23,623 research outputs found
Critical properties of the one-dimensional spin-1/2 antiferromagnetic Heisenberg model in the presence of a uniform field
In the presence of a uniform field the one-dimensional spin-
antiferromagnetic Heisenberg model develops zero frequency excitations at
field-dependent 'soft mode' momenta. We determine three types of critical
quantities, which we extract from the finite-size dependence of the lowest
excitation energies, the singularities in the static structure factors and the
infrared singularities in the dynamical structure factors at the soft mode
momenta. We also compare our results with the predictions of conformal field
theory.Comment: 12 pages, REVTEX, 7 figures, submitted to Physical Review
Fragmentation of spherical radioactive heavy nuclei as a novel probe of transient effects in fission
Peripheral collisions with radioactive heavy-ion beams at relativistic
energies are discussed as an innovative approach for probing the transient
regime experienced by fissile systems evolving towards quasi-equilibrium. A
dedicated experiment using the advanced technical installations of GSI,
Darmstadt, permitted to realize ideal conditions for the investigation of
relaxation effects in the meta-stable well. Combined with a highly sensitive
experimental signature, it provides a measure of the transient effects with
respect to the flux over the fission barrier. Within a two-step reaction
process, 45 proton-rich unstable spherical isotopes produced by
projectile-fragmentation of a stable 238U beam have been used as secondary
projectiles. The fragmentation of the radioactive projectiles on lead results
in nearly spherical compound nuclei which span a wide range in excitation
energy and fissility. The decay of these excited systems by fission is studied
with a dedicated set-up which permits the detection of both fission products in
coincidence and the determination of their atomic numbers with high resolution.
The width of the fission-fragment nuclear charge distribution is shown to be
specifically sensitive to pre-saddle transient effects and is used to establish
a clock for the passage of the saddle point. The comparison of the experimental
results with model calculations points to a fission delay of (3.3+/-0.7).10-21s
for initially spherical compound nuclei, independent of excitation energy and
fissility. This value suggests a nuclear dissipation strength at small
deformation of (4.5+/-0.5).1021s-1. The very specific combination of the
physics and technical equipment exploited in this work sheds light on previous
controversial conclusions.Comment: 38 pages, 15 figure
Bacterial toxins: friends or foes?
Many emerging and reemerging bacterial pathogens synthesize toxins that serve as primary virulence factors. We highlight seven bacterial toxins produced by well-established or newly emergent pathogenic microbes. These toxins, which affect eukaryotic cells by a variety of means, include Staphylococcus aureus alpha-toxin, Shiga toxin, cytotoxic necrotizing factor type 1, Escherichia coli heat-stable toxin, botulinum and tetanus neurotoxins, and S. aureus toxic-shock syndrome toxin. For each, we discuss the information available on its synthesis and structure, mode of action, and contribution to virulence. We also review the role certain toxins have played in unraveling signal pathways in eukaryotic cells and summarize the beneficial uses of toxins and toxoids. Our intent is to illustrate the importance of the analysis of bacterial toxins to both basic and applied sciences
Jet directions in Seyfert galaxies: B and I imaging data
We present the results of broad-band B and I imaging observations for a
sample of 88 Seyfert galaxies (29 Seyfert 1's and 59 Seyfert 2's), selected
from a mostly isotropic property, the flux at 60m. We also present the B
and I imaging results for an additional sample of 20 Seyfert galaxies (7
Seyfert 1's and 13 Seyfert 2's), selected from the literature and known to have
extended radio emission. The I band images are fitted with ellipses to
determine the position angle and ellipticity of the host galaxy major axis.
This information will be used in a future paper, combined with information from
radio observations, to study the orientation of radio jets relative to the
plane of their host galaxies (Kinney et al. 2000). Here we present surface
brightness profiles and magnitudes in the B and I bands, as well as mean
ellipticities and major axis position angles.Comment: To appear in The Astrophysical Journal Supplement Series, June 2000.
48 pages, 7 tables, 19 gif and 11 postscript figures. Better quality figures
can be obtained with the autho
On current ambiguity in the interpretation of fission at intermediate excitation energy
AbstractVarious approaches are currently used to interpret experimental data on fission. We critically examine a wide set of observables measured for fission of Po206,210 nuclei at medium excitation energy, and illustrate the ambiguity in current analysis. Dynamical calculations based on the four-dimensional Langevin equation using a macroscopic potential energy landscape are performed, and found to consistently describe available measurements. This observation calls into question the robustness of recent analysis based on statistical-model calculations and concluding, on the contrary, to substantial shell effects at the fission saddle point in Po206,210. The inconsistency in interpretation reached by the two approaches shows that, depending on the system, the conclusion can be strongly model-dependent. Although this may not be surprising, it emphasizes the today still limited reliability of firmly extracting fundamental nuclear properties from customary analysis
Psychophysiological correlates of peritraumatic dissociative responses in survivors of life-threatening cardiac events
The psychophysiological startle response pattern associated with peritraumatic dissociation (DISS) was studied in 103 survivors of a life-threatening cardiac event (mean age 61.0 years, SD 13.95). Mean time period since the cardiac event was 37 (79 IQD) months. All patients underwent a psychodiagnostic evaluation (including the Peritraumatic Dissociative Experiences Questionnaire) and a psychophysiological startle experience which comprised the delivery of 15 acoustic startle trials. Magnitude and habituation to trials were measured by means of electromyogram (EMG) and skin conductance responses (SCR). Thirty-two (31%) subjects were indexed as patients with a clinically significant level of DISS symptoms. High-level DISS was associated with a higher magnitude of SCR (ANOVA for repeated measures p = 0.017) and EMG (p = 0.055) and an impaired habituation (SCR slope p = 0.064; EMG slope p = 0.005) in comparison to subjects with no or low DISS. In a subgroup analysis, high-level DISS patients with severe post-traumatic stress disorder (PTSD; n = 11) in comparison to high-level DISS patients without subsequent PTSD (n = 19) exhibited higher EMG amplitudes during all trials (repeated measures analysis of variance IF = 5.511, p = 0.026). The results demonstrate exaggerated startle responses in SCR and EMG measures - an abnormal defensive response to high-intensity stimuli which indicates a steady state of increased arousal. DISS patients without PTSD exhibited balanced autonomic responses to the startle trials. DISS may, therefore, unfold malignant properties only in combination with persistent physiological hyperarousability. Copyright (C) 2002 S. Karger AG, Basel
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