of the emitter. In the high-frequency regime,
dips in the spectrum of the spontaneous decay rate exist at the resonance
frequencies in the spectrum of the SWNT conductivity. In the
intermediate-frequency regime, the SWNT conductivity is very low, and the
spontaneous decay rate is practically unaffected by the SWNT. In the
low-frequency regime, the spectrum of the spontaneous decay rate contains
resonances at the antennas resonance frequencies for surface-wave propagation
in the SWNT. Enhancement of both the total and radiative spontaneous decay
rates by several orders in magnitude is predicted at these resonance
frequencies. The strong emitter-field coupling is achieved, in spite of the low
Q factor of the antenna resonances, due to the very high magnitude of the
electromagnetic field in the near-field zone. The vacuum Rabi oscillations of
the population of the excited emitter state are exhibited when the emitter is
coupled to an antenna resonance of the SWNT.Comment: 8 pages, 6 figure
Large radius exciton in single-walled carbon nanotubes
The spectrum of large radius exciton in an individual semiconducting
single-walled carbon nanotube (SWCNT) is described within the framework of
elementary potential model, in which exciton is modeled as bound state of two
oppositely charged quasi-particles confined on the tube surface. Due to the
parity of the interaction potential the exciton states split into the odd and
even series. It is shown that for the bare and screened Coulomb electron-hole
(e-h) potentials the binding energy of even excitons in the ground state well
exceeds the energy gap. The factors preventing the collapse of single-electron
states in isolated semiconducting SWCNTs are discussed.Comment: 14 pages, 1 figure, 5 table
Size quantization of Dirac fermions in graphene constrictions
Quantum point contacts (QPCs) are cornerstones of mesoscopic physics and
central building blocks for quantum electronics. Although the Fermi wave-length
in high-quality bulk graphene can be tuned up to hundreds of nanometers, the
observation of quantum confinement of Dirac electrons in nanostructured
graphene systems has proven surprisingly challenging. Here we show ballistic
transport and quantized conductance of size-confined Dirac fermions in
lithographically-defined graphene constrictions. At high charge carrier
densities, the observed conductance agrees excellently with the Landauer theory
of ballistic transport without any adjustable parameter. Experimental data and
simulations for the evolution of the conductance with magnetic field
unambiguously confirm the identification of size quantization in the
constriction. Close to the charge neutrality point, bias voltage spectroscopy
reveals a renormalized Fermi velocity () in
our graphene constrictions. Moreover, at low carrier density transport
measurements allow probing the density of localized states at edges, thus
offering a unique handle on edge physics in graphene devices.Comment: 24 pages including 20 figures and 1 table. Corrected typos. To appear
in Nature Communication
Theoretical and Experimental Studies of Schottky Diodes That Use Aligned Arrays of Single Walled Carbon Nanotubes
We present theoretical and experimental studies of Schottky diodes that use
aligned arrays of single walled carbon nanotubes. A simple physical model,
taking into account the basic physics of current rectification, can adequately
describe the single-tube and array devices. We show that for as grown array
diodes, the rectification ratio, defined by the
maximum-to-minimum-current-ratio, is low due to the presence of m-SWNT shunts.
These tubes can be eliminated in a single voltage sweep resulting in a high
rectification array device. Further analysis also shows that the channel
resistance, and not the intrinsic nanotube diode properties, limits the
rectification in devices with channel length up to ten micrometer.Comment: Nano Research, 2010, accepte
Photoconductivity of biased graphene
Graphene is a promising candidate for optoelectronic applications such as
photodetectors, terahertz imagers, and plasmonic devices. The origin of
photoresponse in graphene junctions has been studied extensively and is
attributed to either thermoelectric or photovoltaic effects. In addition, hot
carrier transport and carrier multiplication are thought to play an important
role. Here we report the intrinsic photoresponse in biased but otherwise
homogeneous graphene. In this classic photoconductivity experiment, the
thermoelectric effects are insignificant. Instead, the photovoltaic and a
photo-induced bolometric effect dominate the photoresponse due to hot
photocarrier generation and subsequent lattice heating through electron-phonon
cooling channels respectively. The measured photocurrent displays polarity
reversal as it alternates between these two mechanisms in a backgate voltage
sweep. Our analysis yields elevated electron and phonon temperatures, with the
former an order higher than the latter, confirming that hot electrons drive the
photovoltaic response of homogeneous graphene near the Dirac point
Stochastic Bundle Adjustment for Efficient and Scalable 3D Reconstruction
Current bundle adjustment solvers such as the Levenberg-Marquardt (LM)
algorithm are limited by the bottleneck in solving the Reduced Camera System
(RCS) whose dimension is proportional to the camera number. When the problem is
scaled up, this step is neither efficient in computation nor manageable for a
single compute node. In this work, we propose a stochastic bundle adjustment
algorithm which seeks to decompose the RCS approximately inside the LM
iterations to improve the efficiency and scalability. It first reformulates the
quadratic programming problem of an LM iteration based on the clustering of the
visibility graph by introducing the equality constraints across clusters. Then,
we propose to relax it into a chance constrained problem and solve it through
sampled convex program. The relaxation is intended to eliminate the
interdependence between clusters embodied by the constraints, so that a large
RCS can be decomposed into independent linear sub-problems. Numerical
experiments on unordered Internet image sets and sequential SLAM image sets, as
well as distributed experiments on large-scale datasets, have demonstrated the
high efficiency and scalability of the proposed approach. Codes are released at
https://github.com/zlthinker/STBA.Comment: Accepted by ECCV 202
Capillary condensation and quantum vacuum effects on the pull-in voltage of electrostatic switches with self-affine rough plates
Modeling the size dependent pull-in instability of beam-type NEMS using strain gradient theory
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