23,603 research outputs found
Steady-State Two Atom Entanglement in a Pumped Cavity
In this paper we explore the possibility of a steady-state entanglement of
two two-level atoms inside a pumped cavity by taking into account cavity
leakage and the spontaneous emission of photons by the atoms. We describe the
system in the dressed state picture in which the coherence is built into the
dressed states while transitions between the dressed states are incoherent. Our
model assumes the vacuum Rabi splitting of the dressed states to be much larger
than any of the decay parameters of the system which allows atom-field
coherence to build up before any decay process takes over. We show that, under
our model, a pumping field cannot entangle two closed two-level atoms inside
the cavity in the steady-state, but a steady-state entanglement can be achieved
with two open two-level atoms.Comment: 19 pages, 5 figure
Modeling The Time Variability of Accreting Compact Sources
We present model light curves for accreting Black Hole Candidates (BHC) based
on a recently proposed model for their spectro-temporal properties. According
to this model, the observed light curves and aperiodic variability of BHC are
due to a series of soft photon injections at random (Poisson) intervals near
the compact object and their reprocessing into hard radiation in an extended
but non-uniform hot plasma corona surrounding the compact object. We argue that
the majority of the timing characteristics of these light curves are due to the
stochastic nature of the Comptonization process in the extended corona, whose
properties, most notably its radial density dependence, are imprinted in them.
We compute the corresponding Power Spectral Densities (PSD), autocorrelation
functions, time skewness of the light curves and time lags between the light
curves of the sources at different photon energies and compare our results to
observation. Our model light curves compare well with observations, providing
good fits to their overall morphology, as manifest by the autocorrelation and
skewness functions. The lags and PSDs of the model light curves are also in
good agreement with those observed (the model can even accommodate the presence
of QPOs). Finally, while most of the variability power resides at time scales
\gsim a few seconds, at the same time, the model allows also for shots of a
few msec in duration, in accordance with observation. We suggest that
refinements of this type of model along with spectral and phase lag information
can be used to probe the structure of this class of high energy sources.Comment: 23 pages Latex, 15 encapsulated postscript figures, to appear in the
Astrophysical Journa
A mechanism to pin skyrmions in chiral magnets
We propose a mechanism to pin skyrmions in chiral magnets by introducing
local maximum of magnetic exchange strength, which can be realized in chiral
magnetic thin films by engineering the local density of itinerate electrons.
Thus we find a way to artificially control the position of a single skyrmion in
chiral magnetic thin films. The stationary properties and the dynamical pinning
and depinning processes of an isolated skyrmion around a pinning center are
studied. We do a series of simulations to show that the critical current to
depin a skyrmion has linearly dependence on the pinning strength. We also
estimate the critical current to have order of magnitude
10^{7}\sim10^{8}A/m^{2}
Probing the Structure of Accreting Compact Sources Through X-Ray Time Lags and Spectra
We exhibit, by compiling all data sets we can acquire, that the Fourier
frequency dependent hard X-ray lags, first observed in the analysis of
aperiodic variability of the light curves of the black hole candidate Cygnus
X-1, appear to be a property shared by several other accreting black hole
candidate sources and also by the different spectral states of this source. We
then present both analytic and numerical models of these time lags resulting by
the process of Comptonization in a variety of hot electron configurations. We
argue that under the assumption that the observed spectra are due to
Comptonization, the dependence of the lags on the Fourier period provides a
means for mapping the spatial density profile of the hot electron plasma, while
the period at which the lags eventually level--off provides an estimate of the
size of the scattering cloud. We further examine the influence of the location
and spatial extent of the soft photon source on the form of the resulting lags
for a variety of configurations; we conclude that the study of the X-ray hard
lags can provide clues about these parameters of the Comptonization process
too. Fits of the existing data with our models indicate that the size of the
Comptonizing clouds are quite large in extent ( 1 light second) with
inferred radial density profiles which are in many instances inconsistent with
those of the standard dynamical models, while the extent of the source of soft
photons appears to be much smaller than those of the hot electrons by roughly
two orders of magnitude and its location consistent with the center of the hot
electron corona.Comment: 20 pages Latex, 11 postscript figures, to appear in the Astrophysical
Journal, Vol 512, Feb 20 issu
Orbital ordering in the ferromagnetic insulator CsAgF from first principles
We found, using density-functional theory calculations within the generalized
gradient approximation, that CsAgF is stabilized in the insulating
orthorhombic phase rather than in the metallic tetragonal phase. The lattice
distortion present in the orthorhombic phase corresponds to the
/ hole-orbital ordering of the Ag ions, and
this orbital ordering leads to the observed ferromagnetism, as confirmed by the
present total-energy calculations. This picture holds in the presence of
moderate 4d-electron correlation. The results are compared with the picture of
ferromagnetism based on the metallic tetragonal phase.Comment: 5 pages, 4 figures, 1 table; a few energy/moment entries in Table I
are corrected due to a proper treatment of the Ag 4s semicore stat
Semaphorin 4D Promotes Skeletal Metastasis in Breast Cancer.
Bone density is controlled by interactions between osteoclasts, which resorb bone, and osteoblasts, which deposit it. The semaphorins and their receptors, the plexins, originally shown to function in the immune system and to provide chemotactic cues for axon guidance, are now known to play a role in this process as well. Emerging data have identified Semaphorin 4D (Sema4D) as a product of osteoclasts acting through its receptor Plexin-B1 on osteoblasts to inhibit their function, tipping the balance of bone homeostasis in favor of resorption. Breast cancers and other epithelial malignancies overexpress Sema4D, so we theorized that tumor cells could be exploiting this pathway to establish lytic skeletal metastases. Here, we use measurements of osteoblast and osteoclast differentiation and function in vitro and a mouse model of skeletal metastasis to demonstrate that both soluble Sema4D and protein produced by the breast cancer cell line MDA-MB-231 inhibits differentiation of MC3T3 cells, an osteoblast cell line, and their ability to form mineralized tissues, while Sema4D-mediated induction of IL-8 and LIX/CXCL5, the murine homologue of IL-8, increases osteoclast numbers and activity. We also observe a decrease in the number of bone metastases in mice injected with MDA-MB-231 cells when Sema4D is silenced by RNA interference. These results are significant because treatments directed at suppression of skeletal metastases in bone-homing malignancies usually work by arresting bone remodeling, potentially leading to skeletal fragility, a significant problem in patient management. Targeting Sema4D in these cancers would not affect bone remodeling and therefore could elicit an improved therapeutic result without the debilitating side effects
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