699 research outputs found
Self-trapping of Bose-Einstein condensates in optical lattices
The self-trapping phenomenon of Bose-Einstein condensates (BECs) in optical
lattices is studied extensively by numerically solving the Gross-Pitaevskii
equation. Our numerical results not only reproduce the phenomenon that was
observed in a recent experiment [Anker {\it et al.}, Phys. Rev. Lett. {\bf 94}
(2005)020403], but also find that the self-trapping breaks down at long
evolution times, that is, the self-trapping in optical lattices is only
temporary. The analysis of our numerical results shows that the self-trapping
in optical lattices is related to the self-trapping of BECs in a double-well
potential. A possible mechanism of the formation of steep edges in the wave
packet evolution is explored in terms of the dynamics of relative phases
between neighboring wells.Comment: 8 pages, 15 figure
Sensitive frequency-dependence of the carrier-envelope phase effect on bound-bound transition: an interference perspective
We investigate numerically with Hylleraas coordinates the frequency
dependence of the carrier-envelope phase (CEP) effect on bound-bound
transitions of helium induced by an ultrashort laser pulse of few cycles. We
find that the CEP effect is very sensitive to the carrier frequency of the
laser pulse, occurring regularly even at far-off resonance frequencies. By
analyzing a two-level model, we find that the CEP effect can be attributed to
the quantum interference between neighboring multi-photon transition pathways,
which is made possible by the broadened spectrum of the ultrashort laser pulse.
A general picture is developed along this line to understand the sensitivity of
the CEP effect to laser's carrier frequency. Multi-level influence on the CEP
effect is also discussed
The long-lasting optical afterglow plateau of short burst GRB 130912A
The short burst GRB 130912A was detected by Swift, Fermi satellites and
several ground-based optical telescopes. Its X-ray light curve decayed with
time normally. The optical emission, however, displayed a long term plateau,
which is the longest one in current short GRB observations. In this work we
examine the physical origin of the X-ray and optical emission of this peculiar
event. We find that the canonical forward shock afterglow emission model can
account for the X-ray and optical data self-consistently and the energy
injection model that has been widely adopted to interpret the
shallowly-decaying afterglow emission is not needed. We also find that the
burst was born in a very-low density interstellar medium, consistent with the
compact object merger model. Significant fractions of the energy of the forward
shock have been given to accelerate the non-thermal electrons and amplify the
magnetic fields (i.e., and , respectively), which are much larger than those inferred in most short
burst afterglow modeling and can explain why the long-lasting optical afterglow
plateau is rare in short GRBs.Comment: 5 pages, 2 figure
Topological Photonic Phase in Chiral Hyperbolic Metamaterials
Recently the possibility of achieving one-way backscatter immune
transportation of light by mimicking the topological order present within
certain solid state systems, such as topological insulators, has received much
attention. Thus far however, demonstrations of non-trivial topology in
photonics have relied on photonic crystals with precisely engineered lattice
structures, periodic on the scale of the operational wavelength and composed of
finely tuned, complex materials. Here we propose a novel effective medium
approach towards achieving topologically protected photonic surface states
robust against disorder on all length scales and for a wide range of material
parameters. Remarkably, the non-trivial topology of our metamaterial design
results from the Berry curvature arising from the transversality of
electromagnetic waves in a homogeneous medium. Our investigation therefore acts
to bridge the gap between the advancing field of topological band theory and
classical optical phenomena such as the Spin Hall effect of light. The
effective medium route to topological phases will pave the way for highly
compact one-way transportation of electromagnetic waves in integrated photonic
circuits.Comment: 11 pages, 3 figures. To appear in PR
Characteristics of the coal quality and elemental geochemistry in Permian coals from the Xinjier mine in the Huainan Coalfield, north China: Influence of terrigenous inputs
Fifty-six coals along with host rocks of Permian age (including samples from the Shanxi Formation, Lower Shihezi Formation and Upper Shihezi Formation) collected from the active Xinjier mine in the Huainan Coalfield, north China were studied in the present work. The overall object of this study was to characterize the coal quality and elucidate the possible genetic types for trace-element enrichment of Xinjier coals. Based on Chinese National Standards, all the coals can be classified as medium-high-volatile to high-volatile coal and ultra-low sulfur to low sulfur coal. The vitrinite-dominated Xinjier coals show phyiso-chemical properties difference along the coal-bearing strata. Compared to the coals of the Shanxi Formation, vitrinite contents are elevated in the coals of the Lower Shihezi and Upper Shihezi Formations, which indicates that they were probably exposed to a more reducing environment during peat accumulation. Most elements, such as Al2O3, K2O, Y, Se, and Sb, are enriched compared to those of Chinese and World coals. The major-element oxides Al2O3 and K2O as well as trace elements Th and Y increase from the lower to upper seams. Some elements are distinctly concentrated in host rocks (roof, floor, and parting) compared with adjacent coals. These geochemical anomalies and "increasing stratigraphically upward" ash yield trend are attributed to influence from terrigenous inputs. Elements in Xinjier coals were classified into three geochemical groups based on the statistical analysis. In particular, sequential extraction experiments of selected coals found that As, Se, and Sb predominantly occur as organic associations in coal.</p
Influence of magmatic intrusions on organic nitrogen in coal: A case study from the Zhuji mine, the Huainan coalfield, China
Although the influence of magmatic intrusions on coal has been studied extensively at many locations, data on changes of organic nitrogen forms in coal in response to this kind of geological instantaneous heating is still scarce. To fill this information gap, a total of five coal samples, including four coal samples collected along a coal transect approaching a magmatic intrusion and one unaltered coal sample, were collected from the No. 3 coal seam of the Zhuji mine in the Huainan coalfield, China and were analyzed for organic nitrogen forms using X-ray photoelectron spectroscopy (XPS), together with the determination of coal quality parameters and elemental composition. Due to the effect of magmatic intrusion, ash yield and carbon content of the coals increase, whereas moisture, volatile matter, oxygen, nitrogen and total sulfur decrease. The N-5 peak is dominant in unaltered and moderately altered coals, but disappears entirely in the coals adjacent to the magmatic intrusion due to the strong thermal influence. The N-Q peak mainly represents "protonated" quaternary nitrogen in unaltered and moderately altered coals. The N-Q peak can be transformed to the N-6 peak through the deprotonation of "protonated" quaternary nitrogen resulting from the loss of oxygen groups under the thermal influence of the magmatic intrusion. Closer to the magmatic intrusion, the N-Q peak is assigned to "graphitic" quaternary nitrogen, which increases sharply and becomes the predominant form eventually. Magmatic intrusion is responsible for the conversion of less stable nitrogen forms to more stable forms in coal
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