12,424 research outputs found
Theory of the Stark Effect for P donors in Si
We develop a multi-valley effective mass theory for substitutional donors in
silicon in an inhomogeneous environment. Valley-orbit coupling is treated
perturbatively. We apply the theory to the Stark effect in Si:P. The method
becomes more accurate at high fields, and it is designed to give correct
experimental binding energies at zero field. Unexpectedly, the ground state
energy for the donor electron is found to increase with electric field as a
consequence of spectrum narrowing of the 1s manifold. Our results are of
particular importance for the Kane quantum computer.Comment: published versio
Are the Effects of Structure Formation Seen in the Central Metallicity of Galaxy Clusters?
A sample of 46 nearby clusters observed with Chandra is analyzed to produce
radial density, temperature, entropy and metallicity profiles, as well as other
morphological measurements. The entropy profiles are computed to larger radial
extents than in previous Chandra cluster sample analyses. We find that the iron
mass fraction measured in the inner 0.15 R500 shows a larger dispersion across
the sample of low-mass clusters, than it does for the sample of high-mass
clusters. We interpret this finding as the result of the mixing of more haloes
in large clusters than in small clusters, which leads to an averaging of the
metal content in the large clusters, and thus less dispersion of metallicity
for high-mass clusters. This interpretation lends support to the idea that the
low-entropy, metal-rich gas of merging haloes reaches clusters' centers, which
explains observations of Core-Collapse Supernova products metallicity peaks,
and which is seen in hydrodynamical simulations. The gas in these merging
haloes would have to reach the centers of clusters without mixing in the outer
regions, in order to support our interpretation. On the other hand, metallicity
dispersion does not change with mass in the outer regions of clusters,
suggesting that most of the outer metals come from a source with a more uniform
metallicity level, such as during pre-enrichment. We also measure a correlation
between the metal content in low-mass clusters and the degree to which their
Intra-Cluster Medium (ICM) is morphologically disturbed, as measured by
centroid shift. This suggests an alternative interpretation of the large width
of the metallicity distribution in low-mass clusters, whereby a metallicity
boost in the center of low-mass clusters is induced as a transitional state,
during mergers.Comment: Accepted in ApJ, March 9, 201
Sacrificial charge and the spectral resolution performance of the Chandra Advanced CCD Imaging Spectrometer
Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the
focal plane instruments on the Chandra X-ray Observatory, suffered radiation
damage from exposure to soft protons during passages through the Earth's
radiation belts. The ACIS team is continuing to study the properties of the
damage with an emphasis on developing techniques to mitigate charge transfer
inefficiency (CTI) and spectral resolution degradation. A post-facto CTI
corrector has been developed which can effectively recover much of the lost
resolution. Any further improvements in performance will require knowledge of
the location and amount of sacrificial charge - charge deposited along the
readout path of an event which fills electron traps and changes CTI. We report
on efforts by the ACIS Instrument team to characterize which charge traps cause
performance degradation and the properties of the sacrificial charge seen
on-orbit. We also report on attempts to correct X-ray pulseheights for the
presence of sacrificial charge.Comment: 9 pages, 7 figures to be published in Proc. SPIE 485
Gait Verification using Knee Acceleration Signals
A novel gait recognition method for biometric applications is proposed. The approach has the following distinct features. First, gait patterns are determined via knee acceleration signals, circumventing difficulties associated with conventional vision-based gait recognition methods. Second, an automatic procedure to extract gait features from acceleration signals is developed that employs a multiple-template classification method. Consequently, the proposed approach can adjust the sensitivity and specificity of the gait recognition system with great flexibility. Experimental results from 35 subjects demonstrate the potential of the approach for successful recognition. By setting sensitivity to be 0.95 and 0.90, the resulting specificity ranges from 1 to 0.783 and 1.00 to 0.945, respectively
Effective mass theory of monolayer \delta-doping in the high-density limit
Monolayer \delta-doped structures in silicon have attracted renewed interest
with their recent incorporation into atomic-scale device fabrication strategies
as source and drain electrodes and in-plane gates. Modeling the physics of
\delta-doping at this scale proves challenging, however, due to the large
computational overhead associated with ab initio and atomistic methods. Here,
we develop an analytical theory based on an effective mass approximation. We
specifically consider the Si:P materials system, and the limit of high donor
density, which has been the subject of recent experiments. In this case,
metallic behavior including screening tends to smooth out the local disorder
potential associated with random dopant placement. While smooth potentials may
be difficult to incorporate into microscopic, single-electron analyses, the
problem is easily treated in the effective mass theory by means of a jellium
approximation for the ionic charge. We then go beyond the analytic model,
incorporating exchange and correlation effects within a simple numerical model.
We argue that such an approach is appropriate for describing realistic,
high-density, highly disordered devices, providing results comparable to
density functional theory, but with greater intuitive appeal, and lower
computational effort. We investigate valley coupling in these structures,
finding that valley splitting in the low-lying \Gamma band grows much more
quickly than the \Gamma-\Delta band splitting at high densities. We also find
that many-body exchange and correlation corrections affect the valley splitting
more strongly than they affect the band splitting
On hypergeometric series reductions from integral representations, the Kampe de Feriet function, and elsewhere
Single variable hypergeometric functions pFq arise in connection with the
power series solution of the Schrodinger equation or in the summation of
perturbation expansions in quantum mechanics. For these applications, it is of
interest to obtain analytic expressions, and we present the reduction of a
number of cases of pFp and p+1F_p, mainly for p=2 and p=3. These and related
series have additional applications in quantum and statistical physics and
chemistry.Comment: 17 pages, no figure
Dynamic Analysis of a Two Member Manipulator
Attenuating start-up and stopping vibrations when maneuvering large payloads attached to flexible manipulator systems is a great concern for many space missions. To address this concern, it was proposed that the use of smart materials, and their applications in smart structures, may provide an effective method of control for aerospace structures. In this paper, a modified finite element model has been developed to simulate the performance of piezoelectric ceramic actuators, and was applied to a flexible two-arm manipulator system. Connected to a control voltage, the piezoelectric actuators produce control moments based on the optimal control theory. The computer simulation modeled the end-effector vibration suppression of the NASA manipulator testbed for berthing operations of the Space Shuttle to the Space Station. The results of the simulation show that the bonded piezoelectric actuators can effectively suppress follow-up vibrations of the end-effector, stimulated by some external disturbance
Forest structure, stand composition, and climate-growth response in montane forests of Jiuzhaigou National Nature Reserve, China.
Montane forests of western China provide an opportunity to establish baseline studies for climate change. The region is being impacted by climate change, air pollution, and significant human impacts from tourism. We analyzed forest stand structure and climate-growth relationships from Jiuzhaigou National Nature Reserve in northwestern Sichuan province, along the eastern edge of the Tibetan plateau. We conducted a survey to characterize forest stand diversity and structure in plots occurring between 2050 and 3350 m in elevation. We also evaluated seedling and sapling recruitment and tree-ring data from four conifer species to assess: 1) whether the forest appears in transition toward increased hardwood composition; 2) if conifers appear stressed by recent climate change relative to hardwoods; and 3) how growth of four dominant species responds to recent climate. Our study is complicated by clear evidence of 20(th) century timber extraction. Focusing on regions lacking evidence of logging, we found a diverse suite of conifers (Pinus, Abies, Juniperus, Picea, and Larix) strongly dominate the forest overstory. We found population size structures for most conifer tree species to be consistent with self-replacement and not providing evidence of shifting composition toward hardwoods. Climate-growth analyses indicate increased growth with cool temperatures in summer and fall. Warmer temperatures during the growing season could negatively impact conifer growth, indicating possible seasonal climate water deficit as a constraint on growth. In contrast, however, we found little relationship to seasonal precipitation. Projected warming does not yet have a discernible signal on trends in tree growth rates, but slower growth with warmer growing season climates suggests reduced potential future forest growth
Vertex Operators, Screen Currents and Correlation Functions at Arbitrary Level
Bosonized q-vertex operators related to the 4-dimensional evaluation modules
of the quantum affine superalgebra are constructed for
arbitrary level , where is a complex parameter
appearing in the 4-dimensional evaluation representations. They are
intertwiners among the level- highest weight Fock-Wakimoto modules.
Screen currents which commute with the action of up to
total differences are presented. Integral formulae for N-point functions of
type I and type II q-vertex operators are proposed.Comment: Latex file 18 page
Inducing Transport in a Dissipation-Free Lattice with Super Bloch Oscillations
Particles in a perfect lattice potential perform Bloch oscillations when
subject to a constant force, leading to localization and preventing
conductivity. For a weakly-interacting Bose-Einstein condensate (BEC) of Cs
atoms, we observe giant center-of-mass oscillations in position space with a
displacement across hundreds of lattice sites when we add a periodic modulation
to the force near the Bloch frequency. We study the dependence of these "super"
Bloch oscillations on lattice depth, modulation amplitude, and modulation
frequency and show that they provide a means to induce linear transport in a
dissipation-free lattice. Surprisingly, we find that, for an interacting
quantum system, super Bloch oscillations strongly suppress the appearance of
dynamical instabilities and, for our parameters, increase the phase-coherence
time by more than a factor of hundred.Comment: 4 pages, 5 figure
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