5,251 research outputs found
On a class of reductions of Manakov-Santini hierarchy connected with the interpolating system
Using Lax-Sato formulation of Manakov-Santini hierarchy, we introduce a class
of reductions, such that zero order reduction of this class corresponds to dKP
hierarchy, and the first order reduction gives the hierarchy associated with
the interpolating system introduced by Dunajski. We present Lax-Sato form of
reduced hierarchy for the interpolating system and also for the reduction of
arbitrary order. Similar to dKP hierarchy, Lax-Sato equations for (Lax
fuction) due to the reduction split from Lax-Sato equations for (Orlov
function), and the reduced hierarchy for arbitrary order of reduction is
defined by Lax-Sato equations for only. Characterization of the class of
reductions in terms of the dressing data is given. We also consider a waterbag
reduction of the interpolating system hierarchy, which defines
(1+1)-dimensional systems of hydrodynamic type.Comment: 15 pages, revised and extended, characterization of the class of
reductions in terms of the dressing data is give
Spintronics via non-axisymmetric chiral skyrmions
Micromagnetic calculations demonstrate a peculiar evolution of
non-axisymmetric skyrmions driven by an applied magnetic field in confined
helimagnets with longitudinal modulations. We argue that these specific
solitonic states can be employed in nanoelectronic devices as an effective
alternative to the common axisymmetric skyrmions which occur in magnetically
saturated states
`Interpolating' differential reductions of multidimensional integrable hierarchies
We transfer the scheme of constructing differential reductions, developed
recently for the case of the Manakov-Santini hierarchy, to the general
multidimensional case. We consider in more detail the four-dimensional case,
connected with the second heavenly equation and its generalization proposed by
Dunajski. We give a characterization of differential reductions in terms of the
Lax-Sato equations as well as in the framework of the dressing method based on
nonlinear Riemann-Hilbert problem.Comment: Based on the talk at NLPVI, Gallipoli, 15 page
Chiral skyrmions in thin magnetic films: new objects for magnetic storage technologies?
Axisymmetric magnetic lines of nanometer sizes (chiral vortices or skyrmions)
have been predicted to exist in a large group of noncentrosymmetric crystals
more than two decades ago. Recently these magnetic textures have been directly
observed in nanolayers of cubic helimagnets and monolayers of magnetic metals.
We develop a micromagnetic theory of chiral skyrmions in thin magnetic layers
for magnetic materials with intrinsic and induced chirality. Such particle-like
and stable micromagnetic objects can exist in broad ranges of applied magnetic
fields including zero field. Chiral skyrmions can be used as a new type of
highly mobile nanoscale data carriers
Low-temperature kinetics of exciton-exciton annihilation of weakly localized one-dimensional Frenkel excitons
We present results of numerical simulations of the kinetics of
exciton-exciton annihilation of weakly localized one-dimensional Frenkel
excitons at low temperatures. We find that the kinetics is represented by two
well-distinguished components: a fast short-time decay and a very slow
long-time tail. The former arises from excitons that initially reside in states
belonging to the same localization segment of the chain, while the slow
component is caused by excitons created on different localization segments. We
show that the usual bi-molecular theory fails in the description of the
behavior found. We also present a qualitative analytical explanation of the
non-exponential behavior observed in both the short- and the long-time decay
components.Comment: Published in J. Chem. Phys. 114, 1 April (2001
Magnetic structures and reorientation transitions in noncentrosymmetric uniaxial antiferromagnets
A phenomenological theory of magnetic states in noncentrosymmetric tetragonal
antiferromagnets is developed, which has to include homogeneous and
inhomogeneous terms (Lifshitz-invariants) derived from Dzyaloshinskii-Moriya
couplings. Magnetic properties of this class of antiferromagnets with low
crystal symmetry are discussed in relation to its first known members, the
recently detected compounds Ba2CuGe2O7 and K2V3O8. Crystallographic symmetry
and magnetic ordering in these systems allow the simultaneous occurrence of
chiral inhomogeneous magnetic structures and weak ferromagnetism. New types of
incommensurate magnetic structures are possible, namely, chiral helices with
rotation of staggered magnetization and oscillations of the total
magnetization. Field-induced reorientation transitions into modulated states
have been studied and corresponding phase diagrams are constructed. Structures
of magnetic defects (domain-walls and vortices) are discussed. In particular,
vortices, i.e. localized non-singular line defects, are stabilized by the
inhomogeneous Dzyaloshinskii-Moriya interactions in uniaxial noncentrosymmetric
antiferromagnets.Comment: 18 pages RevTeX4, 13 figure
Limb-Darkening of a K Giant in the Galactic Bulge: PLANET Photometry of MACHO 97-BLG-28
We present the PLANET photometric dataset for the binary-lens microlensing
event MACHO 97-BLG-28 consisting of 696 I and V-band measurements, and analyze
it to determine the radial surface brightness profile of the Galactic bulge
source star. The microlensed source, demonstrated to be a K giant by our
independent spectroscopy, crossed the central isolated cusp of the lensing
binary, generating a sharp peak in the light curve that was well-resolved by
dense (3 - 30 minute) and continuous monitoring from PLANET sites in Chile,
South Africa, and Australia. Our modeling of these data has produced stellar
profiles for the source star in the I and V bands that are in excellent
agreement with those predicted by stellar atmospheric models for K giants. The
limb-darkening coefficients presented here are the first derived from
microlensing, among the first for normal giants by any technique, and the first
for any star as distant as the Galactic bulge. Modeling indicates that the
lensing binary has a mass ratio q = 0.23 and an (instantaneous) separation in
units of the angular Einstein ring radius of d = 0.69 . For a lens in the
Galactic bulge, this corresponds to a typical stellar binary with a projected
separation between 1 and 2 AU. If the lens lies closer, the separation is
smaller, and one or both of the lens objects is in the brown dwarf regime.
Assuming that the source is a bulge K2 giant at 8 kpc, the relative lens-source
proper motion is mu = 19.4 +/- 2.6 km/s /kpc, consistent with a disk or bulge
lens. If the non-lensed blended light is due to a single star, it is likely to
be a young white dwarf in the bulge, consistent with the blended light coming
from the lens itself.Comment: 32 Pages, including 1 table and 9 postscript figures. (Revised
version has slightly modified text, corrected typo, and 1 new figure.)
Accepted for publication in 1999 Astrophysical Journal; data are now
available at http://www.astro.rug.nl/~plane
Probing Red Giant Atmospheres with Gravitational Microlensing
Gravitational microlensing provides a new technique for studying the surfaces
of distant stars. Microlensing events are detected in real time and can be
followed up with precision photometry and spectroscopy. This method is
particularly adequate for studying red giants in the Galactic bulge. Recently
we developed an efficient method capable of computing the lensing effect for
thousands of frequencies in a high-resolution stellar spectrum. Here we
demonstrate the effects of microlensing on synthesized optical spectra of red
giant model atmospheres. We show that different properties of the stellar
surface can be recovered from time-dependent photometry and spectroscopy of a
point-mass microlensing event with a small impact parameter. In this study we
concentrate on center-to-limb variation of spectral features. Measuring such
variations can reveal the depth structure of the atmosphere of the source star.Comment: 23 pages with 11 Postscript figures, submitted to ApJ; Section 2
expanded, references added, text revise
Detecting Stellar Spots by Gravitational Microlensing
During microlensing events with a small impact parameter, the amplification
of the source flux is sensitive to the surface brightness distribution of the
source star. Such events provide a means for studying the surface structure of
target stars in the ongoing microlensing surveys, most efficiently for giants
in the Galactic bulge. In this work we demonstrate the sensitivity of
point-mass microlensing to small spots with radii source
radii. We compute the amplification deviation from the light curve of a
spotless source and explore its dependence on lensing and spot parameters.
During source-transit events spots can cause deviations larger than 2%, and
thus be in principle detectable. Maximum relative deviation usually occurs when
the lens directly crosses the spot. Its numerical value for a dark spot with
sufficient contrast is found to be roughly equal to the fractional radius of
the spot, i.e., up to 20% in this study. Spots can also be efficiently detected
by the changes in sensitive spectral lines during the event. Notably, the
presence of a spot can mimic the effect of a low-mass companion of the lens in
some events.Comment: 18 pages with 7 Postscript figures, to appear in ApJ, January 2000;
discussion expanded, references added, minor revisions in tex
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