5,178 research outputs found
Two-loop Critical Fluctuation-Dissipation Ratio for the Relaxational Dynamics of the O(N) Landau-Ginzburg Hamiltonian
The off-equilibrium purely dissipative dynamics (Model A) of the O(N) vector
model is considered at criticality in an up to
O(). The scaling behavior of two-time response and correlation
functions at zero momentum, the associated universal scaling functions, and the
nontrivial limit of the fluctuation-dissipation ratio are determined in the
aging regime.Comment: 21 pages, 6 figures. Discussion enlarged and two figures added. Final
version accepted for publication in Phys. Rev.
Ageing Properties of Critical Systems
In the past few years systems with slow dynamics have attracted considerable
theoretical and experimental interest. Ageing phenomena are observed during
this ever-lasting non-equilibrium evolution. A simple instance of such a
behaviour is provided by the dynamics that takes place when a system is
quenched from its high-temperature phase to the critical point. The aim of this
review is to summarize the various numerical and analytical results that have
been recently obtained for this case. Particular emphasis is put to the
field-theoretical methods that can be used to provide analytical predictions
for the relevant dynamical quantities. Fluctuation-dissipation relations are
discussed and in particular the concept of fluctuation-dissipation ratio (FDR)
is reviewed, emphasizing its connection with the definition of a possible
effective temperature. The Renormalization-Group approach to critical dynamics
is summarized and the scaling forms of the time-dependent non-equilibrium
correlation and response functions of a generic observable are discussed. From
them the universality of the associated FDR follows as an amplitude ratio. It
is then possible to provide predictions for ageing quantities in a variety of
different models. In particular the results for Model A, B, and C dynamics of
the O(N) Ginzburg-Landau Hamiltonian, and Model A dynamics of the weakly dilute
Ising magnet and of a \phi^3 theory, are reviewed and compared with the
available numerical results and exact solutions. The effect of a planar surface
on the ageing behaviour of Model A dynamics is also addressed within the
mean-field approximation.Comment: rvised enlarged version, 72 Pages, Topical Review accepted for
publication on JP
Aging and fluctuation-dissipation ratio for the diluted Ising Model
We consider the out-of-equilibrium, purely relaxational dynamics of a weakly
diluted Ising model in the aging regime at criticality. We derive at first
order in a expansion the two-time response and correlation
functions for vanishing momenta. The long-time limit of the critical
fluctuation-dissipation ratio is computed at the same order in perturbation
theory.Comment: 4 pages, 2 figure
Ictal epileptic headache. an old story with courses and appeals
The term "ictal epileptic headache" has been recently proposed to classify the clinical picture in which headache is the isolated ictal symptom of a seizure. There is emerging evidence from both basic and clinical neurosciences that cortical spreading depression and an epileptic focus may facilitate each other, although with a different degree of efficiency. This review address the long history which lead to the 'migralepsy' concept to the new emerging pathophysiological aspects, and clinical and electroencephalography evidences of ictal epileptic headache. Here, we review and discuss the common physiopathology mechanisms and the historical aspects underlying the link between headache and epilepsy. Either experimental or clinical measures are required to better understand this latter relationship: the development of animal models, molecular studies defining more precise genotype/phenotype correlations as well as multicenter clinical studies with revision of clinical criteria for headache-/epilepsy-related disorders represent the start of future research. Therefore, the definition of ictal epileptic headache should be used to classify the rare events in which headache is the only manifestation of a seizure. Finally, using our recently published criteria, we will be able to clarify if ictal epileptic headache represents an underestimated phenomenon or not
The critical behavior of magnetic systems described by Landau-Ginzburg-Wilson field theories
We discuss the critical behavior of several three-dimensional magnetic
systems, such as pure and randomly dilute (anti)ferromagnets and stacked
triangular antiferromagnets. We also discuss the nature of the multicritical
points that arise in the presence of two distinct O(n)-symmetric order
parameters and, in particular, the nature of the multicritical point in the
phase diagram of high-T_c superconductors that has been predicted by the SO(5)
theory. For each system, we consider the corresponding Landau-Ginzburg-Wilson
field theory and review the field-theoretical results obtained from the
analysis of high-order perturbative series in the frameworks of the epsilon and
of the fixed-dimension d=3 expansions.Comment: 29 pages, 6 fig
Multicritical behavior in frustrated spin systems with noncollinear order
We investigate the phase diagram and, in particular, the nature of the the
multicritical point in three-dimensional frustrated -component spin models
with noncollinear order in the presence of an external field, for instance
easy-axis stacked triangular antiferromagnets in the presence of a magnetic
field along the easy axis. For this purpose we study the renormalization-group
flow in a Landau-Ginzburg-Wilson \phi^4 theory with symmetry O(2)x[Z_2 +O(N-1)]
that is expected to describe the multicritical behavior. We compute its MS
\beta functions to five loops. For N\ge 4, their analysis does not support the
hypothesis of an effective enlargement of the symmetry at the multicritical
point, from O(2) x [Z_2+O(N-1)] to O(2)xO(N). For the physically interesting
case N=3, the analysis does not allow us to exclude the corresponding symmetry
enlargement controlled by the O(2)xO(3) fixed point. Moreover, it does not
provide evidence for any other stable fixed point. Thus, on the basis of our
field-theoretical results, the transition at the multicritical point is
expected to be either continuous and controlled by the O(2)xO(3) fixed point or
to be of first order.Comment: 28 pages, 3 fig
Spin models with random anisotropy and reflection symmetry
We study the critical behavior of a general class of cubic-symmetric spin
systems in which disorder preserves the reflection symmetry ,
for . This includes spin models in the presence of
random cubic-symmetric anisotropy with probability distribution vanishing
outside the lattice axes. Using nonperturbative arguments we show the existence
of a stable fixed point corresponding to the random-exchange Ising universality
class. The field-theoretical renormalization-group flow is investigated in the
framework of a fixed-dimension expansion in powers of appropriate quartic
couplings, computing the corresponding -functions to five loops. This
analysis shows that the random Ising fixed point is the only stable fixed point
that is accessible from the relevant parameter region. Therefore, if the system
undergoes a continuous transition, it belongs to the random-exchange Ising
universality class. The approach to the asymptotic critical behavior is
controlled by scaling corrections with exponent , where
is the specific-heat exponent of the random-exchange
Ising model.Comment: 28 pages, 1 fi
Crossover from random-exchange to random-field critical behavior in Ising models
We compute the crossover exponent describing the crossover from the
random-exchange to the random-field critical behavior in Ising systems. For
this purpose, we consider the field-theoretical approach based on the replica
method, and perform a six-loop calculation in the framework of a
fixed-dimension expansion. The crossover from random-exchange to random-field
critical behavior has been observed in dilute anisotropic antiferromagnets,
such as FeZnF and MnZnF, when applying an
external magnetic field. Our result for the crossover exponent
is in good agreement with the available experimental estimates.Comment: 5 page
Gini estimation under infinite variance
We study the problems related to the estimation of the Gini index in presence
of a fat-tailed data generating process, i.e. one in the stable distribution
class with finite mean but infinite variance (i.e. with tail index
). We show that, in such a case, the Gini coefficient cannot be
reliably estimated using conventional nonparametric methods, because of a
downward bias that emerges under fat tails. This has important implications for
the ongoing discussion about economic inequality.
We start by discussing how the nonparametric estimator of the Gini index
undergoes a phase transition in the symmetry structure of its asymptotic
distribution, as the data distribution shifts from the domain of attraction of
a light-tailed distribution to that of a fat-tailed one, especially in the case
of infinite variance. We also show how the nonparametric Gini bias increases
with lower values of . We then prove that maximum likelihood estimation
outperforms nonparametric methods, requiring a much smaller sample size to
reach efficiency.
Finally, for fat-tailed data, we provide a simple correction mechanism to the
small sample bias of the nonparametric estimator based on the distance between
the mode and the mean of its asymptotic distribution
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