1,364 research outputs found
THE STUDY OF HYPER ELASTIC MATERIAL CHARACTERISTICS IN CASE OF THIN ROD STRUCTURE CALCULATION
The article investigates the deformation hyper elastic material characteristics, i.e. rubber, and determines a mathematical model to calculate the characteristics of test material structure
Structural and optical investigation of non-polar (1-100) GaN grown by the ammonothermal method
We studied the structural and optical properties of state-of-the-art non-polar bulk GaN grown by the ammonothermal method. The investigated samples have an extremely low dislocation density (DD) of less than 5 × 104cm-2, which results in very narrow high-resolution x-ray rocking curves. The a and c lattice parameters of these stress-free GaN samples were precisely determined by using an x-ray diffraction technique based on the modified Bond method. The obtained values are compared to the lattice parameters of free-standing GaN from different methods and sources. The observed differences are discussed in terms of free-electron concentrations, point defects, and DD. Micro Raman spectroscopy revealed a very narrow phonon linewidth and negligible built-in strain in accordance with the high-resolution x-ray diffraction data. The optical transitions were investigated by cathodoluminescence measurements. The analysis of the experimental data clearly demonstrates the excellent crystalline perfection of ammonothermal GaN material and its potential for fabrication of non-polar substrates for homoepitaxial growth of GaN based device structures. © 2013 AIP Publishing LLC
Radio-frequency dressed state potentials for neutral atoms
Potentials for atoms can be created by external fields acting on properties
like magnetic moment, charge, polarizability, or by oscillating fields which
couple internal states. The most prominent realization of the latter is the
optical dipole potential formed by coupling ground and electronically excited
states of an atom with light. Here we present an experimental investigation of
the remarkable properties of potentials derived from radio-frequency (RF)
coupling between electronic ground states. The coupling is magnetic and the
vector character allows to design state dependent potential landscapes. On atom
chips this enables robust coherent atom manipulation on much smaller spatial
scales than possible with static fields alone. We find no additional heating or
collisional loss up to densities approaching atoms / cm compared
to static magnetic traps. We demonstrate the creation of Bose-Einstein
condensates in RF potentials and investigate the difference in the interference
between two independently created and two coherently split condensates in
identical traps. All together this makes RF dressing a powerful new tool for
micro manipulation of atomic and molecular systems
The geography of recent genetic ancestry across Europe
The recent genealogical history of human populations is a complex mosaic
formed by individual migration, large-scale population movements, and other
demographic events. Population genomics datasets can provide a window into this
recent history, as rare traces of recent shared genetic ancestry are detectable
due to long segments of shared genomic material. We make use of genomic data
for 2,257 Europeans (the POPRES dataset) to conduct one of the first surveys of
recent genealogical ancestry over the past three thousand years at a
continental scale. We detected 1.9 million shared genomic segments, and used
the lengths of these to infer the distribution of shared ancestors across time
and geography. We find that a pair of modern Europeans living in neighboring
populations share around 10-50 genetic common ancestors from the last 1500
years, and upwards of 500 genetic ancestors from the previous 1000 years. These
numbers drop off exponentially with geographic distance, but since genetic
ancestry is rare, individuals from opposite ends of Europe are still expected
to share millions of common genealogical ancestors over the last 1000 years.
There is substantial regional variation in the number of shared genetic
ancestors: especially high numbers of common ancestors between many eastern
populations likely date to the Slavic and/or Hunnic expansions, while much
lower levels of common ancestry in the Italian and Iberian peninsulas may
indicate weaker demographic effects of Germanic expansions into these areas
and/or more stably structured populations. Recent shared ancestry in modern
Europeans is ubiquitous, and clearly shows the impact of both small-scale
migration and large historical events. Population genomic datasets have
considerable power to uncover recent demographic history, and will allow a much
fuller picture of the close genealogical kinship of individuals across the
world.Comment: Full size figures available from
http://www.eve.ucdavis.edu/~plralph/research.html; or html version at
http://ralphlab.usc.edu/ibd/ibd-paper/ibd-writeup.xhtm
A Dual-Beam Irradiation Facility for a Novel Hybrid Cancer Therapy
In this paper we present the main ideas and discuss both the feasibility and
the conceptual design of a novel hybrid technique and equipment for an
experimental cancer therapy based on the simultaneous and/or sequential
application of two beams, namely a beam of neutrons and a CW (continuous wave)
or intermittent sub-terahertz wave beam produced by a gyrotron for treatment of
cancerous tumors. The main simulation tools for the development of the computer
aided design (CAD) of the prospective experimental facility for clinical trials
and study of such new medical technology are briefly reviewed. Some tasks for a
further continuation of this feasibility analysis are formulated as well.Comment: 18 pages, 3 tables, 8 figures, 50 reference
Berezinskii-Kosterlitz-Thouless Crossover in a Trapped Atomic Gas
Any state of matter is classified according to its order, and the kind of
order a physical system can posses is profoundly affected by its
dimensionality. Conventional long-range order, like in a ferromagnet or a
crystal, is common in three-dimensional (3D) systems at low temperature.
However, in two-dimensional (2D) systems with a continuous symmetry, true
long-range order is destroyed by thermal fluctuations at any finite
temperature. Consequently, in contrast to the 3D case, a uniform 2D fluid of
identical bosons cannot undergo Bose-Einstein condensation. Nevertheless, it
can form a "quasi-condensate" and become superfluid below a finite critical
temperature. The Berezinskii-Kosterlitz-Thouless (BKT) theory associates this
phase transition with the emergence of a topological order, resulting from the
pairing of vortices with opposite circulations. Above the critical temperature,
proliferation of unbound vortices is expected. Here we report the observation
of a BKT-type crossover in a trapped quantum degenerate gas of rubidium atoms.
Using a matter wave heterodyning technique, we observe both the long-wavelength
fluctuations of the quasi-condensate phase and the free vortices. At low
temperatures, the gas is quasi-coherent on the length scale set by the system
size. As the temperature is increased, the loss of long-range coherence
coincides with the onset of proliferation of free vortices. Our results provide
direct experimental evidence for the microscopic mechanism underlying the BKT
theory, and raise new questions regarding coherence and superfluidity in
mesoscopic systems.Comment: accepted for publication in Natur
Potassium channel gene mutations rarely cause atrial fibrillation
BACKGROUND: Mutations in several potassium channel subunits have been associated with rare forms of atrial fibrillation. In order to explore the role of potassium channels in inherited typical forms of the arrhythmia, we have screened a cohort of patients from a referral clinic for mutations in the channel subunit genes implicated in the arrhythmia. We sought to determine if mutations in KCNJ2 and KCNE1-5 are a common cause of atrial fibrillation. METHODS: Serial patients with lone atrial fibrillation or atrial fibrillation with hypertension were enrolled between June 1, 2001 and January 6, 2005. Each patient underwent a standardized interview and physical examination. An electrocardiogram, echocardiogram and blood sample for genetic analysis were also obtained. Patients with a family history of AF were screened for mutations in KCNJ2 and KCNE1-5 using automated sequencing. RESULTS: 96 patients with familial atrial fibrillation were enrolled. Eighty-three patients had lone atrial fibrillation and 13 had atrial fibrillation and hypertension. Patients had a mean age of 56 years at enrollment and 46 years at onset of atrial fibrillation. Eighty-one percent of patients had paroxysmal atrial fibrillation at enrollment. Unlike patients with an activating mutation in KCNQ1, the patients had a normal QT(c )interval with a mean of 412 ± 42 ms. Echocardiography revealed a normal mean ejection fraction of 62.0 ± 7.2 % and mean left atrial dimension of 39.9 ± 7.0 mm. A number of common polymorphisms in KCNJ2 and KCNE1-5 were identified, but no mutations were detected. CONCLUSION: Mutations in KCNJ2 and KCNE1-5 rarely cause typical atrial fibrillation in a referral clinic population
Circumstellar disks and planets. Science cases for next-generation optical/infrared long-baseline interferometers
We present a review of the interplay between the evolution of circumstellar
disks and the formation of planets, both from the perspective of theoretical
models and dedicated observations. Based on this, we identify and discuss
fundamental questions concerning the formation and evolution of circumstellar
disks and planets which can be addressed in the near future with optical and
infrared long-baseline interferometers. Furthermore, the importance of
complementary observations with long-baseline (sub)millimeter interferometers
and high-sensitivity infrared observatories is outlined.Comment: 83 pages; Accepted for publication in "Astronomy and Astrophysics
Review"; The final publication is available at http://www.springerlink.co
The Spin Structure of the Nucleon
We present an overview of recent experimental and theoretical advances in our
understanding of the spin structure of protons and neutrons.Comment: 84 pages, 29 figure
Effective Rheology of Bubbles Moving in a Capillary Tube
We calculate the average volumetric flux versus pressure drop of bubbles
moving in a single capillary tube with varying diameter, finding a square-root
relation from mapping the flow equations onto that of a driven overdamped
pendulum. The calculation is based on a derivation of the equation of motion of
a bubble train from considering the capillary forces and the entropy production
associated with the viscous flow. We also calculate the configurational
probability of the positions of the bubbles.Comment: 4 pages, 1 figur
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