53,429 research outputs found
On Dust Extinction of Gamma-ray Burst Host Galaxies
Although it is well recognized that gamma-ray burst (GRB) afterglows are
obscured and reddened by dust in their host galaxies, the wavelength-dependence
and quantity of dust extinction are still poorly known. Current studies on this
mostly rely on fitting the afterglow spectral energy distributions (SEDs) with
template extinction models. The inferred extinction (both quantity and
wavelength-dependence) and dust-to-gas ratios are often in disagreement with
that obtained from dust depletion and X-ray spectroscopy studies. We argue that
this discrepancy could result from the prior assumption of a template
extinction law. We propose an analytical formula to approximate the GRB host
extinction law. With the template extinction laws self-contained, and the
capability of revealing extinction laws differing from the conventional ones,
it is shown that this is a powerful approach in modeling the afterglow SEDs to
derive GRB host extinction.Comment: 9 pages, 4 figures; The Astrophysical Journal, in press (2008 Oct 1
issue
Phase diagram of the two-chain Hubbard model
We have calculated the charge gap and spin gap for the two-chain Hubbard
model as a function of the on-site Coulomb interaction and the interchain
hopping amplitude. We used the density matrix renormalization group method and
developed a method to calculate separately the gaps numerically for the
symmetric and antisymmetric modes with respect to the exchange of the chain
indices. We have found very different behaviors for the weak and strong
interaction cases. Our calculated phase diagram is compared to the one obtained
by Balents and Fisher using the weak coupling renormalization group technique.Comment: 4 pages, 6 figures, to appear in PR
To Tube or Not to Tube? The Role of Intubation during Stroke Thrombectomy.
In the 10 years since the FDA first cleared the use of endovascular devices for the treatment of acute stroke, definitive evidence that such therapy improves outcomes remains lacking. The decision to intubate patients undergoing stroke thrombectomy impacts multiple variables that may influence outcomes after stroke. Three main areas where intubation may deleteriously affect acute stroke management include the introduction of delays in revascularization, fluctuations in peri-procedural blood pressure, and hypocapnia, resulting in cerebral vasoconstriction. In this mini-review, we discuss the evidence supporting these limitations of intubation during stroke thrombectomy and encourage neurohospitalists, neurocritical care specialists, and neurointerventionalists to carefully consider the decision to intubate during thrombectomy and provide strategies to avoid potential complications associated with its use in acute stroke
Enhancement of Quantum Tunneling for Excited States in Ferromagnetic Particles
A formula suitable for a quantitative evaluation of the tunneling effect in a
ferromagnetic particle is derived with the help of the instanton method. The
tunneling between n-th degenerate states of neighboring wells is dominated by a
periodic pseudoparticle configuration. The low-lying level-splitting previously
obtained with the LSZ method in field theory in which the tunneling is viewed
as the transition of n bosons induced by the usual (vacuum) instanton is
recovered. The observation made with our new result is that the tunneling
effect increases at excited states. The results should be useful in analyzing
results of experimental tests of macroscopic quantum coherence in ferromagnetic
particles.Comment: 18 pages, LaTex, 1 figur
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Experimental and Numerical Investigation on Progressive Collapse Resistance of Post-tensioned Precast Concrete Beam-Column Sub-assemblages
In this paper, four 1/2 scaled precast concrete (PC) beam-column sub-assemblages with high performance connection were tested under push-down loading procedure to study the load resisting mechanism of PC frames subjected to different column removal scenarios. The parameters investigated include the location of column removal and effective prestress in tendons. The test results indicated that the failure modes of unbonded post-tensioned precast concrete (PTPC) frames were different from that of reinforced concrete (RC) frames: no cracks formed in the beams and wide opening formed near the beam to column interfaces. For specimens without overhanging beams, the failure of side column was eccentric compression failure. Moreover, the load resisting mechanisms in PC frames were significantly different from that of RC frames: the compressive arch action (CAA) developed in concrete during column removal was mainly due to actively applied pre-compressive stress in the concrete; CAA will not vanish when severe crush in concrete occurred. Thus, it may provide negative contribution for load resistance when the displacement exceeds one-beam depth; the tensile force developed in the tendons could provide catenary action from the beginning of the test. Moreover, to deeper understand the behavior of tested specimens, numerical analyses were carried out. The effects of concrete strength, axial compression ratio at side columns, and loading approaches on the behavior of the sub-assemblages were also investigated based on validated numerical analysis
Periodic Bounce for Nucleation Rate at Finite Temperature in Minisuperspace Models
The periodic bounce configurations responsible for quantum tunneling are
obtained explicitly and are extended to the finite energy case for
minisuperspace models of the Universe. As a common feature of the tunneling
models at finite energy considered here we observe that the period of the
bounce increases with energy monotonically. The periodic bounces do not have
bifurcations and make no contribution to the nucleation rate except the one
with zero energy. The sharp first order phase transition from quantum tunneling
to thermal activation is verified with the general criterions.Comment: 17 pages, 5 postscript figures include
A minimal two-band model for the superconducting Fe-pnictides
Following the discovery of the Fe-pnictide superconductors, LDA band
structure calculations showed that the dominant contributions to the spectral
weight near the Fermi energy came from the Fe 3d orbitals. The Fermi surface is
characterized by two hole surfaces around the point and two electron
surfaces around the M point of the 2 Fe/cell Brillouin zone. Here, we describe
a 2-band model that reproduces the topology of the LDA Fermi surface and
exhibits both ferromagnetic and spin density wave (SDW)
fluctuations. We argue that this minimal model contains the essential low
energy physics of these materials.Comment: 5 figures, 5 page
Scalable Quantum Networks based on Few-Qubit Registers
We describe and analyze a hybrid approach to scalable quantum computation
based on an optically connected network of few-qubit quantum registers. We show
that probabilistically connected five-qubit quantum registers suffice for
deterministic, fault-tolerant quantum computation even when state preparation,
measurement, and entanglement generation all have substantial errors. We
discuss requirements for achieving fault-tolerant operation for two specific
implementations of our approach.Comment: 4 pages, 3 figures (new figures 1 and 3
Synthesis of empty bacterial microcompartments, directed organelle protein incorporation, and evidence of filament-associated organelle movement
Compartmentalization is an important process, since it allows the segregation of metabolic activities and, in the era of synthetic biology, represents an important tool by which defined microenvironments can be created for specific metabolic functions. Indeed, some bacteria make specialized proteinaceous metabolic compartments called bacterial microcompartments (BMCs) or metabolosomes. Here we demonstrate that the shell of the metabolosome (representing an empty BMC) can be produced within E. coil cells by the coordinated expression of genes encoding structural proteins. A plethora of diverse structures can be generated by changing the expression profile of these genes, including the formation of large axial filaments that interfere with septation. Fusing GFP to PduC, PduD, or PduV, none of which are shell proteins, allows regiospecific targeting of the reporter group to the empty BMC. Live cell imaging provides unexpected evidence of filament-associated BMC movement within the cell in the presence of Pdu
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