12,122 research outputs found
Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches
A semi-classical Monte Carlo model for studying three-dimensional carrier
dynamics in photoconductive switches is presented. The model was used to
simulate the process of photoexcitation in GaAs-based photoconductive antennas
illuminated with pulses typical of mode-locked Ti:Sapphire lasers. We analyzed
the power and frequency bandwidth of THz radiation emitted from these devices
as a function of bias voltage, pump pulse duration and pump pulse location. We
show that the mechanisms limiting the THz power emitted from photoconductive
switches fall into two regimes: when illuminated with short duration (<40 fs)
laser pulses the energy distribution of the Gaussian pulses constrains the
emitted power, while for long (>40 fs) pulses, screening is the primary
power-limiting mechanism. A discussion of the dynamics of bias field screening
in the gap region is presented. The emitted terahertz power was found to be
enhanced when the exciting laser pulse was in close proximity to the anode of
the photoconductive emitter, in agreement with experimental results. We show
that this enhancement arises from the electric field distribution within the
emitter combined with a difference in the mobilities of electrons and holes.Comment: 7 pages, 7 figure
Multiscale lattice Boltzmann approach to modeling gas flows
For multiscale gas flows, kinetic-continuum hybrid method is usually used to
balance the computational accuracy and efficiency. However, the
kinetic-continuum coupling is not straightforward since the coupled methods are
based on different theoretical frameworks. In particular, it is not easy to
recover the non-equilibrium information required by the kinetic method which is
lost by the continuum model at the coupling interface. Therefore, we present a
multiscale lattice Boltzmann (LB) method which deploys high-order LB models in
highly rarefied flow regions and low-order ones in less rarefied regions. Since
this multiscale approach is based on the same theoretical framework, the
coupling precess becomes simple. The non-equilibrium information will not be
lost at the interface as low-order LB models can also retain this information.
The simulation results confirm that the present method can achieve model
accuracy with reduced computational cost
Excitons and high-order optical transitions in individual carbon nanotubes
We examine the excitonic nature of high-lying optical transitions in
single-walled carbon nanotubes by means of Rayleigh scattering spectroscopy. A
careful analysis of the principal transitions of individual semiconducting and
metallic nanotubes reveals that in both cases the lineshape is consistent with
an excitonic model, but not one of free-carriers. For semiconducting species,
side-bands are observed at ~200 meV above the third and fourth optical
transitions. These features are ascribed to exciton-phonon bound states. Such
side-bands are not apparent for metallic nanotubes,as expected from the reduced
strength of excitonic interactions in these systems
The entanglement in one-dimensional random XY spin chain with Dzyaloshinskii-Moriya interaction
The impurities of exchange couplings, external magnetic fields and
Dzyaloshinskii--Moriya (DM) interaction considered as Gaussian distribution,
the entanglement in one-dimensional random spin systems is investigated by
the method of solving the different spin-spin correlation functions and the
average magnetization per spin. The entanglement dynamics at central locations
of ferromagnetic and antiferromagnetic chains have been studied by varying the
three impurities and the strength of DM interaction. (i) For ferromagnetic spin
chain, the weak DM interaction can improve the amount of entanglement to a
large value, and the impurities have the opposite effect on the entanglement
below and above critical DM interaction. (ii) For antiferromagnetic spin chain,
DM interaction can enhance the entanglement to a steady value. Our results
imply that DM interaction strength, the impurity and exchange couplings (or
magnetic field) play competing roles in enhancing quantum entanglement.Comment: 12 pages, 3 figure
Task Switching in English-Chinese Bilinguals: A Life Span Approach
The current study investigated the developmental trajectory of 124 English-Chinese Singaporean bilinguals (41 6-9-year-olds, 44 18-26-year-olds, and 39 55-79-year-olds) with the Standard (SD), Total Change (TC), Positive Priming (PP), and Negative Priming (NP) versions of the Computerized Dimensional Change Card Sort task. Tasks were administrated in either English or Chinese. Additionally, participants were tested with both English and Chinese versions of the Peabody Picture Vocabulary Test. Separate curve fitting indicated that significant quadratic trends appeared in the local switch costs for accuracy only in the SD and the PP versions. Children had significantly larger local switch costs in all the versions compared to young adults and elderly adults, who had similar local switch costs. These findings suggest that bilingualism may slow down the decay of information maintaining, updating, disinhibition, and task set integration in elderly adults. Results imply that bilingual advantage may accumulate through childhood, and be preserved in late adulthood
Multi-component lattice-Boltzmann model with interparticle interaction
A previously proposed [X. Shan and H. Chen, Phys. Rev. E {\bf 47}, 1815,
(1993)] lattice Boltzmann model for simulating fluids with multiple components
and interparticle forces is described in detail. Macroscopic equations
governing the motion of each component are derived by using Chapman-Enskog
method. The mutual diffusivity in a binary mixture is calculated analytically
and confirmed by numerical simulation. The diffusivity is generally a function
of the concentrations of the two components but independent of the fluid
velocity so that the diffusion is Galilean invariant. The analytically
calculated shear kinematic viscosity of this model is also confirmed
numerically.Comment: 18 pages, compressed and uuencoded postscript fil
A Lattice Boltzmann method for simulations of liquid-vapor thermal flows
We present a novel lattice Boltzmann method that has a capability of
simulating thermodynamic multiphase flows. This approach is fully
thermodynamically consistent at the macroscopic level. Using this new method, a
liquid-vapor boiling process, including liquid-vapor formation and coalescence
together with a full coupling of temperature, is simulated for the first time.Comment: one gzipped tar file, 19 pages, 4 figure
Size-selective nanoparticle growth on few-layer graphene films
We observe that gold atoms deposited by physical vapor deposition onto few
layer graphenes condense upon annealing to form nanoparticles with an average
diameter that is determined by the graphene film thickness. The data are well
described by a theoretical model in which the electrostatic interactions
arising from charge transfer between the graphene and the gold particle limit
the size of the growing nanoparticles. The model predicts a nanoparticle size
distribution characterized by a mean diameter D that follows a scaling law D
proportional to m^(1/3), where m is the number of carbon layers in the few
layer graphene film.Comment: 15 pages, 4 figure
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