1,880 research outputs found
Vibrational signatures for low-energy intermediate-sized Si clusters
We report low-energy locally stable structures for the clusters Si20 and Si21. The structures were obtained by performing geometry optimizations within the local density approximation. Our calculated binding energies for these clusters are larger than any previously reported for this size regime. To aid in the experimental identification of the structures, we have computed the full vibrational spectra of the clusters, along with the Raman and IR activities of the various modes using a recently developed first-principles technique. These represent, to our knowledge, the first calculations of Raman and IR spectra for Si clusters of this size
Density-functional-based predictions of Raman and IR spectra for small Si clusters
We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared (IR) intensities for all of the vibrational modes of several clusters (SiN with N=3-8, 10, 13, 20, and 21) using the local density approximation (LDA). For the smaller clusters (N=3-8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si10 and for the lowest-energy structure of Si13 found to date. For the static response, we compare our calculated polarizabilities for N=10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems
Coupling to haloform molecules in intercalated C60?
For field-effect-doped fullerenes it was reported that the superconducting
transition temperature Tc is markedly larger for C60.2CHX_3 (X=Cl, Br)
crystals, than for pure C60. Initially this was explained by the expansion of
the volume per C60-molecule and the corresponding increase in the density of
states at the Fermi level in the intercalated crystals. On closer examination
it has, however, turned out to be unlikely that this is the mechanism behind
the increase in Tc. An alternative explanation of the enhanced transition
temperatures assumes that the conduction electrons not only couple to the
vibrational modes of the C60-molecule, but also to the modes of the
intercalated molecules. We investigate the possibility of such a coupling. We
find that, assuming the ideal bulk structure of the intercalated crystal, both
a coupling due to hybridization of the molecular levels, and a coupling via
dipole moments should be very small. This suggests that the presence of the
gate-oxide in the field-effect-devices strongly affects the structure of the
fullerene crystal at the interface.Comment: 4 pages, 1 figure, to be published in PRB (rapid communication
Electronic Structure of Superconducting Ba6c60
We report the results of first-principles electronic-structure calculations
for superconducting Ba6C60. Unlike the A3C60 superconductors, this new compound
shows strong Ba-C hybridization in the valence and conduction regions, mixed
covalent/ionic bonding character, partial charge transfer, and insulating
zero-gap band structure.Comment: 11 pages + 4 figures (1 appended, others on request), LaTeX with
REVTE
Coulomb parameters and photoemission for the molecular metal TTF-TCNQ
We employ density-functional theory to calculate realistic parameters for an
extended Hubbard model of the molecular metal TTF-TCNQ. Considering both intra-
and intermolecular screening in the crystal, we find significant longer-range
Coulomb interactions along the molecular stacks, as well as inter-stack
coupling. We show that the long-range Coulomb term of the extended Hubbard
model leads to a broadening of the spectral density, likely resolving the
problems with the interpretation of photoemission experiments using a simple
Hubbard model only.Comment: 4 pages, 2 figure
Theory for transport through a single magnetic molecule: Endohedral N@C60
We consider transport through a single N@C60 molecule, weakly coupled to
metallic leads. Employing a density-matrix formalism we derive rate equations
for the occupation probabilities of many-particle states of the molecule. We
calculate the current-voltage characteristics and the differential conductance
for N@C60 in a break junction. Our results reveal Coulomb-blockade behavior as
well as a fine structure of the Coulomb-blockade peaks due to the exchange
coupling of the C60 spin to the spin of the encapsulated nitrogen atom.Comment: 5 pages, 4 figures, v2: version as publishe
Emission estimates of HCFCs and HFCs in California from the 2010 CalNex study
The CalNex 2010 (California Research at the Nexus of Air Quality and Climate Change) study was designed to evaluate the chemical composition of air masses over key source regions in California. During May to June 2010, air samples were collected on board a National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft over the South Coast Air Basin of California (SoCAB) and the Central Valley (CV). This paper analyzes six effective greenhouse gases - chlorodifluoromethane (HCFC-22), 1,1-dichloro-1-fluoroethane (HCFC-141b), 1-chloro-1,1-difluoroethane (HCFC-142b), 2-chloro-1,1,1,2-tetrafluoroethane (HCFC-124), 1,1,1,2- tetrafluoroethane (HFC-134a), and 1,1-difluoroethane (HFC-152a) - providing the most comprehensive characterization of chlorofluorocarbon (CFC) replacement compound emissions in California. Concentrations of measured HCFCs and HFCs are enhanced greatly throughout the SoCAB and CV, with highest levels observed in the SoCAB: 310 ± 92 pptv for HCFC-22, 30.7 ± 18.6 pptv for HCFC-141b, 22.9 ± 2.0 pptv for HCFC-142b, 4.86 ± 2.56 pptv for HCFC-124, 109 ± 46.4 pptv for HFC-134a, and 91.2 ± 63.9 pptv for HFC-152a. Annual emission rates are estimated for all six compounds in the SoCAB using the measured halocarbon to carbon monoxide (CO) mixing ratios and CO emissions inventories. Emission rates of 3.05 ± 0.70 Gg for HCFC-22, 0.27 ± 0.07 Gg for HCFC-141b, 0.06 ± 0.01 Gg for HCFC-142b, 0.11 ± 0.03 Gg for HCFC-124, 1.89 ± 0.43 Gg for HFC-134a, and 1.94 ± 0.45 Gg for HFC-152b for the year 2010 are calculated for the SoCAB. These emissions are extrapolated from the SoCAB region to the state of California using population data. Results from this study provide a baseline emission rate that will help future studies determine if HCFC and HFC mitigation strategies are successful. Key PointsHCFC and HFC emissions are calculated for the year 2010 for the SoCABEmissions are extrapolated to the state of CaliforniaEmissions are calculated using CalNex field measurements © 2013. American Geophysical Union. All Rights Reserved
Effects of Caffeinated and Non-Caffeinated Gum on Premotor, Motor, and Overall Reaction Time
Chewing gum and caffeine when used independently and concurrently increase neural activity ultimately improving reaction time but less is known about how caffeinated gum influences distinct phases of the reaction time response. Physically active college females (n=14) completed a 60-second reaction time test on a visuomotor board under the following counterbalanced conditions: 1) Baseline, 2) Non-caffeinated gum, 3) Caffeinated gum (300 mg caffeine). Point of application #1: Chewing gum improved premotor reaction time compared to baseline, but caffeine did not provide additional benefit. Point of application #2: Neither non-caffeinated nor caffeinated gum improved motor reaction times from baseline. Point of application #3: Chewing gum improved overall reaction time compared to baseline, but caffeine did not provide additional benefit. Keywords: Chewing, Visuomotor, Visual processin
Discovering and verifying DNA polymorphisms in a mung bean [V. radiata (L.) R. Wilczek] collection by EcoTILLING and sequencing
<p>Abstract</p> <p>Background</p> <p><it>Vigna radiata</it>, which is classified in the family Fabaceae, is an important economic crop and a dietary staple in many developing countries. The species <it>radiata </it>can be further subdivided into varieties of which the variety <it>sublobata </it>is currently acknowledged as the putative progenitor of <it>radiata</it>. EcoTILLING was employed to identify single nucleotide polymorphisms (SNPs) and small insertions/deletions (INDELS) in a collection of <it>Vigna radiata </it>accessions.</p> <p>Findings</p> <p>A total of 157 DNA polymorphisms in the collection were produced from ten primer sets when using <it>V. radiata </it>var. <it>sublobata </it>as the reference. The majority of polymorphisms detected were found in putative introns. The banding patterns varied from simple to complex as the number of DNA polymorphisms between two pooled samples increased. Numerous SNPs and INDELS ranging from 4–24 and 1–6, respectively, were detected in all fragments when pooling <it>V. radiata </it>var. <it>sublobata </it>with <it>V. radiata </it>var. <it>radiata</it>. On the other hand, when accessions of <it>V. radiata </it>var. <it>radiata </it>were mixed together and digested with CEL I relatively few SNPs and no INDELS were detected.</p> <p>Conclusion</p> <p>EcoTILLING was utilized to identify polymorphisms in a collection of mung bean, which previously showed limited molecular genetic diversity and limited morphological diversity in the flowers and pod descriptors. Overall, EcoTILLING proved to be a powerful genetic analysis tool providing the rapid identification of naturally occurring variation.</p
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