23,148 research outputs found
Critical properties of an aperiodic model for interacting polymers
We investigate the effects of aperiodic interactions on the critical behavior
of an interacting two-polymer model on hierarchical lattices (equivalent to the
Migadal-Kadanoff approximation for the model on Bravais lattices), via
renormalization-group and tranfer-matrix calculations. The exact
renormalization-group recursion relations always present a symmetric fixed
point, associated with the critical behavior of the underlying uniform model.
If the aperiodic interactions, defined by s ubstitution rules, lead to relevant
geometric fluctuations, this fixed point becomes fully unstable, giving rise to
novel attractors of different nature. We present an explicit example in which
this new attractor is a two-cycle, with critical indices different from the
uniform model. In case of the four-letter Rudin-Shapiro substitution rule, we
find a surprising closed curve whose points are attractors of period two,
associated with a marginal operator. Nevertheless, a scaling analysis indicates
that this attractor may lead to a new critical universality class. In order to
provide an independent confirmation of the scaling results, we turn to a direct
thermodynamic calculation of the specific-heat exponent. The thermodynamic free
energy is obtained from a transfer matrix formalism, which had been previously
introduced for spin systems, and is now extended to the two-polymer model with
aperiodic interactions.Comment: 19 pages, 6 eps figures, to appear in J. Phys A: Math. Ge
Critical Behavior of a Three-State Potts Model on a Voronoi Lattice
We use the single-histogram technique to study the critical behavior of the
three-state Potts model on a (random) Voronoi-Delaunay lattice with size
ranging from 250 to 8000 sites. We consider the effect of an exponential decay
of the interactions with the distance,, with , and
observe that this system seems to have critical exponents and
which are different from the respective exponents of the three-state Potts
model on a regular square lattice. However, the ratio remains
essentially the same. We find numerical evidences (although not conclusive, due
to the small range of system size) that the specific heat on this random system
behaves as a power-law for and as a logarithmic divergence for
and Comment: 3 pages, 5 figure
Deblocking of interacting particle assemblies: from pinning to jamming
A wide variety of interacting particle assemblies driven by an external force
are characterized by a transition between a blocked and a moving phase. The
origin of this deblocking transition can be traced back to the presence of
either external quenched disorder, or of internal constraints. The first case
belongs to the realm of the depinning transition, which, for example, is
relevant for flux-lines in type II superconductors and other elastic systems
moving in a random medium. The second case is usually included within the
so-called jamming scenario observed, for instance, in many glassy materials as
well as in plastically deforming crystals. Here we review some aspects of the
rich phenomenology observed in interacting particle models. In particular, we
discuss front depinning, observed when particles are injected inside a random
medium from the boundary, elastic and plastic depinning in particle assemblies
driven by external forces, and the rheology of systems close to the jamming
transition. We emphasize similarities and differences in these phenomena.Comment: 20 pages, 8 figures, submitted for a special issue of the Brazilian
Journal of Physics entitled: Statistical Mechanics of Irreversible Stochastic
Models - I
Analytical results for long time behavior in anomalous diffusion
We investigate through a Generalized Langevin formalism the phenomenon of
anomalous diffusion for asymptotic times, and we generalized the concept of the
diffusion exponent. A method is proposed to obtain the diffusion coefficient
analytically through the introduction of a time scaling factor . We
obtain as well an exact expression for for all kinds of diffusion.
Moreover, we show that is a universal parameter determined by the
diffusion exponent. The results are then compared with numerical calculations
and very good agreement is observed. The method is general and may be applied
to many types of stochastic problem
TRAUMATIC SPINAL CORD INJURY: Neurologic and Functional Recovery
Objectivo: Avaliar a recuperação neurológica e funcional dos doentes com
Traumatismo Vertebro-Medular (TVM).
Métodos: Foi realizado um estudo retrospectivo, que incluiu os 93 doentes com o
diagnóstico de TVM admitidos no Serviço de Fisiatria do Hospital Geral de Santo
António, entre Janeiro de 1993 e Dezembro de 2002. Os dados demográficos e médicos
foram recolhidos dos processos clínicos. O registo da avaliação de cada doente
foi efectuado na admissão, à data da alta hospitalar e 1 e 3 anos após a lesão. O
estado neurológico foi classificado de acordo com os critérios da American Spinal
Injury Association (ASIA), e o estado funcional através da Medida de Independência
Funcional (MIF).
Resultados: 87% doentes eram do sexo masculino, sendo a média de idades de 40
anos. Os acidentes de viação e as quedas foram a causa mais frequente da lesão. A
maioria dos traumatismos foi dorsal e cervical (45% e 42% respectivamente). O
tratamento inicial incluiu: administração intravenosa de metilprednisolona, que ocorreu
nas primeiras oito horas após lesão em 54% dos casos; intervenção cirúrgica
precoce, realizada em 77%. Registou-se maior recuperação nos doentes com lesão
medular incompleta, particularmente naqueles classificados como C na escala da
ASIA. Foi encontrada uma associação estatisticamente significativa entre os resultados
obtidos no internamento e os registados no período de três anos de follow-up. A
recuperação neurológica não se correlacionou com a idade dos doentes, o nível de
lesão, administração de corticoides em altas doses ou cirurgia realizada nas primeiras
24 horas após a lesão (p >0,05). Os resultados funcionais registados através da
MIF revelaram-se bastante favoráveis, particularmente os que foram alcançados em
regime de internamento.
Conclusões: Os resultados revelados neste estudo demonstram o considerável potencial
para uma recuperação neurológica após um TVM, enfatizando a importância do
investimento na continuidade dos programas de reabilitação nos anos que se seguem
à lesão, particularmente nas lesões incompletas. Study Design: A retrospective study was conducted
Objective: Evaluate the neurologic and functional recovery in patients with Traumatic
Spinal Cord Injury (TSCI).
Methods: A retrospective study was conducted, including the 93 patients with a
diagnosis of TSCI admitted for rehabilitation in the Physical and Rehabilitation
Medicine Department, Santo António General Hospital, between January 1993 and
December 2002. Demographic and clinical data were collected from the hospital charts. The patients were evaluated at admission for rehabilitation, before discharging,
1 year and 3 years after injury. Neurologic status was registered according to the
standards of the American Spinal Injury Association (ASIA) and functional status
by the Functional Independence Measure (FIM).
Results: The sample population was 87% men, with a mean age of 40 years. Motor
vehicle accidents and falls were the commonest mechanism of
injury. The majority sustained dorsal and cervical lesions (45% and 42% respectively).
The initial management included: intravenous high dose methylprednisolone,
administered within eight hours after injury in 54% of the sample; early surgical
stabilization, performed on 77%. Incomplete cord injury carried a better prognosis
of motor improvement, especially the patients classified as C on the ASIA Impairment
Scale (AIS). A significant statistical association was found between the outcomes
obtained during the initial rehabilitation, and the ones achieved on the 3 year followup
period. The neurologic recovery was not related with the patient’s age, injury
level, high dose steroid administration or surgery performed in the first 24 hours
after injury (p >0,05). The FIM scores demonstrated the favourable functional outcomes
achieved, especially during the inpatient rehabilitation.
Conclusions: The results of this study further demonstrate the considerable potential
for neurologic recovery after TSCI, emphasizing the importance of the rehabilitation
investment continuity, especially for the ones with incomplete cord injury
Non-Local Product Rules for Percolation
Despite original claims of a first-order transition in the product rule model
proposed by Achlioptas et al. [Science 323, 1453 (2009)], recent studies
indicate that this percolation model, in fact, displays a continuous
transition. The distinctive scaling properties of the model at criticality,
however, strongly suggest that it should belong to a different universality
class than ordinary percolation. Here we introduce a generalization of the
product rule that reveals the effect of non-locality on the critical behavior
of the percolation process. Precisely, pairs of unoccupied bonds are chosen
according to a probability that decays as a power-law of their Manhattan
distance, and only that bond connecting clusters whose product of their sizes
is the smallest, becomes occupied. Interestingly, our results for
two-dimensional lattices at criticality shows that the power-law exponent of
the product rule has a significant influence on the finite-size scaling
exponents for the spanning cluster, the conducting backbone, and the cutting
bonds of the system. In all three cases, we observe a continuous variation from
ordinary to (non-local) explosive percolation exponents.Comment: 5 pages, 4 figure
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