273 research outputs found
Electronic structure, magnetic and optical properties of intermetallic compounds R2Fe17 (R=Pr,Gd)
In this paper we report comprehensive experimental and theoretical
investigation of magnetic and electronic properties of the intermetallic
compounds Pr2Fe17 and Gd2Fe17. For the first time electronic structure of these
two systems was probed by optical measurements in the spectral range of 0.22-15
micrometers. On top of that charge carriers parameters (plasma frequency and
relaxation frequency) and optical conductivity s(w) were determined.
Self-consistent spin-resolved bandstructure calculations within the
conventional LSDA+U method were performed. Theoretical interpetation of the
experimental s(w) dispersions indicates transitions between 3d and 4p states of
Fe ions to be the biggest ones. Qualitatively the line shape of the theoretical
optical conductivity coincides well with our experimental data. Calculated by
LSDA+U method magnetic moments per formula unit are found to be in good
agreement with observed experimental values of saturation magnetization.Comment: 16 pages, 5 figures, 1 tabl
Resistive state of superconducting structures with fractal clusters of a normal phase
The effect of morphologic factors on magnetic flux dynamics and critical
currents in percolative superconducting structures is considered. The
superconductor contains the fractal clusters of a normal phase, which act as
pinning centers. The properties of these clusters are analyzed in the general
case of gamma-distribution of their areas. The statistical characteristics of
the normal phase clusters are studied, the critical current distribution is
derived, and the dependencies of the main statistical parameters on the fractal
dimension are found. The effect of fractal clusters of a normal phase on the
electric field induced by the motion of the magnetic flux after the vortices
have been broken away from pinning centers is considered. The voltage-current
characteristics of fractal superconducting structures in a resistive state for
an arbitrary fractal dimension are obtained. It is found that the fractality of
the boundaries of normal phase clusters intensifies magnetic flux trapping and
thereby increases the current-carrying capability of the superconductor.Comment: 15 pages with 8 figures, revtex3, alternative e-mail of author is
[email protected]
Towards the solution of the anomaly in shell-model calculations of muon capture
Recently many authors have performed shell-model calculations of nuclear
matrix elements determining the rates of the ordinary muon capture in light
nuclei. These calculations have employed well-tested effective interactions in
large scale shell-model studies. For one of the nuclei of interest, namely
Si, there exists recent experimental data which can be used to deduce
the value of the ratio by using the calculated matrix elements.
Surprisingly enough, all the abovementioned shell-model results suggest a very
small value () for , quite far from the PCAC prediction
and recent data on muon capture in hydrogen. We show that this rather
disturbing anomaly is solved by employing effective transition operators. This
finding is also very important in studies of the scalar coupling of the weak
charged current of leptons and hadrons.Comment: Revtex, 6 pages, 2 figs include
Constraints for hypothetical interactions from a recent demonstration of the Casimir force and some possible improvements
The Casimir force is calculated in the configuration of a spherical lens and
a disc of finite radius covered by and thin layers which was used in
a recent experiment. The correction to the Casimir force due to finiteness of
the disc radius is shown to be negligible. Also the corrections are discussed
due to the finite conductivity, large-scale and short-scale deviations from the
perfect shape of the bounding surfaces and the temperature correction. They
were found to be essential when confronting the theoretical results with
experimental data. Both Yukawa-type and power-law hypothetical forces are
computed which may act in the configuration under consideration due to the
exchange of light and/or massless elementary particles between the atoms of the
lens and the disc. New constraints on the constants of these forces are
determined which follow from the fact that they were not observed within the
limits of experimental errors. For Yukawa-type forces the new constraints are
up to 30 times stronger than the best ones known up today. A possible
improvement of experimental parameters is proposed which gives the possibility
to strengthen constraints on Yukawa-type interactions up to times and on
power-law interactions up to several hundred times.Comment: 15 pages, 3 figures, subm. to Phys. Rev.
Magnetothermopower and Magnetoresistivity of RuSr2Gd1-xLaxCu2O8 (x=0, 0.1)
We report measurements of magnetothermopower and magnetoresistivity as a
function of temperature on RuSr2Gd1-xLaxCu2O8 (x = 0, 0.1). The normal-state
thermopower shows a dramatic decrease after applying a magnetic field of 5 T,
whereas the resistivity shows only a small change after applying the same
field. Our results suggest that RuO2 layers are conducting and the magnetic
field induced decrease of the overall thermopower is caused by the decrease of
partial thermopower decrease associated with the spin entropy decrease of the
carriers in the RuO2 layers.Comment: 21 pages, 6 figure
Искусственная нейронная сеть для обоснования параметров ходовых систем тракторов
One of the most important properties that determine undercarriage layout on design stage is the soil compaction effect. Existing domestic standards of undercarriages impact to soil do not meet modern agricultural requirements completely. The authors justify the need for analysis of traction and transportation machines travel systems and recommendations for these parameters applied to machines that are on design or modernization stage. The database of crawler agricultural tractors particularly in such parameters as traction class and basic operational weight, engine power rating, average ground pressure, square of track basic branch surface area was modeled. Meanwhile the considered machines were divided into two groups by producing countries: Europe/North America and Russian Federation/CIS. The main graphical dependences for every group of machines are plotted, and the conforming analytical dependences within the ranges with greatest concentration of machines are generated. To make the procedure of obtaining parameters of the soil panning by tractors easier it is expedient to use the program tool - artificial neural network (or perceptron). It is necessary to apply to the solution of this task multilayered perceptron - neutron network of direct distribution of signals (without feedback). To carry out the analysis of parameters of running systems taking into account parameters of the soil panning by them and to recommend the choice of these parameters for newly created machines. The program code of artificial neural network is developed. On the basis of the created base of tractors the artificial neural network was created and tested. Accumulated error was not more than 5 percent. These data indicate the results accuracy and tool reliability. It is possible by operating initial design-data base and using the designed artificial neural network to define missing parameters.Одним из важнейших качеств, определяющих компоновку ходовой системы машины на стадии проектирования, является уплотняющее воздействие на почву. Однако отечественные нормативы этого воздействия не отвечают в полной мере требованиям современного сельского хозяйства. Обоснована необходимость анализа ходовых систем тяговых и транспортных машин, а также рекомендаций по выбору параметров для вновь создаваемой или модернизируемой техники. Разработана база данных гусеничных сельскохозяйственных тракторов с учетом параметров: тягового класса, эксплуатационного веса, номинальной мощности двигателя, среднего давления на почву, площади контактной поверхности опорной ветви гусеницы. Машины были разделены на группы по странам-производителям: Европа/Северная Америка и РФ/СНГ. Построены основные графические зависимости для каждой из групп машин, а также сформированы соответствующие аналитические зависимости в интервалах с наибольшей концентрацией машин. Отметили, что с целью упрощения процедуры получения параметров уплотняющего воздействия тракторов на почву целесообразно использовать программный инструмент - искусственная нейронная сеть (или персептрон). Установили, что для решения этой задачи необходимо применить многослойный персептрон (MLP) - нейтронная сеть прямого распространения сигналов (без обратной связи). Осуществить анализ параметров ходовых систем с учетом их уплотняющих воздействий на почву и рекомендовать выбор этих параметров для вновь создаваемых машин. Разработан программный код искусственной нейронной сети. На основе сформированной базы тракторов создана и протестирована искусственная нейронная сеть. Накопленная погрешность не превышает 5 процентов, что говорит о достоверности полученных в ходе расчета искусственной нейронной сети результатов и надежности инструмента. Показали, что, оперируя исходной информацией из массива заложенных в базу данных, можно определить недостающие данные с помощью разработанной искусственной нейронной сети
Disorder Induced Ferromagnetism in Restricted Geometries
We study the influence of on-site disorder on the magnetic properties of the
ground state of the infinite Hubbard model. We find that for one
dimensional systems disorder has no influence, while for two dimensional
systems disorder enhances the spin polarization of the system. The tendency of
disorder to enhance magnetism in the ground state may be relevant to recent
experimental observations of spin polarized ground states in quantum dots and
small metallic grains.Comment: 4 pages, 4 figure
Measurement of the Pion Form Factor in the Energy Range 1.04-1.38 GeV with the CMD-2 Detector
The cross section for the process is measured in the
c.m. energy range 1.04-1.38 GeV from 995 000 selected collinear events
including 860000 events, 82000 events, and 33000
events. The systematic and statistical errors of measuring the
pion form factor are equal to 1.2-4.2 and 5-13%, respectively.Comment: 5 pages, 2 figure
A High-Energy Study of the Geminga Pulsar
We present the results of deep X-ray and gamma-ray observations of the
Geminga pulsar obtained in the final years of the ASCA and CGRO missions, and
an upper limit from RXTE. A phase-connected ephemeris from the gamma-rays is
derived that spans the years 1973-2000, after allowing for a minor glitch in
frequency of Delta f/f = 6.2 x 10^-10 in late 1996. ASCA observations of the
hard X-ray pulse profile in 1994 and 1999 confirm this glitch. An improved
characterization of the hard X-ray pulse profile and spectrum from the long
ASCA observation of 1999 confirms that there is a non-thermal X-ray component
that is distinct from the gamma-ray spectrum as measured by EGRET. It can be
parameterized as a power-law of photon index Gamma = 1.72 +/- 0.10 with a flux
of 2.62 x 10^-13 ergs/cm^2/s in the 0.7-5 keV band and pulsed fraction 0.54 +/-
0.05, similar to, but more precise than values measured previously. An
extrapolation of this spectrum into the energy band observed by the RXTE PCA is
consistent with the non-detection of pulsed emission from Geminga with that
instrument. These results are interpreted in the context of outer-gap models,
and motivations for future X-ray observations of Geminga are given.Comment: 22 pages including 7 figure
Charging Ultrasmall Tunnel Junctions in Electromagnetic Environment
We have investigated the quantum admittance of an ultrasmall tunnel junction
with arbitrary tunneling strength under an electromagnetic environment. Using
the functional integral approach a close analytical expression of the quantum
admittance is derived for a general electromagnetic environment. We then
consider a specific controllable environment where a resistance is connected in
series with the tunneling junction, for which we derived the dc quantum
conductance from the zero frequency limit of the imaginary part of the quantum
admittance. For such electromagnetic environment the dc conductance has been
investigated in recent experiments, and our numerical results agree
quantitatively very well with the measurements. Our complete numerical results
for the entire range of junction conductance and electromagnetic environmental
conductance confirmed the few existing theoretical conclusions.Comment: 7 pages, 3 ps-figure
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