Physics

The pomegranate fruit moth, Apomyelois ceratoniae (Zeller), is the most important pest of pomegranate orchards in Iran, where infestations lead to 20-80 % fruit loss. Apomyelois ceratoniae overwinters as larvae in several instars. The success in overwintering determines the fruit loss in the following season, thus overwintering physiology of A. ceratoniae could provide insights into population prediction. To this end, overwintering strategy and some seasonal physiological and biochemical changes were investigated in the field-collected larvae of A. ceratoniae. The lowest supercooling point was recorded in November (-14.6 ± 0.91 °C) and the highest in both October and March (-10.2 ± 0.94 °C). The median lethal temperature (LT50) of larvae was higher than supercooling point, suggesting that A. ceratoniae is chill-susceptible. In comparison with summer larvae, accumulation of glycerol and sorbitol in overwintering larvae resulted in lower mortality when exposed to sub-zero temperatures. There were no significant seasonal changes in body water content or hemolymph osmolality. Current winter temperatures in Iranian orchards are higher than the cold tolerance thresholds of A. ceratoniae, suggesting that overwintering mortality is not a key factor in determining A. ceratoniae populations.

A brief introduction to the simulation of stochastic differential equations is presented. Algorithms to simulate rare fluctuations, a topic of interest in the light of recent theoretical work on optimal paths are studied. Problems connected to the treatment of the boundaries and correlated noise will be also discussed.

Stochastic resonance (SR), in which a periodic signal in a nonlinear system can be amplified by added noise, is discussed. The application of circuit modeling techniques to the conventional form of SR, which occurs in static bistable potentials, was considered in a companion paper. Here, the investigation of nonconventional forms of SR in part using similar electronic techniques is described. In the small-signal limit, the results are well described in terms of linear response theory. Some other phenomena of topical interest, closely related to SR, are also treated.

By a joint use of theory and analog simulation the low-•ction regime of the Duffing oscillator is explored. In the weak-temperature case it is shown that the low-friction regime, in turn, must be divided in two well-distinct subregimes. In the former one, characterized by the friction 7 ranging from 20)o (e)0 is the harmonic frequency) up to a lower bound 7r, the method of statistical linearization applies and a continued fraction procedure (CFP) generated by the Zwanzig-Mori projection techniques is shown to provide correct information for both the renormalized frequency of the oscillator and the corresponding line shape. The latter subregime, characterized by the friction 7 ranging from ,/=0 to 7 =~ is fraught with the complete breakdown of the statistical linearization method. The CFP is shown to provide an incorrect picture of the line shape while suggesting a novel mean field approximation which is then proven analytically via an alternative method of calculation. This method consists of expressing the system in a form reminiscent of the model of Kubo's stochastic oscillator. By using this alternative approach we are in a position to account for the residual linewidth which is shown by the analog experiment to survive for 7~0.

Using numerical simulations we show that the response to weak perturbations of a variable of Hamiltonian chaotic systems depends on the number of degrees of freedom: When this is small (ഠ2) the response is not linear, in agreement with the well known objections to the Kubo linear response theory, while, for a larger number of degrees of freedom, the response becomes linear. This is due to the fact that increasing the number of degrees of freedom the shape of the distribution function, projected onto the subspace of the variable of interest, becomes fairly "regular." [S0031-9007 00795-8] PACS numbers: 05.45.+b, 05.20.-y, 05.70.-a

In this article, we present theoretical as well as experimental results on resonantly enhanced tunneling of Bose-Einstein condensates in optical lattices both in the linear case and for small nonlinearities. Our results demonstrate the usefulness of condensates in optical lattices for simulating Hamiltonians originally used for describing solid state phenomena.

Suppose one wants to look at a weak periodic signal within a random noise: a way to improve the Signal to Noise Ratio (SNR) is to use a lock-in amplifier, but the frequency of the periodic component must be known to achieve a high amplification. A different method is offered by the mechanism of Stochastic Resonance (SR) 1.2.

Abstract: Negli ultimi anni si è visto un crescente utilizzo di tecniche che fanno uso delle onde superficiali per la determinazione di alcuni dei parametri fisici che caratterizzano il sottosuolo. In particolare, supponendo che il mezzo investigato sia approssimabile a un modello a strati omogenei, i principali parametri che si possono determinare attraverso l'inversione di dati relativi alla propagazione delle onde superficiali, sono gli spessori e le velocità delle onde S di ciascun strato.

Large rare fluctuations in a nonequilibrium system are investigated theoretically and by analogue electronic experiment. It is emphasized that the optimal paths calculated via the eikonal approximation of the Fokker-Planck equation can be identified with the locus of the ridges of the prehistory probability distributions which can be calculated and measured experimentally for paths terminating at a given final point in configuration sspace.

For a system driven by coloured noise, we investigate the activation energy of escape, and the dynamics during the escape. We have performed analogue experiments to measure the change in activation energy as the power spectrum of the noise varies. An adiabatic approach based on path integral theory allows us to calculate analytically the critical value at which a phase transition in the activation energy occurs.

Noise-induced transitions between coexisting stable states of a periodically driven nonlinear oscillator have been investigated by means of analog experiments and numerical simulations in the nonadiabatic limit for a wide range of oscillator parameters.

Abstract: A new numerical technique is demonstrated and shown to reduce exponentially the time required for Monte Carlo simulations of non-equilibrium systems. The quasi-stationary probability distribution is computed for two model systems, and the results are compared with the asymptotically exact theory in the limit of extremely small noise intensity. Singularities of the non-equilibrium distributions are revealed by the simulations. Keywords: stochastic, non-equilibrium, probability distribution, Monte Carlo simulation