53,101 research outputs found
Unconventional scanning tunneling conductance spectra for graphene
We compute the tunneling conductance of graphene as measured by a scanning
tunneling microscope (STM) with a normal/superconducting tip. We demonstrate
that for undoped graphene with zero Fermi energy, the first derivative of the
tunneling conductance with respect to the applied voltage is proportional to
the density of states of the STM tip. We also show that the shape of the STM
spectra for graphene doped with impurities depends qualitatively on the
position of the impurity atom in the graphene matrix and relate this
unconventional phenomenon to the pseudopsin symmetry of the Dirac
quasiparticles in graphene. We suggest experiments to test our theory.Comment: 6 pages, 3 figure
Eliminating Redundant Training Data Using Unsupervised Clustering Techniques
Training data for supervised learning neural networks can be clustered such that the input/output pairs in each cluster are redundant. Redundant training data can adversely affect training time. In this paper we apply two clustering algorithms, ART2 -A and the Generalized Equality Classifier, to identify training data clusters and thus reduce the training data and training time. The approach is demonstrated for a high dimensional nonlinear continuous time mapping. The demonstration shows six-fold decrease in training time at little or no loss of accuracy in the handling of evaluation data
Study and interpretation of the millimeter-wave spectrum of Venus
The effects of the Venus atmospheric constituents on its millimeter wavelength emission are investigated. Specifically, this research describes the methodology and the results of laboratory measurements which are used to calculate the opacity of some of the major absorbers in the Venus atmosphere. The pressure broadened absorption of gaseous SO2/CO2 and gaseous H2SO4/CO2 has been measured at millimeter wavelengths. We have also developed new formalisms for computing the absorptivities of these gases based on our laboratory work. The complex dielectric constant of liquid sulfuric acid has been measured and the expected opacity from the liquid sulfuric acid cloud layer found in the atmosphere of Venus has been evaluated. The partial pressure of gaseous H2SO4 has been measured which results in a more accurate estimate of the dissociation factor of H2SO4. A radiative transfer model has been developed in order to understand how each atmospheric constituent affects the millimeter wave emissions from Venus. Our results from the radiative transfer model are compared with recent observations of the micro-wave and millimeter wave emissions from Venus. Our main conclusion from this work is that gaseous H2SO4 is the most likely cause of the variation in the observed emission from Venus at 112 GHz
Understanding the variation in the millimeter-wave emission of Venus
Recent observations of the millimeter-wave emission from Venus at 112 GHz (2.6 mm) have shown significant variations in the continuum flux emission that may be attributed to the variability in the abundances of absorbing constituents in the Venus atmosphere. Such constituents include gaseous H2SO4, SO2, and liquid sulfuric acid (cloud condensates). Recently, Fahd and Steffes have shown that the effects of liquid H, SO4, and gaseous SO2 cannot completely account for this measured variability in the millimeter-wave emission of Venus. Thus, it is necessary to study the effect of gaseous H2SO4 on the millimeter-wave emission of Venus. This requires knowledge of the millimeter-wavelength (MMW) opacity of gaseous H2SO4, which unfortunately has never been determined for Venus-like conditions. We have measured the opacity of gaseous H2SO4 in a CO2 atmosphere at 550, 570, and 590 K, at 1 and 2 atm total pressure, and at a frequency of 94.1 GHz. Our results, in addition to previous centimeter-wavelength results are used to verify a modeling formalism for calculating the expected opacity of this gaseous mixture at other frequencies. This formalism is incorporated into a radiative transfer model to study the effect of gaseous H2SO4 on the MMW emission of Venus
Orthorhombic Phase of Crystalline Polyethylene: A Monte Carlo Study
In this paper we present a classical Monte Carlo simulation of the
orthorhombic phase of crystalline polyethylene, using an explicit atom force
field with unconstrained bond lengths and angles and periodic boundary
conditions. We used a recently developed algorithm which apart from standard
Metropolis local moves employs also global moves consisting of displacements of
the center of mass of the whole chains in all three spatial directions as well
as rotations of the chains around an axis parallel to the crystallographic
c-direction. Our simulations are performed in the NpT ensemble, at zero
pressure, and extend over the whole range of temperatures in which the
orthorhombic phase is experimentally known to be stable (10 - 450 K). In order
to investigate the finite-size effects in this extremely anisotropic crystal,
we used different system sizes and different chain lengths, ranging from C_12
to C_96 chains, the total number of atoms in the super-cell being between 432
and 3456. We show here the results for structural parameters, such as the
orthorhombic cell parameters a,b,c, and the setting angle of the chains, as
well as internal parameters of the chains, such as the bond lengths and angles.
Among thermodynamic quantities, we present results for thermal expansion
coefficients, elastic constants and specific heat. We discuss the temperature
dependence of the measured quantities as well as the related finite-size
effects. In case of lattice parameters and thermal expansion coefficients, we
compare our results to those obtained from other theoretical approaches as well
as to some available experimental data. We also suggest some possible ways of
extending this study.Comment: 27 pages, RevTex, 24 figures, submitted to Journal of Chemical
Physic
Estimating European volatile organic compound emissions using satellite observations of formaldehyde from the Ozone Monitoring Instrument
Emission of non-methane Volatile Organic Compounds (VOCs) to the atmosphere
stems from biogenic and human activities, and their estimation is difficult
because of the many and not fully understood processes involved. In order to
narrow down the uncertainty related to VOC emissions, which negatively
reflects on our ability to simulate the atmospheric composition, we exploit
satellite observations of formaldehyde (HCHO), an ubiquitous oxidation
product of most VOCs, focusing on Europe. HCHO column observations from the
Ozone Monitoring Instrument (OMI) reveal a marked seasonal cycle with a
summer maximum and winter minimum. In summer, the oxidation of methane and
other long-lived VOCs supply a slowly varying background HCHO column, while
HCHO variability is dominated by most reactive VOC, primarily biogenic
isoprene followed in importance by biogenic terpenes and anthropogenic VOCs.
The chemistry-transport model CHIMERE qualitatively reproduces the temporal
and spatial features of the observed HCHO column, but display regional
biases which are attributed mainly to incorrect biogenic VOC emissions,
calculated with the Model of Emissions of Gases and Aerosol from Nature
(MEGAN) algorithm. These "bottom-up" or a-priori emissions are corrected
through a
Bayesian inversion of the OMI HCHO observations. Resulting "top-down" or
a-posteriori isoprene emissions are lower than "bottom-up" by 40% over
the Balkans
and by 20% over Southern Germany, and higher by 20% over Iberian
Peninsula, Greece and Italy.
We conclude that OMI
satellite observations of HCHO can provide a quantitative "top-down"
constraint on the European "bottom-up" VOC inventories
Interaction of Nucleosides and Related Compounds with Nucleic Acids as Indicated by the Change of Helix-Coil Transition Temperature
A series of compounds has been tested for effectiveness in lowering the melting temperature of poly A and of thymus DNA. The order of increasing activity was found to be: adonitol, methyl riboside (both negligible) < cyclohexanol < phenol, pyrimidine, uridine < cytidine, thymidine < purine, adenosine, inosine, deoxyguanosine < caffeine, coumarin, 2,6-dichloro-7-methylpurine. Urea was ineffective with poly A and only slightly effective with DNA. At a concentration of 0.3 M, purine lowered the Tm of DNA about 9°
Fe65 Is Phosphorylated on Ser289 after UV-Induced DNA Damage
Fe65 undergoes a phosphatase-sensitive gel mobility shift after DNA damage, consistent with protein phosphorylation. A recent study identified Ser228 as a specific site of phosphorylation, targeted by the ATM and ATR protein kinases, with phosphorylation inhibiting the Fe65-dependent transcriptional activity of the amyloid precursor protein (APP). The direct binding of Fe65 to APP not only regulates target gene expression, but also contributes to secretase-mediated processing of APP, producing cytoactive proteolytic fragments including the APP intracellular domain (AICD) and cytotoxic amyloid β (Aβ) peptides. Given that the accumulation of Aβ peptides in neural plaques is a pathological feature of Alzheimer’s disease (AD), it is essential to understand the mechanisms controlling Aβ production. This will aid in the development of potential therapeutic agents that act to limit the deleterious production of Aβ peptides. The Fe65-APP complex has transcriptional activity and the complex is regulated by multiple post-translational modifications and other protein binding partners. In the present study, we have identified Ser289 as a novel site of UV-induced phosphorylation. Interestingly, this phosphorylation was mediated by ATM, rather than ATR, and occurred independently of APP. Neither phosphorylation nor mutation of Ser289 affected the Fe65-APP interaction, though this was markedly decreased after UV treatment, with a concomitant decrease in the protein levels of APP in cells. Using mutagenesis, we demonstrated that Fe65 Ser289 phosphorylation did not affect the transcriptional activity of the Fe65-APP complex, in contrast to the previously described Ser228 site
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