32,511 research outputs found
The infrared spectra of ABC-stacking tri- and tetra-layer graphenes studied by first-principles calculations
The infrared absorption spectra of ABC-stacking tri- and tetra-layer
graphenes are studied using the density functional theory. It is found that
they exhibit very different characteristic peaks compared with those of
AB-stacking ones, caused by the different stacking sequence and interlayer
coupling. The anisotropy of the spectra with respect to the direction of the
light electric field is significant. The spectra are more sensitive to the
stacking number when the electric field is perpendicular to the graphene plane
due to the interlayer polarization. The high sensitivities make it possible to
identify the stacking sequence and stacking number of samples by comparing
theory and experiment.Comment: 7 pages, 5 figure
Electrodeposition from supercritical fluids
Recent studies have shown that it is possible to electrodeposit a range of materials, such as Cu, Ag and Ge, from various supercritical fluids, including hydrofluorocarbons and mixtures of CO2 with suitable co-solvents. In this perspective we discuss the relatively new field of electrodeposition from supercritical fluids. The perspective focuses on some of the underlying physical chemistry and covers both practical and scientific aspects of electrodeposition from supercritical fluids. We also discuss possible applications for supercritical fluid electrodeposition and suggest some key developments that are required to take the field to the next stage
Spin transfer torque on magnetic insulators
Recent experimental and theoretical studies focus on spin-mediated heat
currents at interfaces between normal metals and magnetic insulators. We
resolve conflicting estimates for the order of magnitude of the spin transfer
torque by first-principles calculations. The spin mixing conductance
G^\uparrow\downarrow of the interface between silver and the insulating
ferrimagnet Yttrium Iron Garnet (YIG) is dominated by its real part and of the
order of 10^14 \Omega^-1m^-2, i.e. close to the value for intermetallic
interface, which can be explained by a local spin model.Comment: 4 pages, 4 figures, 2 table
Sensitivity of Ag/Al Interface Specific Resistances to Interfacial Intermixing
We have measured an Ag/Al interface specific resistance, 2AR(Ag/Al)(111) =
1.4 fOhm-m^2, that is twice that predicted for a perfect interface, 50% larger
than for a 2 ML 50%-50% alloy, and even larger than our newly predicted 1.3
fOhmm^2 for a 4 ML 50%-50% alloy. Such a large value of 2ARAg/Al(111) confirms
a predicted sensitivity to interfacial disorder and suggests an interface
greater than or equal to 4 ML thick. From our calculations, a predicted
anisotropy ratio, 2AR(Ag/Al)(001)/2AR(Ag/Al)(111), of more then 4 for a perfect
interface, should be reduced to less than 2 for a 4 ML interface, making it
harder to detect any such anisotropy.Comment: 3 pages, 2 figures, 1 table. In Press: Journal of Applied Physic
Influence of post-stroke fatigue on reaction times and corticospinal excitability during movement preparation
OBJECTIVES: Reduced corticospinal excitability at rest is associated with post-stroke fatigue (PSF). However, it is not known if corticospinal excitability prior to a movement is also altered in fatigue which may then influence subsequent behaviour. We hypothesized that the levels of PSF can be explained by differences in modulation of corticospinal excitability during movement preparation. METHODS: 73 stroke survivors performed an auditory reaction time task. Corticospinal excitability was measured using transcranial magnetic stimulation. Fatigue was quantified using the fatigue severity scale. The effect of time and fatigue on corticospinal excitability and reaction time was analysed using a mixed effects model. RESULTS: Those with greater levels of PSF showed reduced suppression of corticospinal excitability during movement preparation and increased facilitation immediately prior to movement onset (β = -0.0066, t = -2.22, p = 0.0263). Greater the fatigue, slower the reaction times the closer the stimulation time to movement onset (β = 0.0024, t = 2.47, p = 0.0159). CONCLUSIONS: Lack of pre-movement modulation of corticospinal excitability in high fatigue may indicate poor sensory processing supporting the sensory attenuation model of fatigue. SIGNIFICANCE: We take a systems-based approach and investigate the motor system and its role in pathological fatigue allowing us to move towards gaining a mechanistic understanding of chronic pathological fatigue
Vertex functions for d-wave mesons in the light-front approach
While the light-front quark model (LFQM) is employed to calculate hadronic
transition matrix elements, the vertex functions must be pre-determined. In
this work we derive the vertex functions for all d-wave states in this model.
Especially, since both of and are mesons, the Lorentz
structures of their vertex functions are the same. Thus when one needs to study
the processes where is involved, all the corresponding formulas for
states can be directly applied, only the coefficient of the vertex
function should be replaced by that for . The results would be useful
for studying the newly observed resonances which are supposed to be d-wave
mesons and furthermore the possible 2S-1D mixing in with the LFQM.Comment: 12 pages, 2 figures, some typos corrected and more discussions added.
Accepted by EPJ
On Silicon Carbide Grains as the Carrier of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars
The mysterious 21mu emission feature seen in 12 proto-planetary nebulae
(PPNe) remains unidentified since its first detection in 1989. Over a dozen of
candidate materials have been proposed within the past decade, but none of them
has received general acceptance. Very recently, silicon carbide (SiC) grains
with impurities were suggested to be the carrier of this enigmatic feature,
based on recent laboratory data that doped SiC grains exhibit a resonance at
\~21mu. This proposal gains strength from the fact that SiC is a common dust
species in carbon-rich circumstellar envelopes. However, SiC dust has a strong
vibrational band at ~11.3mu. We show in this Letter that in order to be
consistent with the observed flux ratios of the 11.3mu feature to the 21mu
feature, the band strength of the 21mu resonance has to be very strong, too
strong to be consistent with current laboratory measurements. But this does not
yet readily rule out the SiC hypothesis since recent experimental results have
demonstrated that the 21mu resonance of doped SiC becomes stronger as the C
impurity increases. Further laboratory measurements of SiC dust with high
fractions of C impurity are urgently needed to test the hypothesis of SiC as
the carrier of the 21mu feature.Comment: 14 pages, 3 figures, accepted for publication in ApJ
Symmetry of Dirac Equation and Corresponding Phenomenology
It has been suggested that the high symmetries in the Schr\"odinger equation
with the Coulomb or harmonic oscillator potentials may remain in the
corresponding relativistic Dirac equation. If the principle is correct, in the
Dirac equation the potential should have a form as
where is for hydrogen atom and for harmonic
oscillator. However, in the case of hydrogen atom, by this combination the
spin-orbit coupling term would not exist and it is inconsistent with the
observational spectra of hydrogen atom, so that the symmetry of SO(4) must
reduce into SU(2). The governing mechanisms QED and QCD which induce potential
are vector-like theories, so at the leading order only vector potential exists.
However, the higher order effects may cause a scalar fraction. In this work, we
show that for QED, the symmetry restoration is very small and some discussions
on the symmetry breaking are made. At the end, we briefly discuss the QCD case
and indicate that the situation for QCD is much more complicated and
interesting.Comment: 15pages, 3 figures, accepted by International Journal of Modern
Physics
Absorbate-Induced Piezochromism in a Porous Molecular Crystal
Atmospherically stable porous frameworks
and materials are interesting
for heterogeneous solid–gas applications. One motivation is
the direct and selective uptake of pollutant/hazardous gases, where
the material produces a measurable response in the presence of the
analyte. In this report, we present a combined experimental and theoretical
rationalization for the piezochromic response of a robust and porous
molecular crystal built from an extensively fluorinated trispyrazole.
The electronic response of the material is directly determined by
analyte uptake, which provokes a subtle lattice contraction and an
observable bathochromic shift in the optical absorption onset. Selectivity
for fluorinated absorbates is demonstrated, and toluene is also found
to crystallize within the pore. Furthermore, we demonstrate the application
of electronic structure calculations to predict a physicochemical
response, providing the foundations for the design of electronically
tunable porous solids with the chemical properties required for development
of novel gas-uptake media
Structural Studies on a Mitochondrial Glyoxalase II
Glyoxalase 2 is a β-lactamase fold-containing enzyme that appears to be involved with cellular chemical detoxification. Although the cytoplasmic isozyme has been characterized from several organisms, essentially nothing is known about the mitochondrial proteins. As a first step in understanding the structure and function of mitochondrial glyoxalase 2 enzymes, a mitochondrial isozyme (GLX2-5) from Arabidopsis thaliana was cloned, overexpressed, purified, and characterized using metal analyses, EPR and 1H NMR spectroscopies, and x-ray crystallography. The recombinant enzyme was shown to bind 1.04 ± 0.15 eq of iron and 1.31 ± 0.05 eq of Zn(II) and to exhibit kcat and Km values of 129 ± 10 s-1 and 391 ± 48 μm, respectively, when using S-d-lactoylglutathione as the substrate. EPR spectra revealed that recombinant GLX2-5 contains multiple metal centers, including a predominant Fe(III)Z-n(II) center and an anti-ferromagnetically coupled Fe(III)Fe(II) center. Unlike cytosolic glyoxalase 2 from A. thaliana, GLX2-5 does not appear to specifically bind manganese. 1H NMR spectra revealed the presence of at least eight paramagnetically shifted resonances that arise from protons in close proximity to a Fe(III)Fe(II) center. Five of these resonances arose from solvent-exchangeable protons, and four of these have been assigned to NH protons on metal-bound histidines. A 1.74-Å resolution crystal structure of the enzyme revealed that although GLX2-5 shares a number of structural features with human GLX2, several important differences exist. These data demonstrate that mitochondrial glyoxalase 2 can accommodate a number of different metal centers and that the predominant metal center is Fe(III)Zn(II)
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