80 research outputs found
Spin glasses without time-reversal symmetry and the absence of a genuine structural glass transition
We study the three-spin model and the Ising spin glass in a field using
Migdal-Kadanoff approximation. The flows of the couplings and fields indicate
no phase transition, but they show even for the three-spin model a slow
crossover to the asymptotic high-temperature behaviour for strong values of the
couplings. We also evaluated a quantity that is a measure of the degree of
non-self-averaging, and we found that it can become large for certain ranges of
the parameters and the system sizes. For the spin glass in a field the maximum
of non-self-averaging follows for given system size a line that resembles the
de Almeida-Thouless line. We conclude that non-self-averaging found in
Monte-Carlo simulations cannot be taken as evidence for the existence of a
low-temperature phase with replica-symmetry breaking. Models similar to the
three-spin model have been extensively discussed in order to provide a
description of structural glasses. Their theory at mean-field level resembles
the mode-coupling theory of real glasses. At that level the one-step replica
symmetry approach breaking predicts two transitions, the first transition being
dynamical and the second thermodynamical. Our results suggest that in real
finite dimensional glasses there will be no genuine transitions at all, but
that some features of mean-field theory could still provide some useful
insights.Comment: 11 pages, 11 figure
Comment on "General Method to Determine Replica Symmetry Breaking Transitions"
In a recent letter Marinari et al [Phys. Rev. Lett. 81, 1698 (1998)]
introduced a new method to study spin glass transitions and argued that by
probing replica symmetry (RS) as opposed to time reversal symmetry (TRS), their
method unambiguously shows that replica symmetry breaking (RSB) occurs in
short-range spin glasses. In this comment we show that while the new method is
indeed useful for studying transitions in systems where TRS is absent (such as
the p-spin model studied by them), the conclusion that it shows the existence
of RSB in short-range spin glasses is wrong.Comment: 1 page, RevTe
CD4-Lck Through TCR and in the Absence of Vav Exchange Factor Induces Bax Increase and Mitochondrial Damage
AbstractIn the present study, we aimed to demonstrate that CD4 may represent a critical turning point that governs the apoptotic and survival programs in T cells, without modifying the physical association with the TCR-CD3 complex. To address this issue, we have explored the possibility that the activation of CD4 may transduce apoptotic signals unless signaling effectors neutralize them. Our data show that in Jurkat T cells CD4 engagement by Leu3a mAb results in a rapid and strong increase of Lck kinase activity, subsequent alterations of mitochondrial membrane potential, and apoptosis. Critical parameters are coassociation of CD4/Lck with TCR/CD3 and up-regulation of the proapoptotic protein Bax. Indeed, Leu3a-mediated Lck activation failed to induce apoptotic features in Jurkat cells either defective for TCR/CD3 or overexpressing the antiapoptotic protein Bcl-2. Furthermore, we demonstrate that Leu3a treatment of Jurkat cells overexpressing Vav results in the inhibition of mitochondrial damage and apoptosis; this rescue effect is accompanied with a significant decrease of Bax expression observed in apoptotic cells. Our evidence that the activation of Lck activates in T cells apoptotic pathways which are counteracted by Vav, a signaling molecule that cooperates with CD28 to boost TCR signals, suggests a novel role for costimulation in protecting T cells from CD4-mediated cell death
Lennard-Jones binary mixture: a thermodynamical approach to glass transition
We study the liquid-glass transition of the Lennard-Jones binary mixture
introduced by Kob and Andersen from a thermodynamic point of view. By means of
the replica approach, translating the problem in the study of a molecular
liquid, we study the phase transition due to the entropy crisis and we find
that the Kauzmann's temperature \tk is . At the end we compare
analytical predictions with numerical results.Comment: 24 pages, 11 postscript figures. Revised version accepted for
pubblication on J. Chem. Phys. Numerical precision of the computations
improve
The activation of Csk by CD4 interferes with TCR-mediated activatory signaling
CD4-Lck recruitment to TCR/CD3, as well as Lck activation is essential for T cell activation. Indeed, the blockage of CD4-Lck recruitment to TCR during antigen recognition exerts a drastic inhibitory effect on T cell activation by interfering with both early and late phases of T cell signaling. In the present work, we report a novel inhibitory mechanism by which CD4 can shut down proximal T cell-activating signals. Indeed, we show that upon ligation of CD4 by antibodies the inhibitory kinase, p50csk, is strongly induced and prolonged during the time. In contrast, p50csk was not activated when TCR and CD4 were properly engaged by their ligands. We also demonstrate that anti-CD4 treatment stimulated Csk kinase associated to the membrane adapter, PAG/Cbp, without affecting the total amount of Csk bound to PAG/Cbp. As a consequence, early tyrosine phosphorylation events as well as downstream signaling pathways leading to IL-2 gene expression induced by TCR were inhibited in anti-CD4 pretreated cells. We suggest a new model to explain the activation of negative signals by CD4 molecule
New evidence for super-roughening in crystalline surfaces with disordered substrate
We study the behavior of the Binder cumulant related to long distance
correlation functions of the discrete Gaussian model of disordered substrate
crystalline surfaces. We exhibit numerical evidence that the non-Gaussian
behavior in the low- region persists on large length scales, in agreement
with the broken phase being super-rough.Comment: 10 pages and 4 figures, available at
http://chimera.roma1.infn.it/index_papers_complex.html . We have extended the
RG discussion and minor changes in the tex
On the energy minima of the Sherrington-Kirkpatrick model
We study properties of the energy minima obtained by quenching equilibrium configurations of the Sherrington-Kirkpatrick (SK) mean-field spin glass. We measure the probability distribution of the overlap among quenched configurations and the quenched energy, looking at the dependence on the starting equilibrium temperature, and performing a systematic analysis of finite-size effects
On the Approach to the Equilibrium and the Equilibrium Properties of a Glass-Forming Model
In this note we apply some theoretical predictions that arise in the mean
field framework for a large class of infinite range models to structural
glasses and we present a first comparison of these predictions with numerical
results.Comment: 22 pages, 15 figure
Numerical study of a short-range p-spin glass model in three dimensions
In this work we study numerically a short range p-spin glass model in three
dimensions. The behaviour of the model appears to be remarkably different from
mean field predictions. In fact it shares some features typical of models with
full replica-symmetry breaking (FRSB). Nevertheless, we believe that the
transition that we study is intrinsically different from the FRSB and basically
due to non-perturbative contributions. We study both the statics and the
dynamics of the system which seem to confirm our conjectures.Comment: 20 pages, 15 figure
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