5,922 research outputs found
When Uncle Sam Gets Fighting Mad
https://digitalcommons.library.umaine.edu/mmb-vp/5363/thumbnail.jp
Observation of Surface-Avoiding Waves: A New Class of Extended States in Periodic Media
Coherent time-domain optical experiments on GaAs-AlAs superlattices reveal
the exis-tence of an unusually long-lived acoustic mode at ~ 0.6 THz, which
couples weakly to the environment by evading the sample boundaries. Classical
as well as quantum states that steer clear of surfaces are generally shown to
occur in the spectrum of periodic struc-tures, for most boundary conditions.
These surface-avoiding waves are associated with frequencies outside forbidden
gaps and wavevectors in the vicinity of the center and edge of the Brillouin
zone. Possible consequences for surface science and resonant cavity
ap-plications are discussed.Comment: 16 pages, 3 figure
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Birth seasonality studies in a large Prader-Willi syndrome cohort.
Prader-Willi syndrome (PWS) is generally due to sporadic paternal deletions of the chromosome 15q11-q13 region followed by maternal disomy 15. Advanced maternal age is more commonly seen in those with maternal disomy 15. Environmental factors (e.g., drug use, occupational chemical exposure, infectious agents, and irradiation) could account for chromosome changes. Previous evidence of differences in male and female gametogenesis could suggest an environmental role in the causation of the paternal 15q11-q13 deletion seen in PWS. Certain occupations such as hydrocarbon-exposing occupations (e.g., landscaping, farming, and painting) and viral exposure (e.g., human coronavirus 229E causing upper respiratory infections in adults with an incorporation site in the human genome at chromosome 15q11) can be seasonal in nature and contribute to chromosome damage. To assess, we reviewed birth seasonality data in a large cohort of individuals with PWS recruited nationally (N = 355) but no significant differences were seen by month between those with the 15q11-q13 deletion compared with maternal disomy 15 when analyzing quarterly seasonal patterns. Although early evidence supported birth seasonality differences in PWS, a larger number of individuals in our recent study using advanced genetic testing methods did not find this observation
Plasduino: an inexpensive, general purpose data acquisition framework for educational experiments
Based on the Arduino development platform, Plasduino is an open-source data
acquisition framework specifically designed for educational physics
experiments. The source code, schematics and documentation are in the public
domain under a GPL license and the system, streamlined for low cost and ease of
use, can be replicated on the scale of a typical didactic lab with minimal
effort. We describe the basic architecture of the system and illustrate its
potential with some real-life examples.Comment: 11 pages, 10 figures, presented at the XCIX conference of the
Societ\`a Italiana di Fisic
Photon Diffusion in Microscale Solids
This paper presents a theoretical and experimental investigation of photon
diffusion in highly absorbing microscale graphite. An Nd:YAG continuous wave
(CW) laser is used to heat the graphite samples with thicknesses of 40 microns
and 100 microns. Optical intensities of 10 kW/cm^2 and 20 kW/cm^2 are used in
laser heating. The graphite samples are heated to temperatures of thousands of
kelvins within milliseconds, which are recorded by a 2-color, high-speed
pyrometer. To compare the observed temperatures, the differential equation of
heat conduction is solved across the samples with proper initial and boundary
conditions. In addition to lattice vibrations, photon diffusion is incorporated
into the analytical model of thermal conductivity for solving the heat
equation. The numerical simulations showed close matching between experiment
and theory only when including the photon diffusion equations and existing
material properties data found in the previously published works with no
fitting constants. The results indicate that the commonly-overlooked mechanism
of photon diffusion dominates the heat transfer of many microscale structures
near their evaporation temperatures. In addition, the treatment explains the
discrepancies between thermal conductivity measurements and theory that were
previously described in the scientific literature.Comment: 8 pages, 7 figures, (N.B. there is a typo and minor correction in
Table 1 and References in the online version of the journal, corrected and
highlighted in this PDF
Exciton Optical Absorption in Self-Similar Aperiodic Lattices
Exciton optical absorption in self-similar aperiodic one-dimensional systems
is considered, focusing our attention on Thue-Morse and Fibonacci lattices as
canonical examples. The absorption line shape is evaluated by solving the
microscopic equations of motion of the Frenkel-exciton problem on the lattice,
in which on-site energies take on two values, according to the Thue-Morse or
Fibonacci sequences. Results are compared to those obtained in random lattices
with the same stechiometry and size. We find that aperiodic order causes the
occurrence of well-defined characteristic features in the absorption spectra
which clearly differ from the case of random systems, indicating a most
peculiar exciton dynamics. We successfully explain the obtained spectra in
terms of the two-center problem. This allows us to establish the origin of all
the absorption lines by considering the self-similar aperiodic lattices as
composed of two-center blocks, within the same spirit of the renormalization
group ideas.Comment: 16 pages in REVTeX 3.0. 2 figures on request to F. D-A
([email protected]
Novel approach to a perfect lens
Within the framework of an exact analytical solution of Maxwell equations in
a space domain, it is shown that optical scheme based on a slab with negative
refractive index () (Veselago lens or Pendry lens) does not possess
focusing properties in the usual sense . In fact, the energy in such systems
does not go from object to its "image", but from object and its "image" to an
intersection point inside a metamaterial layer, or vice versa. A possibility of
applying this phenomenon to a creation of entangled states of two atoms is
discussed.Comment: 4 pages, 6 figure
Orbital mechanisms of electron spin manipulation by an electric field
A theory of spin manipulation of quasi-two-dimensional (2D) electrons by a
time-dependent gate voltage applied to a quantum well is developed. The
Dresselhaus and Rashba spin-orbit coupling mechanisms are shown to be rather
efficient for this purpose. The spin response to a perpendicular-to-plane
electric field is due to a deviation from the strict 2D limit and is controlled
by the ratios of the spin, cyclotron and confinement frequencies. The
dependence of this response on the magnetic field direction is indicative of
the strenghts of the competing spin-orbit coupling mechanisms
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Early Diagnosis in Prader-Willi Syndrome Reduces Obesity and Associated Co-Morbidities.
Prader-Willi syndrome (PWS) is an imprinting genetic disorder characterized by lack of expression of genes on the paternal chromosome 15q11-q13 region. Growth hormone (GH) replacement positively influences stature and body composition in PWS. Our hypothesis was that early diagnosis delays onset of obesity in PWS. We studied 352 subjects with PWS, recruited from the NIH Rare Disease Clinical Research Network, to determine if age at diagnosis, ethnicity, gender, and PWS molecular class influenced the age they first become heavy, as determined by their primary care providers, and the age they first developed an increased appetite and began seeking food. The median ages that children with PWS became heavy were 10 years, 6 years and 4 years for age at diagnosis < 1 year, between 1 and 3 years, and greater than 3 years of age, respectively. The age of diagnosis and ethnicity were significant factors influencing when PWS children first became heavy (p < 0.01), however gender and the PWS molecular class had no influence. Early diagnosis delayed the onset of becoming heavy in individuals with PWS, permitting early GH and other treatment, thus reducing the risk of obesity-associated co-morbidities. Non-white individuals had an earlier onset of becoming heavy
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