902 research outputs found
Percolative shunting on electrified surface
The surface discharge of electrified dielectrics at high humidity is
considered. The percolative nature of charge transport in electrets is
established. Particular attention is given to the phenomena of adsorption and
nucleation of electrically conducting phase in the cause of percolation cluster
growth on electrified surface. The critical index of the correlation lenght for
percolation cluster is found, and its value is in good agreement with the known
theoretical estimations.Comment: 4 pages with 1 figure, revtex, published in Tech. Phys. Lett. 25
(1999) 877-879 with one additional figur
Stability properties of periodically driven overdamped pendula and their implications to physics of semiconductor superlattices and Josephson junctions
We consider the first order differential equation with a sinusoidal
nonlinearity and periodic time dependence, that is, the periodically driven
overdamped pendulum. The problem is studied in the case that the explicit
time-dependence has symmetries common to pure ac-driven systems. The only
bifurcation that exists in the system is a degenerate pitchfork bifurcation,
which describes an exchange of stability between two symmetric nonlinear modes.
Using a type of Prufer transform to a pair of linear differential equations, we
derive an approximate condition of the bifurcation. This approximation is in
very good agreement with our numerical data. In particular, it works well in
the limit of large drive amplitudes and low external frequencies. We
demonstrate the usefulness of the theory applying it to the models of pure
ac-driven semiconductor superlattices and Josephson junctions. We show how the
knowledge of bifurcations in the overdamped pendulum model can be utilized to
describe effects of rectification and amplification of electric fields in these
microstructures.Comment: 15 pages, 7 figures, Revtex 4.1. Revised and expanded following
referee's report. Submitted to journal Chaos
Co dimers on hexagonal carbon rings proposed as subnanometer magnetic storage bits
It is demonstrated by means of density functional and ab-initio quantum
chemical calculations, that transition metal - carbon systems have the
potential to enhance the presently achievable area density of magnetic
recording by three orders of magnitude. As a model system, Co_2-benzene with a
diameter of 0.5 nm is investigated. It shows a magnetic anisotropy in the order
of 0.1 eV per molecule, large enough to store permanently one bit of
information at temperatures considerably larger than 4 K. A similar performance
can be expected, if cobalt dimers are deposited on graphene or on graphite. It
is suggested that the subnanometer bits can be written by simultaneous
application of a moderate magnetic and a strong electric field.Comment: 13 pages, 4 figure
Do the precise measurements of the Casimir force agree with the expectations?
An upper limit on the Casimir force is found using the dielectric functions
of perfect crystalline materials which depend only on well defined material
constants. The force measured with the atomic force microscope is larger than
this limit at small separations between bodies and the discrepancy is
significant. The simplest modification of the experiment is proposed allowing
to make its results more reliable and answer the question if the discrepancy
has any relation with the existence of a new force.Comment: 9 pages, LaTeX, 2 Postscript figure
On the Strength of Spin-Isospin Transitions in A=28 Nuclei
The relations between the strengths of spin-isospin transition operators
extracted from direct nuclear reactions, magnetic scattering of electrons and
processes of semi-leptonic weak interactions are discussed.Comment: LaTeX, 8 pages, 1Postscript with figur
Термодинамическая устойчивость интерметаллических соединений в техническом алюминии
For the following systems Al-Ti, Al-Ni, Al-Zr, Al-Cr, Al-Fe, Al-V the wide range of intermetallic
compounds based on aluminum was considered. Thermodynamic parameters such as Gibbs energy
and formation enthalpy characterizing the properties of resulting intermetallic compounds were
calculated. The relations of these two characteristics versus temperature were established in a wide
range. On the basis of the obtained relations the comparative assessment of the compound stability for
each binary system was carried out. The level of each intermetallic compound stability was revealed
on the ground of the affinity with aluminum. The possibilities of practical application of obtained data
for actual metallurgical tasks were definedРассмотрен широкий ряд интерметаллических соединений систем Al-Ti, Al-Ni, Al-Zr, Al-Cr,
Al-Fe, Al-V. Рассчитаны такие термодинамические характеристики, как энергия Гиббса и
энтальпия образования, характеризующие свойства образующихся интерметаллических
соединений. Установлены зависимости данных характеристик от температуры в широком
диапазоне. На основании полученных зависимостей проведена сравнительная оценка
устойчивости интерметаллических соединений для каждой системы. Показана степень
устойчивости каждого интерметаллида исходя из его сродства к алюмини
Electron Correlations in the Low Carrier Density LaFeAsO0.89F0.11 Superconductor (Tc = 28 K)
The crystal structure and numerous normal and superconducting state
properties of layered tetragonal (P4/nmm) LaFeAsO, with F-doping of 11 %, are
reported. Resistivity measurements give an onset transition temperature Tc =
28.2 K, and low field magnetic susceptibility data indicate bulk
superconductivity. In applied magnetic field, analysis of the resistive
transition results in a critical field Hc2 = 30 T and a coherence length 35 A.
An upper limit for the electron carrier concentration of 1 x 10^21 cm-3 is
inferred from Hall data just above Tc. Strong electron-electron correlations
are suggested from temperature-dependent resistivity, Seebeck coefficient, and
thermal conductivity data. Anomalies near Tc are observed in both Seebeck
coefficient and thermal conductivity data.Comment: 13 pages, 8 figure
Послеоперационный делирий и инфаркт миокарда у пациентов пожилого и старческого возраста с переломами шейки бедренной кости: роль послеоперационной анемии
Objective. To assess the impact of anemia on the incidence of myocardial infarction and delirium in the early postoperative period in patients with hip fractures who underwent total hip replacement. Materials and methods. A retrospective study of 303 patients with hip fractures who underwent total hip replacement was performed. Patients were divided into two groups according to the level of hemoglobin (Hb) on the 2nd postoperative day: Group 1 (n = 116) with Hb levels above 100 g/L and Group 2 (n = 187) with Hb levels below 100 g/L. Results and conclusion. Transfusion in Group 1 was performed more often intraoperatively or in the early postoperative hours (67 vs. 35 patients; p 0.001). Hb levels on the 2nd day were significantly higher in Group 1 (108 [104; 117] vs. 87 [80; 92] g/L; p 0.001). Moderate and severe anemia in the early postoperative period was noted in 53.5% and 4.3% of the patients of Group 2, respectively. The incidence of myocardial infarction and delirium was higher in Group 2 than in Group 1 (6.4% vs. 0%; p = 0,013 and 7.5% vs 0.8%; p = 0.021, respectively). The study has shown that moderate and severe anemia in the early postoperative period is associated with an increase in the incidence of myocardial infarction and delirium.Цель исследования: оценка влияния анемии на частоту инфаркта миокарда и делирия в раннем послеоперационном периоде у пациентов с переломами шейки бедренной кости, которым выполнено тотальное эндопротезирование тазобедренного сустава. Материалы и методы. Проведено ретроспективное исследование 303 пациентов с переломами шейки бедренной кости, которым было выполнено тотальное эндопротезирование тазобедренного сустава. Пациенты были разделены на две группы по уровню гемоглобина на 2-е сутки после операции: 1-я группа (n = 116) с уровнем гемоглобина выше 100 г/л; 2-я группа (n = 187) с уровнем гемоглобина ниже 100 г/л. Результаты и заключение. Трансфузия в первой группе проводилась чаще во время операции и в первые часы после операции (67 против 35 пациентов; p0,001). уровень гемоглобина на вторые сутки был существенно выше в первой группе (108 [104;117] против 87 [80;92] г/л; p0,001). Анемия умеренной и тяжелой степени в раннем послеоперационном периоде отмечена соответственно у 53,5% и 4,3% пациентов второй группы. Во второй группе чаще, чем в первой группе, наблюдался инфаркт миокарда (6,4% против 0% ; p = 0,013) и делирий (7,5% против 0,8%; p = 0,021). Было установлено, что анемия средней и тяжелой степени в раннем послеоперационном периоде сопровождается увеличением частоты развития инфаркта миокарда и делирия
A VLA Search for the Geminga Pulsar: A Bayesian Limit on a Scintillating Source
We derive an upper limit of 3 mJy (95% confidence) for the flux density at
317 MHz of the Geminga pulsar (J0633+1746). Our results are based on 7 hours of
fast-sampled VLA data, which we averaged synchronously with the pulse period
using a period model based on CGRO/EGRET gamma-ray data. Our limit accounts for
the fact that this pulsar is most likely subject to interstellar scintillations
on a timescale much shorter than our observing span. Our Bayesian method is
quite general and can be applied to calculate the fluxes of other scintillated
sources. We also present a Bayesian technique for calculating the flux in a
pulsed signal of unknown width and phase.
Comparing our upper limit of 3 mJy with the quoted flux density of Geminga at
102 MHz, we calculate a lower limit to its spectral index of 2.7. We discuss
some possible reasons for Geminga's weakness at radio wavelengths, and the
likelihood that many of the unidentified EGRET sources are also radio-quiet or
radio-weak Geminga-like pulsars.Comment: 27 pages, including figures. Published in Ap
A CLEAN-based Method for Deconvolving Interstellar Pulse Broadening from Radio Pulses
Multipath propagation in the interstellar medium distorts radio pulses, an
effect predominant for distant pulsars observed at low frequencies. Typically,
broadened pulses are analyzed to determine the amount of propagation-induced
pulse broadening, but with little interest in determining the undistorted pulse
shapes. In this paper we develop and apply a method that recovers both the
intrinsic pulse shape and the pulse broadening function that describes the
scattering of an impulse. The method resembles the CLEAN algorithm used in
synthesis imaging applications, although we search for the best pulse
broadening function, and perform a true deconvolution to recover intrinsic
pulse structre. As figures of merit to optimize the deconvolution, we use the
positivity and symmetry of the deconvolved result along with the mean square
residual and the number of points below a given threshold. Our method makes no
prior assumptions about the intrinsic pulse shape and can be used for a range
of scattering functions for the interstellar medium. It can therefore be
applied to a wider variety of measured pulse shapes and degrees of scattering
than the previous approaches. We apply the technique to both simulated data and
data from Arecibo observations.Comment: 9 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
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