702 research outputs found
A simple spin model for three steps relaxation and secondary proccesses in glass formers
A number of general trends are known to occur in systems displaying secondary
processes in glasses and glass formers. Universal features can be identified as
components of large and small cooperativeness whose competition leads to excess
wings or apart peaks in the susceptibility spectrum. To the aim of
understanding such rich and complex phenomenology we analyze the behavior of a
model combining two apart glassy components with a tunable different
cooperativeness. The model salient feature is, indeed, based on the competition
of the energetic contribution of groups of dynamically relevant variables,
e.g., density fluctuations, interacting in small and large sets. We investigate
how the model is able to reproduce the secondary processes physics without
further ad hoc ingredients, displaying known trends and properties under
cooling or pressing.Comment: 11 Pages, 11 Figure
Reply to Comment on ``Spherical 2+p spin-glass model: an analytically solvable model with a glass-to-glass transition''
In his Comment, Krakoviack [Phys. Rev. B (2007)] finds that the phase
behavior of the s+p spin-glass model is different from what proposed by
Crisanti and Leuzzi [Phys. Rev. B 73, 014412 (2006)] if s and p are larger than
two and are separated well enough. He proposes a trial picture, based on a one
step replica symmetry breaking solution, displaying a mode-coupling-like
glass-to-glass transition line ending in a A3 singularity. However, actually,
the physics of these systems changes when p-s is large, the instability of
which the one step replica symmetry breaking glassy phase suffers turns out to
be so wide ranging that the whole scenario proposed by Krakoviack must be
seriously reconsidered.Comment: 4 pages, 5 figure; reply to arXiv:0705.3187. To be published in Phys
Rev B 76 (2007
An alternative wind profile formulation for urban areas in neutral conditions
On the basis of meteorological observations conducted within the city of Rome, Italy, a new formulation of the wind-speed profile valid in urban areas and neutral conditions is developed. It is found that the role played by the roughness length in the canonical log-law profile can be taken by a local length scale, depending on both the surface cover and the distance above the ground surface, which follows a pattern of exponential decrease with
height. The results show that the proposed model leads to increased performance compared with that obtained by using other approaches found in the literature
The Complexity of the Spherical -spin spin glass model, revisited
Some questions concerning the calculation of the number of ``physical''
(metastable) states or complexity of the spherical -spin spin glass model
are reviewed and examined further. Particular attention is focused on the
general calculation procedure which is discussed step-by-step.Comment: 13 pages, 3 figure
Complexity of the Sherrington-Kirkpatrick Model in the Annealed Approximation
A careful critical analysis of the complexity, at the annealed level, of the
Sherrington-Kirkpatrick model has been performed. The complexity functional is
proved to be always invariant under the Becchi-Rouet-Stora-Tyutin
supersymmetry, disregarding the formulation used to define it. We consider two
different saddle points of such functional, one satisfying the supersymmetry
[A. Cavagna {\it et al.}, J. Phys. A {\bf 36} (2003) 1175] and the other one
breaking it [A.J. Bray and M.A. Moore, J. Phys. C {\bf 13} (1980) L469]. We
review the previews studies on the subject, linking different perspectives and
pointing out some inadequacies and even inconsistencies in both solutions.Comment: 20 pages, 4 figure
The random Blume-Capel model on cubic lattice: first order inverse freezing in a 3D spin-glass system
We present a numerical study of the Blume-Capel model with quenched disorder
in 3D. The phase diagram is characterized by spin-glass/paramagnet phase
transitions of both first and second order in the thermodynamic sense.
Numerical simulations are performed using the Exchange-Monte Carlo algorithm,
providing clear evidence for inverse freezing. The main features at criticality
and in the phase coexistence region are investigated. The whole inverse
freezing transition appears to be first order. The second order transition
appears to be in the same universality class of the Edwards-Anderson model. The
nature of the spin-glass phase is analyzed by means of the finite size scaling
behavior of the overlap distribution functions and the four-spins real-space
correlation functions. Evidence for a replica symmetry breaking-like
organization of states is provided.Comment: 18 pages, 24 figures, 7 table
Neural-Based Nonlinear Device Models for Intermodulation Analysis
A new procedure to learn a nonlinear model together with its derivative parameters using a composite neural network is presented.So far neural networks have never been used to extract large-signal device model accounting for distortion parameters.Applying this method to FET devices leads to nonlinear models for current- voltage functions which allow improved prediction of weak and mildly device nonlinearities in the whole bias region. The resulting models have demonstrated to be suitable for both small-signal and large-signal analyses,including intermodulation distortion prediction
On Equilibrium Dynamics of Spin-Glass Systems
We present a critical analysis of the Sompolinsky theory of equilibrium
dynamics. By using the spherical spin glass model we test the asymptotic
static limit of the Sompolinsky solution showing that it fails to yield a
thermodynamically stable solution. We then present an alternative formulation,
based on the Crisanti, H\"orner and Sommers [Z. f\"ur Physik {\bf 92}, 257
(1993)] dynamical solution of the spherical -spin spin glass model,
reproducing a stable static limit that coincides, in the case of a one step
Replica Symmetry Breaking Ansatz, with the solution at the dynamic free energy
threshold at which the relaxing system gets stuck off-equilibrium. We formally
extend our analysis to any number of Replica Symmetry Breakings . In the
limit both formulations lead to the Parisi anti-parabolic
differential equation. This is the special case, though, where no dynamic
blocking threshold occurs. The new formulation does not contain the additional
order parameter of the Sompolinsky theory.Comment: 24 pages, 6 figure
Complexity in Mean-Field Spin-Glass Models: Ising -spin
The Complexity of the Thouless-Anderson-Palmer (TAP) solutions of the Ising
-spin is investigated in the temperature regime where the equilibrium phase
is one step Replica Symmetry
Breaking. Two solutions of the resulting saddle point equations are found.
One is supersymmetric (SUSY) and includes the equilibrium value of the free
energy while the other is non-SUSY. The two solutions cross exactly at a value
of the free energy where the replicon eigenvalue is zero; at low free energy
the complexity is described by the SUSY solution while at high free energy it
is described by the non-SUSY solution. In particular the non-SUSY solution
describes the total number of solutions, like in the
Sherrington-Kirkpatrick (SK) model. The relevant TAP solutions corresponding
to the non-SUSY solution share the same feature of the corresponding solutions
in the SK model, in particular their Hessian has a vanishing isolated
eigenvalue. The TAP solutions corresponding to the SUSY solution, instead, are
well separated minima.Comment: 13 pages, 9 figure
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