895 research outputs found
Міжнародна наукова конференція «Шляхи розвитку науково-технічного співробітництва Росії, України і Білорусі»
7 жовтня 2011 року у Москві, у приміщенні Президії РАН на Ленінському проспекті 32А, відбулася міжнародна наукова конференція «Шляхи розвитку науково-технічного співробітництва Росії, України і Білорусі». Конференція була організована Інститутом проблем розвитку науки РАН (ІПРАН РАН) за підтримки Президії РАН. У ролі партнерської організації1виступив Російський гуманітарний науковий фонд
Gamma-ray emission from the solar halo and disk: a study with EGRET data
Context: The Sun has recently been predicted to be an extended source of
gamma-ray emission, produced by inverse-Compton (IC) scattering of cosmic-ray
(CR) electrons on the solar radiation field. The emission was predicted to be
extended and a confusing foreground for the diffuse extragalactic background
even at large angular distances from the Sun. The solar disk is also expected
to be a steady gamma-ray source. While these emissions are expected to be
readily detectable in the future by GLAST, the situation for available EGRET
data is more challenging. Aims: The theory of gamma-ray emission from IC
scattering on the solar radiation field by Galactic CR electrons is given in
detail. This is used as the basis for detection and model verification using
EGRET data. Methods: We present a detailed study of the solar emission using
the EGRET database, accounting for the effect of the emission from 3C 279, the
moon, and other sources, which interfere with the solar emission. The analysis
was performed for 2 energy ranges, above 300 MeV and for 100-300 MeV, as well
as for the combination to improve the detection statistics. The technique was
tested on the moon signal, with our results consistent with previous work.
Results: Analyzing the EGRET database, we find evidence of emission from the
solar disk and its halo. The observations are compared with our model for the
extended emission. The spectrum of the solar disk emission and the spectrum of
the extended emission have been obtained. The spectrum of the moon is also
given. Conclusions: The observed intensity distribution and the flux are
consistent with the predicted model of IC gamma-rays from the halo around the
Sun.Comment: Corrected typos, added acknowledgements. A&A in pres
``Flux'' state in double exchange model
We study the ground state properties of the double-exchange systems. The
phase factor of the hopping matrix elements arises from spin texture
in two or more dimensions. A novel ``flux'' state is stabilized against the
canted antiferromagnetic and spiral spin states. In a certain range of hole
doping, the phase separation occurs between the ``flux'' state and
antiferromagnetic states. Constructing a trial state which provides the
rigorous upper bound on the ground state, we show that the metallic canted
antiferromagnetic state is not stable in the double exchange model.Comment: REVTEX, 8 pages and 4 PS figure
Modification of the Landau-Lifshitz Equation in the Presence of a Spin-Polarized Current in CMR and GMR Materials
We derive a continuum equation for the magnetization of a conducting
ferromagnet in the presence of a spin-polarized current. Current effects enter
in the form of a topological term in the Landau-Lifshitz equation . In the
stationary situation the problem maps onto the motion of a classical charged
particle in the field of a magnetic monopole. The spatial dependence of the
magnetization is calculated for a one-dimensional geometry and suggestions for
experimental observation are made. We also consider time-dependent solutions
and predict a spin-wave instability for large currents.Comment: 4 two-column pages in RevTex, 3 ps-figure
Numerical Study of Excited States in the Shastry-Sutherland Model
We investigate excited states of the Shastry-Sutherland model using a kind of
variational method. Starting from various trial states which include one or two
triplet dimers, we numerically pursue the best evaluation of the energy for
each set of quantum numbers. We present the energy difference as a function of
either the coupling ratio or the momentum and compare them with the
perturbative calculations. Our data suggest that the helical order phase exists
between the singlet dimer phase and the magnetically ordered phase. In
comparison with the experimental data we can estimate the intra-dimer coupling
J and the inter-dimer coupling J' for
SrCu2(BO3)2 : J'/J =0.65 and J = 87K.Comment: 15pages, 5figures to be published in JPS
Dual-frequency VLBI study of Centaurus A on sub-parsec scales
Centaurus A is the closest active galactic nucleus. High resolution imaging
using Very Long Baseline Interferometry (VLBI) enables us to study the spectral
and kinematic behavior of the radio jet-counterjet system on sub-parsec scales,
providing essential information for jet emission and formation models. Our aim
is to study the structure and spectral shape of the emission from the
central-parsec region of Cen A. As a target of the Southern Hemisphere VLBI
monitoring program TANAMI (Tracking Active Galactic Nuclei with Milliarcsecond
Interferometry), VLBI observations of Cen A are made regularly at 8.4 and 22.3
GHz with the Australian Long Baseline Array (LBA) and associated telescopes in
Antarctica, Chile, and South Africa. The first dual-frequency images of this
source are presented along with the resulting spectral index map. An angular
resolution of 0.4 mas x 0.7 mas is achieved at 8.4 GHz, corresponding to a
linear scale of less than 0.013 pc. Hence, we obtain the highest resolution
VLBI image of Cen A, comparable to previous space-VLBI observations. By
combining with the 22.3 GHz image, which has been taken without contributing
transoceanic baselines at somewhat lower resolution, we present the
corresponding dual-frequency spectral index distribution along the sub-parsec
scale jet revealing the putative emission regions for recently detected
gamma-rays from the core region by Fermi/LAT. We resolve the innermost
structure of the milliarcsecond scale jet and counterjet system of Cen A into
discrete components. The simultaneous observations at two frequencies provide
the highest resolved spectral index map of an AGN jet allowing us to identify
multiple possible sites as the origin of the high energy emission.Comment: 5 pages, 3 figures (1 color); A&A, accepte
Cosmological dynamics of R^n gravity
A detailed analysis of dynamics of cosmological models based on
gravity is presented. We show that the cosmological equations can be written as
a first order autonomous system and analyzed using the standard techniques of
dynamical system theory. In absence of perfect fluid matter, we find exact
solutions whose behavior and stability are analyzed in terms of the values of
the parameter . When matter is introduced, the nature of the (non-minimal)
coupling between matter and higher order gravity induces restrictions on the
allowed values of . Selecting such intervals of values and following the
same procedure used in the vacuum case, we present exact solutions and analyze
their stability for a generic value of the parameter . From this analysis
emerges the result that for a large set of initial conditions an accelerated
expansion is an attractor for the evolution of the cosmology. When matter
is present a transient almost-Friedman phase can also be present before the
transition to an accelerated expansion.Comment: revised and extended version, 35 pages, 12 tables, 14 figures which
are not included and can be found at http://www.mth.uct.ac.za/~peter/R
Quantum phase transition in a minimal model for the Kondo effect in a Josephson junction
We propose a minimal model for the Josephson current through a quantum dot in
a Kondo regime. We start with the model that consists of an Anderson impurity
connected to two superconducting (SC) leads with the gaps
, where for the lead at left and right. We show that, when one of the SC gaps is
much larger than the others , the starting model can
be mapped exactly onto the single-channel model, which consists of the right
lead of and the Anderson impurity with an extra onsite SC gap of
. Here and are
defined with respect to the starting model, and is the level width
due to the coupling with the left lead. Based on this simplified model, we
study the ground-state properties for the asymmetric gap, , using the numerical renormalization group (NRG) method. The
results show that the phase difference of the SC gaps , which induces the Josephson current, disturbs the screening of the
local moment to destabilize the singlet ground state typical of the Kondo
system. It can also drive the quantum phase transition to a magnetic doublet
ground state, and at the critical point the Josephson current shows a
discontinuous change. The asymmetry of the two SC gaps causes a re-entrant
magnetic phase, in which the in-gap bound state lies close to the Fermi level.Comment: 23 pages, 13 figures, typos are correcte
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