54,885 research outputs found
Energy levels of a parabolically confined quantum dot in the presence of spin-orbit interaction
We present a theoretical study of the energy levels in a parabolically
confined quantum dot in the presence of the Rashba spin-orbit interaction
(SOI). The features of some low-lying states in various strengths of the SOI
are examined at finite magnetic fields. The presence of a magnetic field
enhances the possibility of the spin polarization and the SOI leads to
different energy dependence on magnetic fields applied. Furthermore, in high
magnetic fields, the spectra of low-lying states show basic features of
Fock-Darwin levels as well as Landau levels.Comment: 6 pages, 4 figures, accepted by J. Appl. Phy
Optical properties of Si/Si0.87Ge0.13 multiple quantum well wires
Nanometer-scale wires cut into a Si/Si0.87Ge0.13 multiple quantum well structure were fabricated and characterized by using photoluminescence and photoreflectance at temperatures between 4 and 20 K. It was found that, in addition to a low-energy broadband emission at around 0.8 eV and other features normally observable in photoluminescence measurements, fabrication process induced strain relaxation and enhanced electron-hole droplets emission together with a new feature at 1.131 eV at 4 K were observed. The latter was further identified as a transition related to impurities located at the Si/Si0.87Ge0.13 heterointerfaces
SATMC: Spectral Energy Distribution Analysis Through Markov Chains
We present the general purpose spectral energy distribution (SED) fitting
tool SED Analysis Through Markov Chains (SATMC). Utilizing Monte Carlo Markov
Chain (MCMC) algorithms, SATMC fits an observed SED to SED templates or models
of the user's choice to infer intrinsic parameters, generate confidence levels
and produce the posterior parameter distribution. Here we describe the key
features of SATMC from the underlying MCMC engine to specific features for
handling SED fitting. We detail several test cases of SATMC, comparing results
obtained to traditional least-squares methods, which highlight its accuracy,
robustness and wide range of possible applications. We also present a sample of
submillimetre galaxies that have been fitted using the SED synthesis routine
GRASIL as input. In general, these SMGs are shown to occupy a large volume of
parameter space, particularly in regards to their star formation rates which
range from ~30-3000 M_sun yr^-1 and stellar masses which range from
~10^10-10^12 M_sun. Taking advantage of the Bayesian formalism inherent to
SATMC, we also show how the fitting results may change under different
parametrizations (i.e., different initial mass functions) and through
additional or improved photometry, the latter being crucial to the study of
high-redshift galaxies.Comment: 17 pages, 11 figures, MNRAS accepte
Polarization as a Probe to the Production Mechanisms of Charmonium in Collisions
Measurements of the polarization of \jp produced in pion-nucleus collisions
are in disagreement with leading twist QCD prediction where \jp is observed
to have negligible polarization whereas theory predicts substantial
polarization. We argue that this discrepancy cannot be due to poorly known
structure functions nor the relative production rates of \jp and .
The disagreement between theory and experiment suggests important higher twist
corrections, as has earlier been surmised from the anomalous non-factorized
nuclear -dependence of the \jp cross section.Comment: 8 page
Lagrangian Based Methods for Coherent Structure Detection
There has been a proliferation in the development of Lagrangian analytical methods for detecting coherent structures in fluid flow transport, yielding a variety of qualitatively different approaches. We present a review of four approaches and demonstrate the utility of these methods via their application to the same sample analytic model, the canonical double-gyre flow, highlighting the pros and cons of each approach. Two of the methods, the geometric and probabilistic approaches, are well established and require velocity field data over the time interval of interest to identify particularly important material lines and surfaces, and influential regions, respectively. The other two approaches, implementing tools from cluster and braid theory, seek coherent structures based on limited trajectory data, attempting to partition the flow transport into distinct regions. All four of these approaches share the common trait that they are objective methods, meaning that their results do not depend on the frame of reference used. For each method, we also present a number of example applications ranging from blood flow and chemical reactions to ocean and atmospheric flows. (C) 2015 AIP Publishing LLC.ONR N000141210665Center for Nonlinear Dynamic
Throughput analysis for cognitive radio networks with multiple primary users and imperfect spectrum sensing
In cognitive radio networks, the licensed frequency bands of the primary users (PUs) are available to the secondary user (SU) provided that they do not cause significant interference to the PUs. In this study, the authors analysed the normalised throughput of the SU with multiple PUs coexisting under any frequency division multiple access communication protocol. The authors consider a cognitive radio transmission where the frame structure consists of sensing and data transmission slots. In order to achieve the maximum normalised throughput of the SU and control the interference level to the legal PUs, the optimal frame length of the SU is found via simulation. In this context, a new analytical formula has been expressed for the achievable normalised throughput of SU with multiple PUs under prefect and imperfect spectrum sensing scenarios. Moreover, the impact of imperfect sensing, variable frame length of SU and the variable PU traffic loads, on the normalised throughput has been critically investigated. It has been shown that the analytical and simulation results are in perfect agreement. The authors analytical results are much useful to determine how to select the frame duration length subject to the parameters of cognitive radio network, such as network traffic load, achievable sensing accuracy and number of coexisting PUs
After heat distribution of a mobile nuclear power plant
A computer program was developed to analyze the transient afterheat temperature and pressure response of a mobile gas-cooled reactor power plant following impact. The program considers (in addition to the standard modes of heat transfer) fission product decay and transport, metal-water reactions, core and shield melting and displacement, and pressure and containment vessel stress response. Analyses were performed for eight cases (both deformed and undeformed models) to verify operability of the program options. The results indicated that for a 350 psi (241 n/sq cm) initial internal pressure, the containment vessel can survive over 100,000 seconds following impact before creep rupture occurs. Recommendations were developed as to directions for redesign to extend containment vessel life
Multi-Modes Phonon Softening in Two-Dimensional Electron-Lattice System
Phonon dispersion in a two-dimensional electron-lattice system described by a
two-dimensional square-lattice version of Su-Schrieffer-Heeger's model and
having the half-filled electronic band is studied theoretically at temperatures
higher than the mean field critical temperature of the Peierls transition. When
the temperature is lowered from the higher region down to the critical one,
softening of multi phonon modes which have wave vectors equal to the nesting
vector \vv{Q}=(\pi/a,\pi/a) with the lattice constant or parallel to
\vv{Q} is observed. Although both of the transverse and longitudinal modes
are softened at the critical temperature in the case of the wave vector equal
to \vv{Q}, only the transverse modes are softened for other wave vectors
parallel to \vv{Q}. This behavior is consistent with the Peierls distortions
at lower temperatures.Comment: 10 pages, 5 Figure
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