59 research outputs found
Large Deviations of the Free-Energy in Diluted Mean-Field Spin-Glass
Sample-to-sample free energy fluctuations in spin-glasses display a markedly
different behaviour in finite-dimensional and fully-connected models, namely
Gaussian vs. non-Gaussian. Spin-glass models defined on various types of random
graphs are in an intermediate situation between these two classes of models and
we investigate whether the nature of their free-energy fluctuations is Gaussian
or not. It has been argued that Gaussian behaviour is present whenever the
interactions are locally non-homogeneous, i.e. in most cases with the notable
exception of models with fixed connectivity and random couplings . We confirm these expectation by means of various analytical
results. In particular we unveil the connection between the spatial
fluctuations of the populations of populations of fields defined at different
sites of the lattice and the Gaussian nature of the free-energy fluctuations.
On the contrary on locally homogeneous lattices the populations do not
fluctuate over the sites and as a consequence the small-deviations of the free
energy are non-Gaussian and scales as in the Sherrington-Kirkpatrick model
Analysis of the infinity-replica symmetry breaking solution of the Sherrington-Kirkpatrick model
In this work we analyse the Parisi's infinity-replica symmetry breaking
solution of the Sherrington - Kirkpatrick model without external field using
high order perturbative expansions. The predictions are compared with those
obtained from the numerical solution of the infinity-replica symmetry breaking
equations which are solved using a new pseudo-spectral code which allows for
very accurate results. With this methods we are able to get more insight into
the analytical properties of the solutions. We are also able to determine
numerically the end-point x_{max} of the plateau of q(x) and find that lim_{T
--> 0} x_{max}(T) > 0.5.Comment: 15 pages, 11 figures, RevTeX 4.
Spin glass transition in a magnetic field: a renormalization group study
We study the transition of short range Ising spin glasses in a magnetic
field, within a general replica symmetric field theory, which contains three
masses and eight cubic couplings, that is defined in terms of the fields
representing the replicon, anomalous and longitudinal modes. We discuss the
symmetry of the theory in the limit of replica number n to 0, and consider the
regular case where the longitudinal and anomalous masses remain degenerate.
The spin glass transitions in zero and non-zero field are analyzed in a
common framework. The mean field treatment shows the usual results, that is a
transition in zero field, where all the modes become critical, and a transition
in non-zero field, at the de Almeida-Thouless (AT) line, with only the replicon
mode critical. Renormalization group methods are used to study the critical
behavior, to order epsilon = 6-d. In the general theory we find a stable
fixed-point associated to the spin glass transition in zero field. This
fixed-point becomes unstable in the presence of a small magnetic field, and we
calculate crossover exponents, which we relate to zero-field critical
exponents. In a finite magnetic field, we find no physical stable fixed-point
to describe the AT transition, in agreement with previous results of other
authors.Comment: 36 pages with 4 tables. To be published in Phys. Rev.
Multifractal Analysis of the Coupling Space of Feed-Forward Neural Networks
Random input patterns induce a partition of the coupling space of
feed-forward neural networks into different cells according to the generated
output sequence. For the perceptron this partition forms a random multifractal
for which the spectrum can be calculated analytically using the
replica trick. Phase transition in the multifractal spectrum correspond to the
crossover from percolating to non-percolating cell sizes. Instabilities of
negative moments are related to the VC-dimension.Comment: 10 pages, Latex, submitted to PR
On the scaling and ageing behaviour of the alternating susceptibility in spin glasses and local scale-invariance
The frequency-dependent scaling of the dispersive and dissipative parts of
the alternating susceptibility is studied for spin glasses at criticality. An
extension of the usual -scaling is proposed. Simulational data from
the three-dimensional Ising spin glass agree with this new scaling form and
moreover reproduce well the scaling functions explicitly calculated for systems
satisfying local scale-invariance. There is also a qualitative agreement with
existing experimental data.Comment: 19 pages, 2 figures, to appear in special issue of J. Phys. Cond.
Matt. dedicated to Lothar Schaefer on the occasion of his 60th birthday,
final form with IOP macro
The Phase Diagram of an Anisotropic Potts Model
A study is made of an anisotropic Potts model in three dimensions where the
coupling depends on both the Potts state on each site but also the direction of
the bond between them using both analytical and numerical methods. The phase
diagram is mapped out for all values of the exchange interactions. Six distinct
phases are identified. Monte Carlo simulations have been used to obtain the
order parameter and the values for the energy and entropy in the ground state
and also the transition temperatures. Excellent agreement is found between the
simulated and analytic results. We find one region where there are two phase
transitions with the lines meeting in a triple point. The orbital ordering that
occurs in occurs as one of the ordered phases.Comment: 30 pages, 19 figures, one tabl
Static chaos and scaling behaviour in the spin-glass phase
We discuss the problem of static chaos in spin glasses. In the case of
magnetic field perturbations, we propose a scaling theory for the spin-glass
phase. Using the mean-field approach we argue that some pure states are
suppressed by the magnetic field and their free energy cost is determined by
the finite-temperature fixed point exponents. In this framework, numerical
results suggest that mean-field chaos exponents are probably exact in finite
dimensions. If we use the droplet approach, numerical results suggest that the
zero-temperature fixed point exponent is very close to
. In both approaches is the lower critical dimension in
agreement with recent numerical simulations.Comment: 28 pages + 6 figures, LateX, figures uuencoded at the end of fil
Field Theory of Fluctuations in Glasses
We develop a field-theoretical description of dynamical heterogeneities and
fluctuations in supercooled liquids close to the (avoided) MCT singularity.
Using quasi-equilibrium arguments we eliminate time from the description and we
completely characterize fluctuations in the beta regime. We identify different
sources of fluctuations and show that the most relevant ones are associated to
variations of "self-induced disorder" in the initial condition of the dynamics.
It follows that heterogeneites can be describes through a cubic field theory
with an effective random field term. The phenomenon of perturbative dimensional
reduction ensues, well known in random field problems, which implies an upper
critical dimension of the theory equal to 8. We apply our theory to finite size
scaling for mean-field systems and we test its prediction against numerical
simulations
Functional Characterization of the Plasmodium falciparum Chloroquine-Resistance Transporter (PfCRT) in Transformed Dictyostelium discoideum Vesicles
Chloroquine (CQ)-resistant Plasmodium falciparum malaria has been a global health catastrophe, yet much about the CQ resistance (CQR) mechanism remains unclear. Hallmarks of the CQR phenotype include reduced accumulation of protonated CQ as a weak base in the digestive vacuole of the erythrocyte-stage parasite, and chemosensitization of CQ-resistant (but not CQ-sensitive) P. falciparum by agents such as verapamil. Mutations in the P. falciparum CQR transporter (PfCRT) confer CQR; particularly important among these mutations is the charge-loss substitution K→T at position 76. Dictyostelium discoideum transformed with mutant PfCRT expresses key features of CQR including reduced drug accumulation and verapamil chemosensitization.We describe the isolation and characterization of PfCRT-transformed, hematin-free vesicles from D. discoideum cells. These vesicles permit assessments of drug accumulation, pH, and membrane potential that are difficult or impossible with hematin-containing digestive vacuoles from P. falciparum-infected erythrocytes. Mutant PfCRT-transformed D. discoideum vesicles show features of the CQR phenotype, and manipulations of vesicle membrane potential by agents including ionophores produce large changes of CQ accumulation that are dissociated from vesicular pH. PfCRT in its native or mutant form blunts the ability of valinomycin to reduce CQ accumulation in transformed vesicles and decreases the ability of K(+) to reverse membrane potential hyperpolarization caused by valinomycin treatment.Isolated vesicles from mutant-PfCRT-transformed D. discoideum exhibit features of the CQR phenotype, consistent with evidence that the drug resistance mechanism operates at the P. falciparum digestive vacuole membrane in malaria. Membrane potential apart from pH has a major effect on the PfCRT-mediated CQR phenotype of D. discoideum vesicles. These results support a model of PfCRT as an electrochemical potential-driven transporter in the drug/metabolite superfamily that (appropriately mutated) acts as a saturable simple carrier for the facilitated diffusion of protonated CQ
Influence of socioeconomic factors on pregnancy outcome in women with structural heart disease
OBJECTIVE: Cardiac disease is the leading cause of indirect maternal mortality. The aim of this study was to analyse to what extent socioeconomic factors influence the outcome of pregnancy in women with heart disease. METHODS: The Registry of Pregnancy and Cardiac disease is a global prospective registry. For this analysis, countries that enrolled ≥10 patients were included. A combined cardiac endpoint included maternal cardiac death, arrhythmia requiring treatment, heart failure, thromboembolic event, aortic dissection, endocarditis, acute coronary syndrome, hospitalisation for cardiac reason or intervention. Associations between patient characteristics, country characteristics (income inequality expressed as Gini coefficient, health expenditure, schooling, gross domestic product, birth rate and hospital beds) and cardiac endpoints were checked in a three-level model (patient-centre-country). RESULTS: A total of 30 countries enrolled 2924 patients from 89 centres. At least one endpoint occurred in 645 women (22.1%). Maternal age, New York Heart Association classification and modified WHO risk classification were associated with the combined endpoint and explained 37% of variance in outcome. Gini coefficient and country-specific birth rate explained an additional 4%. There were large differences between the individual countries, but the need for multilevel modelling to account for these differences disappeared after adjustment for patient characteristics, Gini and country-specific birth rate. CONCLUSION: While there are definite interregional differences in pregnancy outcome in women with cardiac disease, these differences seem to be mainly driven by individual patient characteristics. Adjustment for country characteristics refined the results to a limited extent, but maternal condition seems to be the main determinant of outcome
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