1,164 research outputs found
Tunnelling in quantum superlattices with variable lacunarity
Quantum fractal superlattices are microelectronic devices consisting of a
series of thin layers of two semiconductor materials deposited alternately on
each other over a substrate following the rules of construction of a fractal
set, here, a symmetrical polyadic Cantor fractal. The scattering properties of
electrons in these superlattices may be modeled by using that of quantum
particles in piecewise constant potential wells. The twist plots representing
the reflection coefficient as function of the lacunarity parameter show the
appearance of black curves with perfectly transparent tunnelling which may be
classified as vertical, arc, and striation nulls. Approximate analytical
formulae for these reflection-less curves are derived using the transfer matrix
method. Comparison with the numerical results show their good accuracy.Comment: 12 pages, 3 figure
Constructing a rational kinetic model of the selective propane oxidation over a mixed metal oxide catalyst
This research presents a kinetic investigation of the selective oxidation of propane to acrylic acid over a MoVTeNb oxide (M1 phase) catalyst. The paper contains both an overview of the related literature, and original results with a focus on kinetic aspects. Two types of kinetic experiments were performed in a plug flow reactor, observing (i) steady-state conditions (partial pressure variations) and (ii) the catalyst evolution as a function of time-on-stream. For this, the catalyst was treated in reducing atmosphere, before re-oxidising it. These observations in long term behaviour were used to distinguish different catalytic routes, namely for the formation of propene, acetic acid, acrylic acid, carbon monoxide and carbon dioxide. A partial carbon balance was introduced, which is a ‘kinetic fingerprint’, that distinguishes one type of active site from another. Furthermore, an ‘active site’ was found to consist of one or more ‘active centres’. A rational mechanism was developed based on the theory of graphs and includes two time scales belonging to (i) the catalytic cycle and (ii) the catalyst evolution. Several different types of active sites exist, at least as many, as kinetically independent product molecules are formed over a catalyst surface
Enhanced observability of quantum post-exponential decay using distant detectors
We study the elusive transition from exponential to post-exponential
(algebraic) decay of the probability density of a quantum particle emitted by
an exponentially decaying source, in one dimension. The main finding is that
the probability density at the transition time, and thus its observability,
increases with the distance of the detector from the source, up to a critical
distance beyond which exponential decay is no longer observed. Solvable models
provide explicit expressions for the dependence of the transition on resonance
and observational parameters, facilitating the choice of optimal conditions
Spatial properties of conjugated network in semicrystalline polymer thin films studied by intensity x-ray cross-correlation functions
We present results of x-ray study of spatial properties of
conjugated networks in polymer thin films. We applied the x-ray
cross-correlation analysis to x-ray scattering data from blends of
poly(3-hexylthiophene) (P3HT) and gold nanoparticles. The Fourier spectra of
the intensity cross-correlation functions for different films contain non-zero
components of orders and measuring the degree of structural order
in the system.Comment: 6 pages, 2 figures, Proceedings ICXOM22 Conference, 2-6 September
2013, Hamburg, German
Nuclear Charge Density Distributions from Elastic Electron Scattering Data
The model independent procedure of extracting charge density distributions
from elastic electron scattering data is investigated. The charge density
distributions are expanded on an orthonormal basis and the parameters of the
expansions are fixed by the comparison with the experimental data. Two bases
with different analytical properties (Fourier-Bessel and Hermite) are used.
This allows us to disentangle the uncertainties coming from the choice of the
expansion basis from those intrinsic to the extraction procedure. We design a
set of tests to select the number of the expansion coefficients adequate for a
proper description of the data. The procedure is applied to elastic data
measured on C, Ca and Pb nuclei.Comment: 17 pages, 11 figures available on request to [email protected], to
be published in Nucl.Phy
Neighborhood crime is differentially associated with cardiovascular risk factors as a function of race and sex
Background: Neighborhood crime may be an important factor contributing to cardiovascular health disparities, and these relations may vary by race and sex. The present investigation evaluated (a) potential differential associations between neighborhood crime and cardiovascular disease (CVD) risk factors within subgroups of African American (AA) and White men and women, and (b) potential mediation by negative affect.
Design and Methods: Participants were 1,718 AAs and Whites (58% AA; 54% female; 59% above poverty; ages 30-64 years) living in Baltimore, Maryland who completed the first wave of the Healthy Aging in Neighborhoods of Diversity across the Life Span study from 2004-2009. CVD risk factors included body mass index, total serum cholesterol, glucose, and systolic and diastolic blood pressure. A negative affect composite was comprised of self-reported depression, anxiety, anger, vigilance, and perceived stress. Hierarchical multiple regression analyses were used to examine associations between per capita overall and violent crime rates, negative affect, and CVD risk factors.
Results: There were significant associations of greater overall crime rate with higher fasting glucose (b=.192, P<0.05), and greater violent crime rate with higher systolic (b=86.50, P<0.05) and diastolic (b=60.12, P<0.05) blood pressure in AA women, but not men. These associations were not explained by negative affect. In Whites, there were no significant associations of overall or violent crime rates with cardiovascular risk factors.
Conclusions: AA women may be particularly vulnerable to the negative impact of crime on cardiovascular risk. Preventative efforts aimed toward this group may help to deter the detrimental effects that living in a high crime area may have on one’s cardiovascular health
Long time deviation from exponential decay: non-integral power laws
Quantal systems are predicted to show a change-over from exponential decay to
power law decay at very long times. Although most theoretical studies predict
integer power-law exponents, recent measurements by Rothe et al. of decay
luminescence of organic molecules in solution {Phys. Rev. Lett. 96 (2006)
163601} found non-integer exponents in most cases. We propose a physical
mechanism, within the realm of scattering from potentials with long tails,
which produces a continuous range of power law exponents. In the tractable case
of the repulsive inverse square potential, we demonstrate a simple relation
between the strength of the long range tail and the power law exponent. This
system is amenable to experimental scrutiny
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