123085141
Intermittency and Multifractality in Magnetic Dynamos
We study the convection of magnetic fields in the presence of Lagrangian chaos, i.e. when nearby fluid elements diverge exponentially in time. We show that the magnetic field is of a multifractal nature as a consequence of the spatial fluctuations in the exponential growth of the field. We provide numerical and theoretical evidence that the magnetic fields can concentrate in small domains of a fluid with weak Lagrangian chaos. Moreover, these properties are relevant also for the study of the case of small but nonzero magnetic diffusivity.
120014368
A new proposal for the determination of the renormalization group trajectories by the Monte Carlo renormalization group method
Abstract We present a new method for calculating block renormalized couplings by Monte Carlo renormalization group. This method has several advantages with respect to the existing ones and can be applied for any value of the coupling constants. A preliminary numerical study of the 2-dimensional O(3) non linear σ-model is also presented.
121504553
On the masses of the glueballs in pure SU(2) lattice gauge theory
Abstract Using a Monte Carlo method to measure the correlation functions of different quantities, the mass spectrum of the glue-ball is investigated for an SU(2) gauge model in an 8 4 lattice.
121490138
On the effects of an uncertainty on the evolution law in dynamical systems
We show that a small perturbation of the evolution law of chaotic dynamical systems is practically equivalent to the finite precision on the knowledge of the initial condition. In spite of the short predictability time tp on a single trajectory we found that the statistical properties are not sensitive to small changes in the evolution law or initial probability distribution. This feature holds also for correlation functions at a delay larger than tp. We discuss the roundoff effects on integrable systems.
15060511
Transition from regular to complex behaviour in a discrete deterministic asymmetric neural network model
The authors study the long-time behaviour of the transient before the collapse on the periodic attractors of a discrete deterministic asymmetric neural network model. The system has a finite number of possible states so it is not possible to use the term chaos in the usual sense of sensitive dependence on the initial condition. Nevertheless, on varying the asymmetry parameter, k, one observes a transition from ordered motion (i.e. short transients and short periods on the attractors) to 'complex' temporal behaviour. This transition takes place for the same value kc at which one has a change in behaviour for the mean transient length from a power law in the size of the system (N) to an exponential law in N. The 'complex' behaviour during the transient shows strong analogies with chaotic behaviour: decay of temporal correlations, positive Shannon entropy, nonconstant Renyi entropies of different orders. Moreover the transition is very similar to the intermittent transition in chaotic systems: a scaling law for the Shannon entropy and strong fluctuations of the 'effective Shannon entropy' along the transient, for k>kc.
121714112
A Monte Carlo simulation with an “improved” action for the O(3) non-linear sigma model
Abstract We present the results of a Monte Carlo simulation for a lattice action improved a la Symanzik. The results seem to give a faster approach of the lattice physics to its continuum limit.
16350340
Two Ising models coupled to two-dimensional gravity
Abstract To investigate the properties of c = 1 matter coupled to 2d-gravity we have performed large-scale simulations ot two copies of the Ising model on a dynamical lattice. We measure spin susceptibility and percolation critical exponents using finite-size scaling. We show explicity how algorithmic corrections are needed for a proper comparison with theoretical exponents. We also exhibit correlations, mediated by gravity, between the energy and magnetic properties of the two Ising species. The prospects for extending this work beyond c = 1 are dressed.
8220387
Phantom Tubules
We provide the first numerical evidence for the existence of the tubule phase, predicted by Radzihovsky and Toner (RT), for anisotropic tethered membranes. Incorporating anisotropy into the bending rigidity of a simple model of a phantom tethered membrane with the topology of a disk and free boundary conditions we show that the model indeed has two phase transitions corresponding to the flat-to-tubule and tubule-to-crumpled transitions. For the tubule phase we measure the Flory exponent νF , describing the growth of the tubule radius with the extent of the tubule, and the roughness exponent ζ, which measures the r.m.s. height fluctuations in the extended direction. We find νF = 0.305(14) and ζ = 0.895(60) which are in reasonable agreement with the theoretical predictions of RT, νF = 1/4 and ζ = 1. PACS numbers: 64.60.Fr, 05.40.+j, 82.65.Dp Typeset using REVTEX
14633114
The phase diagram of crystalline surfaces
We report the status of a high-statistics Monte Carlo simulation of non-self-avoiding crystalline surfaces with extrinsic curvature on lattices of size up to 128 2 nodes. We impose free boundary conditions. The free energy is a gaussian spring tethering potential together with a normal-normal bending energy. Particular emphasis is given to the behavior of the model in the cold phase where we measure the decay of the normal-normal correlation function.
119499105
Anisotropic Membranes
We describe the statistical behavior of anisotropic crystalline membranes. In particular we give the phase diagram and critical exponents for phantom membranes and discuss the generalization to self-avoiding membranes.
12041139
The flat phase of fixed-connectivity membranes
Abstract The statistical mechanics of flexible two-dimensional surfaces (membranes) appears in a wide variety of physical settings. In this talk we discuss the simplest case of fixed-connectivity surfaces. We first review the current theoretical understanding of the remarkable flat phase of such membranes. We then summarize the results of a recent large scale Monte Carlo simulation of the simplest conceivable discrete realization of this system [1]. We verify the existence of long-range order, determine the associated critical exponents of the flat phase and compare the results to the predictions of various theoretical models.