15,795 research outputs found
Geometrical effects on spin injection: 3D spin drift diffusion model
We discuss a three-dimensional (3D) spin drift diffusion (SDD) model to
inject spin from a ferromagnet (FM) to a normal metal (N) or semiconductor
(SC). Using this model we investigate the problem of spin injection into
isotropic materials like GaAs and study the effect of FM contact area and SC
thickness on spin injection. We find that in order to achieve detectable spin
injection a small contact area or thick SC samples are essential for direct
contact spin injection devices. We investigate the use of thin metal films (Cu)
proposed by S.B. Kumar et al. and show that they are an excellent substitute
for tunnelling barriers (TB) in the regime of small contact area. Since most
tunnelling barriers are prone to pinhole defects, we study the effect of
pinholes in AlO tunnelling barriers and show that the reduction in the
spin-injection ratio () is solely due to the effective area of the
pinholes and there is no correlation between the number of pinholes and the
spin injection ratio.Comment: 5 pages, 6 figures. Accepted by JA
Charge Transport in a Quantum Electromechanical System
We describe a quantum electromechanical system(QEMS) comprising a single
quantum dot harmonically bound between two electrodes and facilitating a
tunneling current between them. An example of such a system is a fullerene
molecule between two metal electrodes [Park et al., Nature, 407, 57 (2000)].
The description is based on a quantum master equation for the density operator
of the electronic and vibrational degrees of freedom and thus incorporates the
dynamics of both diagonal (population) and off diagonal (coherence) terms. We
derive coupled equations of motion for the electron occupation number of the
dot and the vibrational degrees of freedom, including damping of the vibration
and thermo-mechanical noise. This dynamical description is related to
observable features of the system including the stationary current as a
function of bias voltage.Comment: To appear in Phys. Rev. B., 13 pages, single colum
Tunneling Qubit Operation on a Protected Josephson Junction Array
We discuss a protected quantum computation process based on a hexagon
Josephson junction array. Qubits are encoded in the punctured array, which is
topologically protected. The degeneracy is related to the number of holes. The
topological degeneracy is lightly shifted by tuning the flux through specific
hexagons. We also show how to perform single qubit operation and basic quantum
gate operations in this system.Comment: 8 pages, 4 figures. The published version in Phys. Rev.,
A81(2010)01232
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Exposure to hazardous volatile organic compounds, PM 10 and CO while walking along streets in urban Guangzhou, China
Toxic air pollutants in street canyons are important issues concerning public health especially in some large Asian cities like Guangzhou. In 1998 <18% of Guangzhou citizens used public transportation modes, with a majority commuting on foot (42%) or by bicycle (22%). Of the pedestrians, 57% were either senior citizens or students. In the present study, we measured toxic air pollutants while walking along urban streets in Guangzhou to evaluate pedestrian exposure. Volatile organic compounds (VOCs) were collected with sorbent tubes, and PM 10 and CO were measured simultaneously with portable analyzers. Our results showed that pedestrian exposure to PM 10 (with an average of 303 μg m -3 for all samples) and some toxic VOCs (for example, benzene) was relatively high. Monocyclic aromatic hydrocarbons were found to be the most abundant VOCs, and 71% of the samples had benzene levels higher than 30 μg m -3. Benzene, PM 10 and CO in walk-only streets were significantly lower (p<0.05) than in traffic streets, and the differences in exposure levels between new urban streets and old urban streets were highly significant (p<0.01). Pedestrian exposure to toxic VOCs and PM 10 was higher than those reported in other public transportation modes (bus and subway). The good correlations between BTEX, PM 10 and CO in the streets indicated that automotive emission might be their major source. Our study also showed that the risk to pedestrians due to air pollution was misinterpreted by the reported air quality index based on measurement of SO 2, NO x and PM 10 in the government monitoring stations. An urban roadside monitoring station might be needed by air quality monitoring networks in large Asian cities like Guangzhou, in order to survey exposure to air toxics in urban roadside microenvironments. © 2004 Elsevier Ltd. All rights reserved
When Things Matter: A Data-Centric View of the Internet of Things
With the recent advances in radio-frequency identification (RFID), low-cost
wireless sensor devices, and Web technologies, the Internet of Things (IoT)
approach has gained momentum in connecting everyday objects to the Internet and
facilitating machine-to-human and machine-to-machine communication with the
physical world. While IoT offers the capability to connect and integrate both
digital and physical entities, enabling a whole new class of applications and
services, several significant challenges need to be addressed before these
applications and services can be fully realized. A fundamental challenge
centers around managing IoT data, typically produced in dynamic and volatile
environments, which is not only extremely large in scale and volume, but also
noisy, and continuous. This article surveys the main techniques and
state-of-the-art research efforts in IoT from data-centric perspectives,
including data stream processing, data storage models, complex event
processing, and searching in IoT. Open research issues for IoT data management
are also discussed
Manipulation of heat current by the interface between graphene and white graphene
We investigate the heat current flowing across the interface between graphene
and hexagonal boron nitride (so-called white graphene) using both molecular
dynamics simulation and nonequilibrium Green's function approaches. These two
distinct methods discover the same phenomena that the heat current is reduced
linearly with increasing interface length, and the zigzag interface causes
stronger reduction of heat current than the armchair interface. These phenomena
are interpreted by both the lattice dynamics analysis and the transmission
function explanation, which both reveal that the localized phonon modes at
interfaces are responsible for the heat management. The room temperature
interface thermal resistance is about mK/W in zigzag
interface and mK/W in armchair interface, which
directly results in stronger heat reduction in zigzag interface. Our
theoretical results provide a specific route for experimentalists to control
the heat transport in the graphene and hexagonal boron nitride compound through
shaping the interface between these two materials.Comment: accepted by EP
Tuning the exciton g-factor in single InAs/InP quantum dots
Photoluminescence data from single, self-assembled InAs/InP quantum dots in
magnetic fields up to 7 T are presented. Exciton g-factors are obtained for
dots of varying height, corresponding to ground state emission energies ranging
from 780 meV to 1100 meV. A monotonic increase of the g-factor from -2 to +1.2
is observed as the dot height decreases. The trend is well reproduced by sp3
tight binding calculations, which show that the hole g-factor is sensitive to
confinement effects through orbital angular momentum mixing between the
light-hole and heavy-hole valence bands. We demonstrate tunability of the
exciton g-factor by manipulating the quantum dot dimensions using pyramidal InP
nanotemplates
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