265,139 research outputs found
Power Spectra of X-ray Binaries
The interpretation of Fourier spectra in the time domain is critically
examined. Power density spectra defined and calculated in the time domain are
compared with Fourier spectra in the frequency domain for three different types
of variability: periodic signals, Markov processes and random shots. The power
density spectra for a sample of neutron stars and black hole binaries are
analyzed in both the time and the frequency domains. For broadband noise, the
two kinds of power spectrum in accreting neutron stars are usually consistent
with each other, but the time domain power spectra for black hole candidates
are significantly higher than corresponding Fourier spectra in the high
frequency range (10--1000 Hz). Comparing the two kinds of power density spectra
may help to probe the intrinsic nature of timing phenomena in compact objects.Comment: 21 pages, 10 figures, to appear in Astrophysical Journa
Adiabatic passage of collective excitations in atomic ensembles
We describe a theoretical scheme that allows for transfer of quantum states
of atomic collective excitation between two macroscopic atomic ensembles
localized in two spatially-separated domains. The conception is based on the
occurrence of double-exciton dark states due to the collective destructive
quantum interference of the emissions from the two atomic ensembles. With an
adiabatically coherence manipulation for the atom-field couplings by stimulated
Ramann scattering, the dark states will extrapolate from an exciton state of an
ensemble to that of another. This realizes the transport of quantum information
among atomic ensembles.Comment: 7 pages, 2 figure
Transplantation of olfactory ensheathing cells into spinal cord lesions restores breathing and climbing
One of the most devastating effects of damage to the upper spinal cord is the loss of the ability to breathe; patients suffering these injuries can be kept alive only with assisted ventilation. No known method for repairing these injuries exists. We report here the return of supraspinal control of breathing and major improvements in climbing after the application of a novel endogenous matrix transfer method. This method permits efficient transfer and retention of cultured adult rat olfactory ensheathing cells when transplanted into large lesions that destroy all tracts on one side at the upper cervical level of the adult rat spinal cord. This demonstrates that transplantation can produce simultaneous repair of two independent spinal functions
Evaluation of ASTER GDEM ver2 using GPS measurements and SRTM ver4.1 in China
The freely available ASTER GDEM ver2 was released by NASA and METI on October 17, 2011. As one of the most complete high resolution digital topographic data sets of the world to date, the ASTER GDEM covers land surfaces between 83°N and 83°S at a spatial resolution of 1 arc-second and will be a useful product for many applications, such as relief analysis, hydrological studies and radar interferometry. The stated improvements in the second version of ASTER GDEM benefit from finer horizontal resolution, offset adjustment and water body detection in addition to new observed ASTER scenes. This study investigates the absolute vertical accuracy of the ASTER GDEM ver2 at five study sites in China using ground control points (GCPs) from high accuracy GPS benchmarks, and also using a DEM-to-DEM comparison with the Consultative Group for International Agriculture Research Consortium for Spatial Information (CGIAR-CSI) SRTM DEM (Version 4.1). And then, the results are separated into GlobCover land cover classes to derive the spatial pattern of error. It is demonstrated that the RMSE (19m) and mean (-13m) values of ASTER GDEM ver2 against GPS-GCPs in the five study areas is lower than its first version ASTER GDEM ver1 (26m and -21m) as a result of the adjustment of the elevation offsets in the new version. It should be noted that the five study areas in this study are representative in terms of terrain types and land covers in China, and even for most of mid-latitude zones. It is believed that the ASTER GDEM offers a major alternative in accessibility to high quality elevation data
The Coupled Cluster Method Applied to Quantum Magnets: A New LPSUB Approximation Scheme for Lattice Models
A new approximation hierarchy, called the LPSUB scheme, is described for
the coupled cluster method (CCM). It is applicable to systems defined on a
regular spatial lattice. We then apply it to two well-studied prototypical
(spin-1/2 Heisenberg antiferromagnetic) spin-lattice models, namely: the XXZ
and the XY models on the square lattice in two dimensions. Results are obtained
in each case for the ground-state energy, the ground-state sublattice
magnetization and the quantum critical point. They are all in good agreement
with those from such alternative methods as spin-wave theory, series
expansions, quantum Monte Carlo methods and the CCM using the alternative
LSUB and DSUB schemes. Each of the three CCM schemes (LSUB, DSUB
and LPSUB) for use with systems defined on a regular spatial lattice is
shown to have its own advantages in particular applications
Controlling diffusive transport in confined geometries
We analyze the diffusive transport of Brownian particles in narrow channels
with periodically varying cross-section. The geometrical confinements lead to
entropic barriers, the particle has to overcome in order to proceed in
transport direction. The transport characteristics exhibit peculiar behaviors
which are in contrast to what is observed for the transport in potentials with
purely energetic barriers. By adjusting the geometric parameters of the channel
one can effectively tune the transport and diffusion properties. A prominent
example is the maximized enhancement of diffusion for particular channel
parameters. The understanding of the role of channel-shape provides the
possibility for a design of stylized channels wherein the quality of the
transport can be efficiently optimized.Comment: accepted for publication in Acta Physica Polonica
Transverse Magnetic Susceptibility of a Frustrated Spin- ---- Heisenberg Antiferromagnet on a Bilayer Honeycomb Lattice
We use the coupled cluster method (CCM) to study a frustrated
spin- ---- Heisenberg antiferromagnet
on a bilayer honeycomb lattice with stacking. Both nearest-neighbor (NN)
and frustrating next-nearest-neighbor antiferromagnetic (AFM) exchange
interactions are present in each layer, with respective exchange coupling
constants and . The two layers are
coupled with NN AFM exchanges with coupling strength . We calculate to high orders of approximation within the CCM
the zero-field transverse magnetic susceptibility in the N\'eel phase.
We thus obtain an accurate estimate of the full boundary of the N\'eel phase in
the plane for the zero-temperature quantum phase diagram. We
demonstrate explicitly that the phase boundary derived from is fully
consistent with that obtained from the vanishing of the N\'eel magnetic order
parameter. We thus conclude that at all points along the N\'eel phase boundary
quasiclassical magnetic order gives way to a nonclassical paramagnetic phase
with a nonzero energy gap. The N\'eel phase boundary exhibits a marked
reentrant behavior, which we discuss in detail
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