1,600 research outputs found
Critical behavior of an Ising model with aperiodic interactions
We write exact renormalization-group recursion relations for a ferromagnetic
Ising model on the diamond hierarchical lattice with an aperiodic distribution
of exchange interactions according to a class of generalized two-letter
Fibonacci sequences. For small geometric fluctuations, the critical behavior is
unchanged with respect to the uniform case. For large fluctuations, the uniform
fixed point in the parameter space becomes fully unstable. We analyze some
limiting cases, and propose a heuristic criterion to check the relevance of the
fluctuations.Comment: latex file, 5 figures, accepted by Braz. Jour. Phy
Critical exponents for the long-range Ising chain using a transfer matrix approach
The critical behavior of the Ising chain with long-range ferromagnetic
interactions decaying with distance , , is investigated
using a numerically efficient transfer matrix (TM) method. Finite size
approximations to the infinite chain are considered, in which both the number
of spins and the number of interaction constants can be independently
increased. Systems with interactions between spins up to 18 sites apart and up
to 2500 spins in the chain are considered. We obtain data for the critical
exponents associated with the correlation length based on the Finite
Range Scaling (FRS) hypothesis. FRS expressions require the evaluation of
derivatives of the thermodynamical properties, which are obtained with the help
of analytical recurrence expressions obtained within the TM framework. The Van
den Broeck extrapolation procedure is applied in order to estimate the
convergence of the exponents. The TM procedure reduces the dimension of the
matrices and circumvents several numerical matrix operations.Comment: 10 pages, 2 figures, Conference NEXT Sigma Ph
Interrelationship between facial pattern, malocclusion, TMDs, head and neck posture and type of breathing in young people
Objectives: To compare occlusal, facial, and craniocervical postural characteristics according to the breathing pattern, study the association between temporomandibular disorders’ (TMDs) class and severity, gauge the influence of the breathing pattern, head and neck posture, occlusal class, and facial pattern on TMDs severity and the lower cervicofacial ratio, and identify any prevalent differences in TMDs severity by gender. Methods: This cross-sectional study included a convenience sample of 139 individuals, 81 females (58.3%) and 58 males (41.7%), with a mean age of 13.0±0.72 years old. Data were collected from observations, medical forms and photographic records. We classified TMDs severity according to Fonseca Anamnestic Index and used Software for Postural Assessment. Results: Compared to nasal breathers, oral breathers exhibited a predominance of Class II occlusion (p<0.01), a convex profile (p<0.05), increased cervicofacial ratio (p<0.01), and a tendency for head anteriorization (p<0.05). An association between TMDs and individuals with Class II occlusion was also found (p<0.01). Oral breathers showed a greater risk of increased lower cervicofacial ratio and mild TMDs (OR: 9.64 and 4.01, respectively). Signs and symptoms of TMDs appeared in 60% of young females, though the difference between genders was not significant (p=0.290). Conclusions: We detected associations between oral breathing and head anteriorization, TMDs, Class II malocclusion, convex facial profile, and increased lower cervicofacial ratio. TMDs were associated with occlusal Class II, and oral breathing increased the risk of developing mild TMDs and increased lower cervicofacial ratio
Aperiodic quantum XXZ chains: Renormalization-group results
We report a comprehensive investigation of the low-energy properties of
antiferromagnetic quantum XXZ spin chains with aperiodic couplings. We use an
adaptation of the Ma-Dasgupta-Hu renormalization-group method to obtain
analytical and numerical results for the low-temperature thermodynamics and the
ground-state correlations of chains with couplings following several two-letter
aperiodic sequences, including the quasiperiodic Fibonacci and other
precious-mean sequences, as well as sequences inducing strong geometrical
fluctuations. For a given aperiodic sequence, we argue that in the easy-plane
anisotropy regime, intermediate between the XX and Heisenberg limits, the
general scaling form of the thermodynamic properties is essentially given by
the exactly-known XX behavior, providing a classification of the effects of
aperiodicity on XXZ chains. We also discuss the nature of the ground-state
structures, and their comparison with the random-singlet phase, characteristic
of random-bond chains.Comment: Minor corrections; published versio
On the random neighbor Olami-Feder-Christensen slip-stick model
We reconsider the treatment of Lise and Jensen (Phys. Rev. Lett. 76, 2326
(1996)) on the random neighbor Olami-Feder-Christensen stik-slip model, and
examine the strong dependence of the results on the approximations used for the
distribution of states p(E).Comment: 6pages, 3 figures. To be published in PRE as a brief repor
Abrupt transitions from reinfections in social contagions
The study of social contagion processes is of utmost importance for understanding the emergence of collective social states. Here we introduce reinfections in the Susceptible-Infected-Recovered (SIR) model so to incorporate the possibility that an individual that ceases its activity (recovered) can resume it due to secondary infections from its active (infected) peers. We show that, when primary infection is less frequent than secondary ones, a typical situation in many social contagion processes, the epidemic transition turns from smooth to abrupt. As a consequence, macroscopic collective states can be triggered from the inactive (healthy) regime by a small increment of the primary contagion rate
Universal Critical Behavior of Aperiodic Ferromagnetic Models
We investigate the effects of geometric fluctuations, associated with
aperiodic exchange interactions, on the critical behavior of -state
ferromagnetic Potts models on generalized diamond hierarchical lattices. For
layered exchange interactions according to some two-letter substitutional
sequences, and irrelevant geometric fluctuations, the exact recursion relations
in parameter space display a non-trivial diagonal fixed point that governs the
universal critical behavior. For relevant fluctuations, this fixed point
becomes fully unstable, and we show the apperance of a two-cycle which is
associated with a novel critical behavior. We use scaling arguments to
calculate the critical exponent of the specific heat, which turns out
to be different from the value for the uniform case. We check the scaling
predictions by a direct numerical analysis of the singularity of the
thermodynamic free-energy. The agreement between scaling and direct
calculations is excellent for stronger singularities (large values of ). The
critical exponents do not depend on the strengths of the exchange interactions.Comment: 4 pages, 1 figure (included), RevTeX, submitted to Phys. Rev. E as a
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