77,762 research outputs found
Non-neutral theory of biodiversity
We present a non-neutral stochastic model for the dynamics taking place in a
meta-community ecosystems in presence of migration. The model provides a
framework for describing the emergence of multiple ecological scenarios and
behaves in two extreme limits either as the unified neutral theory of
biodiversity or as the Bak-Sneppen model. Interestingly, the model shows a
condensation phase transition where one species becomes the dominant one, the
diversity in the ecosystems is strongly reduced and the ecosystem is
non-stationary. This phase transition extend the principle of competitive
exclusion to open ecosystems and might be relevant for the study of the impact
of invasive species in native ecologies.Comment: 4 pages, 3 figur
Electroexcitation of the Roper resonance in the relativistic quark models
The amplitudes of the transition gamma* N -> P11(1440) are calculated within
light-front relativistic quark model assuming that the P11(1440) is the first
radial excitation of the 3q ground state. The results are presented along with
the predictions obtained in other relativistic quark models. In comparison with
the previous calculations, we have extended the range of Q^2 up to 4.5 GeV^2 to
cover the kinematic interval of the forthcoming experimental data. Using
approach based on PCAC, we have checked the relative sign between quark model
predictions for the N and P11(1440) contributions to the pion electroproduction
found in previous investigations.Comment: 5 pages, 1 figure, accepted by PR
Origin of the high piezoelectric response in PbZr(1-x)TixO3
High resolution x-ray powder diffraction measurements on poled PbZr(1-x)TixO3
(PZT) ceramic samples close to the rhombohedral-tetragonal phase boundary (the
so-called morphotropic phase boundary, MPB) have shown that for both
rhombohedral and tetragonal compositions, the piezoelectric elongation of the
unit cell does not occur along the polar directions but along those directions
associated with the monoclinic distortion. This work provides the first direct
evidence for the origin of the very high piezoelectricity in PZT.Comment: 4 pages, 4 EPS figures embedded. More specific title and abstract. To
appear in Phys. Rev. Let
The suppression of hidden order and onset of ferromagnetism in URu2Si2 via Re substitution
Substitution of Re for Ru in the heavy fermion compound URu2Si2 suppresses
the hidden order transition and gives rise to ferromagnetism at higher
concentrations. The hidden order transition of URu(2-x)Re(x)Si2, tracked via
specific heat and electrical resistivity measurements, decreases in temperature
and broadens, and is no longer observed for x>0.1. A critical scaling analysis
of the bulk magnetization indicates that the ferromagnetic ordering temperature
and ordered moment are suppressed continuously towards zero at a critical
concentration of x = 0.15, accompanied by the additional suppression of the
critical exponents gamma and (delta-1) towards zero. This unusual trend appears
to reflect the underlying interplay between Kondo and ferromagnetic
interactions, and perhaps the proximity of the hidden order phase.Comment: 8 pgs, 5 figs, ICM 2009; please refer to Phys. Rev. Lett. 103, 076404
(2009), arXiv:0908.1809 for details on magnetic scaling and phase diagram
(reference added to this version
Anxious to see you: Neuroendocrine mechanisms of social vigilance and anxiety during adolescence.
Social vigilance is a behavioral strategy commonly used in adverse or changing social environments. In animals, a combination of avoidance and vigilance allows an individual to evade potentially dangerous confrontations while monitoring the social environment to identify favorable changes. However, prolonged use of this behavioral strategy in humans is associated with increased risk of anxiety disorders, a major burden for human health. Elucidating the mechanisms of social vigilance in animals could provide important clues for new treatment strategies for social anxiety. Importantly, during adolescence the prevalence of social anxiety increases significantly. We hypothesize that many of the actions typically characterized as anxiety behaviors begin to emerge during this time as strategies for navigating more complex social structures. Here, we consider how the social environment and the pubertal transition shape neural circuits that modulate social vigilance, focusing on the bed nucleus of the stria terminalis and prefrontal cortex. The emergence of gonadal hormone secretion during adolescence has important effects on the function and structure of these circuits, and may play a role in the emergence of a notable sex difference in anxiety rates across adolescence. However, the significance of these changes in the context of anxiety is still uncertain, as not enough studies are sufficiently powered to evaluate sex as a biological variable. We conclude that greater integration between human and animal models will aid the development of more effective strategies for treating social anxiety
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