14,002 research outputs found
Static and dynamic traversable wormhole geometries satisfying the Ford-Roman constraints
It was shown by Ford and Roman in 1996 that quantum field theory severely
constrains wormhole geometries on a macroscopic scale. The first part of this
paper discusses a wide class of wormhole solutions that meet these constraints.
The type of shape function used is essentially generic. The constraints are
then discussed in conjunction with various redshift functions. Violations of
the weak energy condition and traversability criteria are also considered. The
second part of the paper analyzes analogous time-dependent (dynamic) wormholes
with the aid of differential forms. It is shown that a violation of the weak
energy condition is not likely to be avoidable even temporarily.Comment: 16 pages AMSTe
Annular electroconvection with shear
We report experiments on convection driven by a radial electrical force in
suspended annular smectic A liquid crystal films. In the absence of an
externally imposed azimuthal shear, a stationary one-dimensional (1D) pattern
consisting of symmetric vortex pairs is formed via a supercritical transition
at the onset of convection. Shearing reduces the symmetries of the base state
and produces a traveling 1D pattern whose basic periodic unit is a pair of
asymmetric vortices. For a sufficiently large shear, the primary bifurcation
changes from supercritical to subcritical. We describe measurements of the
resulting hysteresis as a function of the shear at radius ratio . This simple pattern forming system has an unusual combination of
symmetries and control parameters and should be amenable to quantitative
theoretical analysis.Comment: 12 preprint pages, 3 figures in 2 parts each. For more info, see
http://mobydick.physics.utoronto.c
Boronic Acid Derivatives Targeting HIV-1
A series of novel boronic acid derivatives containing either a pyrimidine or purine base was synthesized. The preparation involved the condensation of 4-bromobutyl boronic acid with the appropriate base. These acyclic nucleosides were designed as potential antiviral agents especially targeting the human immunodeficiency virus. Two analogues, 6-chloro-9-(4-dihydroxyborylbutyl)purine and 2,6-dichloro-9-(4-dihydroxyborylbutyl)purine, exhibited EC50 values of 7.7 µM and 0.99 µM, respectively, in an HIV-1 syncytial plaque reduction assay
Muon spin rotation measurements of the superfluid density in fresh and aged superconducting PuCoGa
We have measured the temperature dependence and magnitude of the superfluid
density via the magnetic field penetration depth
in PuCoGa (nominal critical temperature K) using the muon
spin rotation technique in order to investigate the symmetry of the order
parameter, and to study the effects of aging on the superconducting properties
of a radioactive material. The same single crystals were measured after 25 days
( K) and 400 days ( K) of aging at room temperature.
The temperature dependence of the superfluid density is well described in both
materials by a model using d-wave gap symmetry. The magnitude of the muon spin
relaxation rate in the aged sample, , where is the effective mass, is reduced by
about 70% compared to fresh sample. This indicates that the scattering from
self-irradiation induced defects is not in the limit of the conventional
Abrikosov-Gor'kov pair-breaking theory, but rather in the limit of short
coherence length (about 2 nm in PuCoGa) superconductivity.Comment: 11 page
Quantification of the effect of changes in steel microstructural parameters on EM sensor signals
In an accompanying paper being presented in NDESAI, the utility of Multifrequency electromagnetic sensors has been shown to be able to detect steel microstructure changes, for example the austenite to ferrite transformation and
the presence of decarburisation in high carbon steel rods. In the present study, steels with various carbon contents have been used to study the effect of phase balance changes (ferrite, pearlite, un-tempered martensite and tempered martensite) on the EM readings. With an increase in pearlite content in ferrite/ pearlite microstructures, the relative permeability and hence inductance value dec-reases. Changing the microstructural state from pearlite to martensite, in a high carbon steel, decreases the rel-ative permeability and hence inductance value, whilst tempering increases these values. In addition, steel wires, with a fully pearlitic microstructure have been used to determine whether the EM sensor can be used to quantify interlamellar spacing changes. The low frequency inductance value was found to increase approximately linearly with an increase in the interlamellar spacing for the range of values investigated
Monitoring of Steel Microstructures using Electromagnetic Sensors
The characterization of steel microstructures is an important tool for metallurgists as mechanical properties are controlled by microstructural parameters such as grain size, phase balance and precipitates. This paper descr-ibes multi frequency electromagnetic(EM)sensors that have been designed to detect changes in the relative permea-bility and resistivity of steel on-line during steel processing, which can be directly related to changes in
microstructure. Examples presented in this paper include both laboratory hot tests and industrial field trials for monitoring ofphase transformation in steels, detection of decarburization on a steel rod surface, and imaging of molten steel in the submerged entry nozzle (SEN) during continuous casting
Polaron Effects on Superexchange Interaction: Isotope Shifts of , , and in Layered Copper Oxides
A compact expression has been obtained for the superexchange coupling of
magnetic ions via intermediate anions with regard to polaron effects at both
magnetic ions and intermediate anions. This expression is used to analyze the
main features of the behavior of isotope shifts for temperatures of three types
in layered cuprates: the Neel temperatures (), critical temperatures of
transitions to a superconducting state (), and characteristic temperatures
of the pseudogap in the normal state ().Comment: 4 pages, 1 figur
Chronology Protection in Generalized Godel Spacetime
The effective action of a free scalar field propagating in the generalized
Godel spacetime is evaluated by the zeta-function regularization method. From
the result we show that the renormalized stress energy tensor may be divergent
at the chronology horizon. This gives a support to the chronology protection
conjecture.Comment: Latex 6 pages, typos correcte
Statistical-Mechanical Measure of Stochastic Spiking Coherence in A Population of Inhibitory Subthreshold Neurons
By varying the noise intensity, we study stochastic spiking coherence (i.e.,
collective coherence between noise-induced neural spikings) in an inhibitory
population of subthreshold neurons (which cannot fire spontaneously without
noise). This stochastic spiking coherence may be well visualized in the raster
plot of neural spikes. For a coherent case, partially-occupied "stripes"
(composed of spikes and indicating collective coherence) are formed in the
raster plot. This partial occupation occurs due to "stochastic spike skipping"
which is well shown in the multi-peaked interspike interval histogram. The main
purpose of our work is to quantitatively measure the degree of stochastic
spiking coherence seen in the raster plot. We introduce a new spike-based
coherence measure by considering the occupation pattern and the pacing
pattern of spikes in the stripes. In particular, the pacing degree between
spikes is determined in a statistical-mechanical way by quantifying the average
contribution of (microscopic) individual spikes to the (macroscopic)
ensemble-averaged global potential. This "statistical-mechanical" measure
is in contrast to the conventional measures such as the "thermodynamic" order
parameter (which concerns the time-averaged fluctuations of the macroscopic
global potential), the "microscopic" correlation-based measure (based on the
cross-correlation between the microscopic individual potentials), and the
measures of precise spike timing (based on the peri-stimulus time histogram).
In terms of , we quantitatively characterize the stochastic spiking
coherence, and find that reflects the degree of collective spiking
coherence seen in the raster plot very well. Hence, the
"statistical-mechanical" spike-based measure may be used usefully to
quantify the degree of stochastic spiking coherence in a statistical-mechanical
way.Comment: 16 pages, 5 figures, to appear in the J. Comput. Neurosc
Towards the statistical detection of the warm-hot intergalactic medium in intercluster filaments of the cosmic web.
Modern analyses of structure formation predict a universe tangled in a ‘cosmic web’ of dark matter and diffuse baryons. These theories further predict that at low z, a significant fraction of the baryons will be shock-heated to T ∼ 105–107 K yielding a warm–hot intergalactic medium (WHIM), but whose actual existence has eluded a firm observational confirmation. We present a novel experiment to detect the WHIM, by targeting the putative filaments connecting galaxy clusters. We use HST/COS to observe a remarkable quasi-stellar object (QSO) sightline that passes within Δd = 3 Mpc from the seven intercluster axes connecting seven independent cluster pairs at redshifts 0.1 ≤ z ≤ 0.5. We find tentative excesses of total H I, narrow H I (NLA; Doppler parameters b < 50 km s−1), broad H I (BLA; b ≥ 50 km s−1) and O VI absorption lines within rest-frame velocities of Δv ≲ 1000 km s−1 from the cluster-pairs redshifts, corresponding to ∼2, ∼1.7, ∼6 and ∼4 times their field expectations, respectively. Although the excess of O VI likely comes from gas close to individual galaxies, we conclude that most of the excesses of NLAs and BLAs are truly intergalactic. We find the covering fractions, fc, of BLAs close to cluster pairs are ∼4–7 times higher than the random expectation (at the ∼2σ c.l.), whereas the fc of NLAs and O VI are not significantly enhanced. We argue that a larger relative excess of BLAs compared to those of NLAs close to cluster pairs may be a signature of the WHIM in intercluster filaments. By extending this analysis to tens of sightlines, our experiment offers a promising route to detect the WHIM
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