2,792 research outputs found
Spin-Coupled Local Distortions in Multiferroic Hexagonal HoMnO3
Local structural measurements have been performed on hexagonal HoMnO3 in
order to ascertain the specific changes in bond distances which accompany
magnetic ordering transitions. The transition from paramagnetic to the
antiferromagetic (noncollinear) phase near ~70 K is dominated by changes in the
a-b plane Mn-Mn bond distances. The spin rotation transition near ~40 K
involves both Mn-Mn and nearest neighbor Ho-Mn interactions while the low
temperature transition below 10 K involves all interactions, Mn-Mn, Ho-Mn
(nearest and next nearest) and Ho-Ho correlations. These changes in bond
distances reveal strong spin-lattice coupling. The similarity in magnitude of
the change in J(Mn-Mn) and J(Ho-Mn) enhances the system frustration. The
structural changes are interpreted in terms of a model of competing spin order
and local structural distortions. Density functional calculations are used to
estimate the energies associated with ionic displacements. The calculations
also reveal asymmetric polarization of the charge density of Ho, O3 and O4
sites along the z-axis in the ferroelectric phase. This polarization
facilitates coupling between Ho atoms on neighboring planes normal to the
z-axis.Comment: 8 figure
Adaptive thermal compensation of test masses in advanced LIGO
As the first generation of laser interferometric gravitational wave detectors
near operation, research and development has begun on increasing the
instrument's sensitivity while utilizing the existing infrastructure. In the
Laser Interferometer Gravitational Wave Observatory (LIGO), significant
improvements are being planned for installation in ~2007, increasing strain
sensitivity through improved suspensions and test mass substrates, active
seismic isolation, and higher input laser power. Even with the highest quality
optics available today, however, finite absorption of laser power within
transmissive optics, coupled with the tremendous amount of optical power
circulating in various parts of the interferometer, result in critical
wavefront deformations which would cripple the performance of the instrument.
Discussed is a method of active wavefront correction via direct thermal
actuation on optical elements of the interferometer. A simple nichrome heating
element suspended off the face of an affected optic will, through radiative
heating, remove the gross axisymmetric part of the original thermal distortion.
A scanning heating laser will then be used to remove any remaining
non-axisymmetric wavefront distortion, generated by inhomogeneities in the
substrate's absorption, thermal conductivity, etc. A proof-of-principle
experiment has been constructed at MIT, selected data of which are presented.Comment: 11 pages, 7 figures, submitted to Classical and Quantum Gravit
Observational Constraints on Exponential Gravity
We study the observational constraints on the exponential gravity model of
f(R)=-beta*Rs(1-e^(-R/Rs)). We use the latest observational data including
Supernova Cosmology Project (SCP) Union2 compilation, Two-Degree Field Galaxy
Redshift Survey (2dFGRS), Sloan Digital Sky Survey Data Release 7 (SDSS DR7)
and Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP7) in our analysis.
From these observations, we obtain a lower bound on the model parameter beta at
1.27 (95% CL) but no appreciable upper bound. The constraint on the present
matter density parameter is 0.245< Omega_m^0<0.311 (95% CL). We also find out
the best-fit value of model parameters on several cases.Comment: 14pages, 3 figures, accepted by PR
Schwarzschild black hole lensing
We study strong gravitational lensing due to a Schwarzschild black hole.
Apart from the primary and the secondary images we find a sequence of images on
both sides of the optic axis; we call them {\em relativistic images}. These
images are formed due to large bending of light near r = 3M (the closest
distance of approach r_o is greater than 3M). The sources of the entire
universe are mapped in the vicinity of the black hole by these images. For the
case of the Galactic supermassive ``black hole'' they are formed at about 17
microarcseconds from the optic axis. The relativistic images are not resolved
among themselves, but they are resolved from the primary and secondary images.
However the relativistic images are very much demagnified unless the observer,
lens and source are very highly aligned. Due to this and some other
difficulties the observation of these images does not seem to be feasible in
near future. However, it would be a great success of the general theory of
relativity in a strong gravitational field if they ever were observed and it
would also give an upper bound, r_o = 3.21 M, to the compactness of the lens,
which would support the black hole interpretation of the lensing object.Comment: RevTex, 5 eps files are included, observational difficulties are
discussed and there are some changes in presentatio
Exploring Dark Energy with Next-Generation Photometric Redshift Surveys
The coming decade will be an exciting period for dark energy research, during which astronomers will address the question of what drives the accelerated cosmic expansion as first revealed by type Ia supernova (SN) distances, and confirmed by later observations. The mystery of dark energy poses a challenge of such magnitude that, as stated by the Dark Energy Task Force (DETF), nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration. The lack of multiple complementary precision observations is a major obstacle in developing lines of attack for dark energy theory. This lack is precisely what next-generation surveys will address via the powerful techniques of weak lensing (WL) and baryon acoustic oscillations (BAO) -- galaxy correlations more generally -- in addition to SNe, cluster counts, and other probes of geometry and growth of structure. Because of their unprecedented statistical power, these surveys demand an accurate understanding of the observables and tight control of systematics. This white paper highlights the opportunities, approaches, prospects, and challenges relevant to dark energy studies with wide-deep multiwavelength photometric redshift surveys. Quantitative predictions are presented for a 20000 sq. deg. ground-based 6-band (ugrizy) survey with 5-sigma depth of r~27.5, i.e., a Stage 4 survey as defined by the DETF
Direct Measurement of Kirkwood-Rihaczek distribution for spatial properties of coherent light beam
We present direct measurement of Kirkwood-Rihaczek (KR) distribution for
spatial properties of coherent light beam in terms of position and momentum
(angle) coordinates. We employ a two-local oscillator (LO) balanced heterodyne
detection (BHD) to simultaneously extract distribution of transverse position
and momentum of a light beam. The two-LO BHD could measure KR distribution for
any complex wave field (including quantum mechanical wave function) without
applying tomography methods (inverse Radon transformation). Transformation of
KR distribution to Wigner, Glauber Sudarshan P- and Husimi or Q- distributions
in spatial coordinates are illustrated through experimental data. The direct
measurement of KR distribution could provide local information of wave field,
which is suitable for studying particle properties of a quantum system. While
Wigner function is suitable for studying wave properties such as interference,
and hence provides nonlocal information of the wave field. The method developed
here can be used for exploring spatial quantum state for quantum mapping and
computing, optical phase space imaging for biomedical applications.Comment: 27 pages, 14 figure
Boolean network model predicts cell cycle sequence of fission yeast
A Boolean network model of the cell-cycle regulatory network of fission yeast
(Schizosaccharomyces Pombe) is constructed solely on the basis of the known
biochemical interaction topology. Simulating the model in the computer,
faithfully reproduces the known sequence of regulatory activity patterns along
the cell cycle of the living cell. Contrary to existing differential equation
models, no parameters enter the model except the structure of the regulatory
circuitry. The dynamical properties of the model indicate that the biological
dynamical sequence is robustly implemented in the regulatory network, with the
biological stationary state G1 corresponding to the dominant attractor in state
space, and with the biological regulatory sequence being a strongly attractive
trajectory. Comparing the fission yeast cell-cycle model to a similar model of
the corresponding network in S. cerevisiae, a remarkable difference in
circuitry, as well as dynamics is observed. While the latter operates in a
strongly damped mode, driven by external excitation, the S. pombe network
represents an auto-excited system with external damping.Comment: 10 pages, 3 figure
Parametrization and Classification of 20 Billion LSST Objects: Lessons from SDSS
The Large Synoptic Survey Telescope (LSST) will be a large, wide-field
ground-based system designed to obtain, starting in 2015, multiple images of
the sky that is visible from Cerro Pachon in Northern Chile. About 90% of the
observing time will be devoted to a deep-wide-fast survey mode which will
observe a 20,000 deg region about 1000 times during the anticipated 10
years of operations (distributed over six bands, ). Each 30-second long
visit will deliver 5 depth for point sources of on average.
The co-added map will be about 3 magnitudes deeper, and will include 10 billion
galaxies and a similar number of stars. We discuss various measurements that
will be automatically performed for these 20 billion sources, and how they can
be used for classification and determination of source physical and other
properties. We provide a few classification examples based on SDSS data, such
as color classification of stars, color-spatial proximity search for wide-angle
binary stars, orbital-color classification of asteroid families, and the
recognition of main Galaxy components based on the distribution of stars in the
position-metallicity-kinematics space. Guided by these examples, we anticipate
that two grand classification challenges for LSST will be 1) rapid and robust
classification of sources detected in difference images, and 2) {\it
simultaneous} treatment of diverse astrometric and photometric time series
measurements for an unprecedentedly large number of objects.Comment: Presented at the "Classification and Discovery in Large Astronomical
Surveys" meeting, Ringberg Castle, 14-17 October, 200
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