47,948 research outputs found
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
Degeneracy of Ground State in Two-dimensional Electron-Lattice System
We discuss the ground state of a two dimensional electron-lattice system
described by a Su-Schrieffer-Heeger type Hamiltonian with a half-filled
electronic band, for which it has been pointed out in the previous paper [J.
Phys. Soc. Jpn. 69 (2000) 1769-1776] that the ground state distortion pattern
is not unique in spite of a unique electronic energy spectrum and the same
total energy. The necessary and sufficient conditions to be satisfied by the
distortion patterns in the ground state are derived numerically. As a result
the degrees of degeneracy in the ground state is estimated to be about
for with the linear dimension of the system.Comment: 2pages, 2figure
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
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
Severity classification of ground-glass opacity via 2-D convolutional neural network and lung CT scans: a 3-day exploration
Ground-glass opacity is a hallmark of numerous lung diseases, including
patients with COVID19 and pneumonia, pulmonary fibrosis, and tuberculosis. This
brief note presents experimental results of a proof-of-concept framework that
got implemented and tested over three days as driven by the third challenge
entitled "COVID-19 Competition", hosted at the AI-Enabled Medical Image
Analysis Workshop of the 2023 IEEE International Conference on Acoustics,
Speech and Signal Processing (ICASSP 2023). Using a newly built virtual
environment (created on March 17, 2023), we investigated various pre-trained
two-dimensional convolutional neural networks (CNN) such as Dense Neural
Network, Residual Neural Networks (ResNet), and Vision Transformers, as well as
the extent of fine-tuning. Based on empirical experiments, we opted to
fine-tune them using ADAM's optimization algorithm with a standard learning
rate of 0.001 for all CNN architectures and apply early-stopping whenever the
validation loss reached a plateau. For each trained CNN, the model state with
the best validation accuracy achieved during training was stored and later
reloaded for new classifications of unseen samples drawn from the validation
set provided by the challenge organizers. According to the organizers, few of
these 2D CNNs yielded performance comparable to an architecture that combined
ResNet and Recurrent Neural Network (Gated Recurrent Units). As part of the
challenge requirement, the source code produced during the course of this
exercise is posted at https://github.com/lisatwyw/cov19. We also hope that
other researchers may find this light prototype consisting of few Python files
based on PyTorch 1.13.1 and TorchVision 0.14.1 approachable
Properties of Resonating-Valence-Bond Spin Liquids and Critical Dimer Models
We use Monte Carlo simulations to study properties of Anderson's
resonating-valence-bond (RVB) spin-liquid state on the square lattice (i.e.,
the equal superposition of all pairing of spins into nearest-neighbor singlet
pairs) and compare with the classical dimer model (CDM). The latter system also
corresponds to the ground state of the Rokhsar-Kivelson quantum dimer model at
its critical point. We find that although spin-spin correlations decay
exponentially in the RVB, four-spin valence-bond-solid (VBS) correlations are
critical, qualitatively like the well-known dimer-dimer correlations of the
CDM, but decaying more slowly (as with , compared with
for the CDM). We also compute the distribution of monomer (defect) pair
separations, which decay by a larger exponent in the RVB than in the CDM. We
further study both models in their different winding number sectors and
evaluate the relative weights of different sectors. Like the CDM, all the
observed RVB behaviors can be understood in the framework of a mapping to a
"height" model characterized by a gradient-squared stiffness constant . Four
independent measurements consistently show a value , with the same kinds of numerical evaluations of give
results in agreement with the rigorously known value . The
background of a nonzero winding number gradient introduces spatial
anisotropies and an increase in the effective K, both of which can be
understood as a consequence of anharmonic terms in the height-model free
energy, which are of relevance to the recently proposed scenario of "Cantor
deconfinement" in extended quantum dimer models. We also study ensembles in
which fourth-neighbor (bipartite) bonds are allowed, at a density controlled by
a tunable fugacity, resulting (as expected) in a smooth reduction of K.Comment: 26 pages, 21 figures. v3: final versio
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