Model of populations colonizing a new habitat

We propose a model suitable for investigating the conditions under which a population, adapted to a given environment, may colonize a new neighbouring, spatially separated, habitat. We find out how similar the two regions must be for such a colonization to succeed and what will be the spatial distribution of genetic pools of the two populations after the process. It is found that between the two populations each adapted to the different region, a hybrid zone, characterized by increased heterozygosity, may be formed. The dependence of the shape of the hybrid zone on the external (environmental) parameters is determined. When the differences between the two regions are significant, the populations are separated by a depopulated zone. We show that the conditions in the colonized habitat influence the genetic pool of the population living in the first region. Computer simulations based on the standard Monte Carlo technique are used. Received 11 December 1998 and Received in final form 17 December 1998     

Multiparticle correlations in resonance-matter formation

The effect of multiparticle correlations on resonance and pion populations, in relativistic nuclear reactions, is calculated in the context of an intranuclear cascade model which includes N-body (N > 2) collisional processes. The resonance-matter population present in the highly-compressed phase of nucleus-nucleus collisions is investigated, in reactions between different intermediate-mass nuclear systems. Received: 29 December 1998 / Revised version: 20 December 1999     

Twin-correlations in atoms

We discuss the possibility of preparing an atomic sample of atoms with minimum fluctuations in the difference between populations of two levels. A first scheme involves absorption of twin beams of light, and it presents a variant of a recent proposal for atomic spin squeezing within an excited state manifold [Kuzmich et al., Phys. Rev. Lett. 79, 4782 (1997)]. A second scheme involves atoms with two stable states, and we suggest that by use of quantum non-demolition detection and feed-back optical pumping, we may ensure a perfect agreement between the number of atoms in these two states. Received: 14 May 1998 / Revised: 10 August 1998 / Accepted: 8 October 1998     

Self-organisation of female menopause in populations with child-care and reproductive risk

In this paper, by using a computer model for the evolution of age-structured populations, we show that in a sexual population subject to: (i) senescence, (ii) age-increasing reproductive risks, and (iii) long juvenile dependence, menopause arises spontaneously, in order to guarantee the survival of the offspring. Received: 12 September 1998 / Accepted: 14 September 1998     

Dissipation, decoherence and preparation effects in the spin-boson system

The dynamics of the reduced density matrix of the driven dissipative two-state system is studied for a general diagonal/off-diagonal initial state. We derive exact formal series expressions for the populations and coherences and show that they can be cast into the form of coupled nonconvolutive exact master equations and integral relations. We show that neither the asymptotic distributions, nor the transition temperature between coherent and incoherent motion, nor the dephasing rate and relaxation rate towards the equilibrium state depend on the particular initial state chosen. However, in the underdamped regime, effects of the particular initial preparation, e.g. in an off-diagonal state of the density matrix, strongly affect the transient dynamics. We find that an appropriately tuned external ac-field can slow down decoherence and thus allow preparation effects to persist for longer times than in the absence of driving. Received 23 October 1998 and Received in final form 26 February 1999     

Forward Raman scattering in GaAs/AlAs superlattices: Study of optical phonon anisotropy

We present the forward Raman scattering study of zone-centre optical phonon anisotropy in short-period GaAs/AlAs superlattices. Experiments were performed on specially prepared superlattice structures having anti-reflection dielectric coatings and removed substrates. The experimental data are compared with the angular dispersion of superlattice optical phonons calculated within the dielectric susceptibility model. We have found a good agreement between the experimental data and the calculations taking into account interface disorder. Received 9 September 1998 and Received in final form 22 October 1998     

Towards an universal classification of scale invariant processes

We consider fields which take random values over several decades. Starting from physical examples, we postulate that scale is not an absolute quantity. We then establish the equivalence between two existing approaches based on scale symmetry arguments as general as possible. This yields a classification of log-infinitely divisible laws, possibly universal. The physical significance of the parameters entering in the classification is discussed. Received: 7 November 1997 / Received in final form: 26 March 1998 / Accepted: 30 March 1998     

Effect of exciton dephasing in a semiconductor microcavity

We study the interaction between delocalized excitons in a semiconductor quantum well and a longitudinal mode of the radiation field in a semiconductor microcavity with Bragg mirrors. The drastic enhancement of the spontaneous emission rate, that occurs under strong coupling conditions, is found to be surprisingly robust with respect to incoherent processes leading to dephasing of the exciton mode. Received date: 3 June 1998 / Received in final form: 19 November 1998     

Force fluctuations in a vertically pushed granular column

We present series of experiments on the resistance force encountered by a bottom piston pushing a vertical granular column confined in a two-dimensional cell. We show that, due to the presence of friction at the boundaries and between the grains, the signal shows many complex features. At slow driving velocities, we observe a transition to a stick-slip dynamic instability. Depending on the granular material used, the elementary stick-slip events may either be well characterized or largely distributed. We present a statistical study on the waiting time between events and the distribution of energy release as a function of the spring stiffness and the driving velocity. Received 5 August 1998 and Received in final form 22 October 1998     

Interpolating between soft and hard dynamics in deep inelastic scattering

An explicit model for the proton structure function is suggested, interpolating between low- vector meson dominance and Regge behavior, on the one hand, and the high- solution of the Gribov-Lipatov-Altarelli-Parisi evolution equation, on the other hand. The model is fitted to the experimental data in a wide range of the kinematical variables with emphasis on the low-x HERA data. The boundaries, transition region and interface between various regimes are quantified. Received: 9 March 1998 / Revised version: 26 May 1998 / Published online: 5 October 1998     

Mean magnetic field and noise cross-correlation in magnetohydrodynamic turbulence: results from a one-dimensional model

We show that a recently proposed [J. Fleischer, P.H. Diamond, Phys. Rev. E 58, R2709 (1998)] one-dimensional Burgers-like model for magnetohydrodynamics (MHD) is in effect identical to existing models for drifting lines and sedimenting lattices. We use the model to demonstrate, contrary to claims in the literature, that the energy spectrum of MHD turbulence should be independent of mean magnetic field and that cross-correlations between the noise sources for the velocity and magnetic fields cannot change the structure of the equations under renormalisation. We comment on the scaling and the multiscaling properties of the stochastically forced version of the model. Received 29 October 1998 and Received in final form 8 December 1998     

Summing up the perturbation series in the Schwinger Model

Perturbation series for the electron propagator in the Schwinger Model is summed up in a direct way by adding contributions coming from individual Feynman diagrams. The calculation, performed entirely in momentum space, shows the complete agreement between nonperturbative and perturbative approaches. Received: 2 March 1998 / Revised version: 12 May 1998 / Published online: 8 September 1998     

Random copolymers at a selective interface: Many chains with excluded volume interactions

We investigate numerically, using the bond-fluctuation model, the adsorption of many random AB-copolymers with excluded volume interactions at the interface between two solvents. We find two regimes, controlled by the total number of polymers. In the first (dilute) regime, the copolymers near the interface extend parallel to it, while in the second regime they extend perpendicular to it. The density at the interface and the density in the bulk depend differently on the total number of copolymers: In the first regime the density at the interface increases more rapidly than in the bulk, whereas the opposite is true in the second regime. Received 4 March 1998 and Received in final form 22 September 1998     

Charge density wave transport in submicron antidot arrays in NbSe3

We demonstrate for the first time that a periodic array of submicrometer holes (antidots) can be patterned into thin single NbSe₃ crystals. We report on the study of Charge Density Wave (CDW) transport of the network of mesoscopic units between antidots. Size of the elementary unit can be as small as 0.5 μm along the chain axis and in cross-section. We observe size effects for Ohmic residual resistance and in CDW transport current-voltage characteristics in submicronic networks. Received: 25 November 1997 / Received in final form: 30 March 1998 / Accepted: 6 April 1998     

Spontaneous emission from a four-level system

We examine the decay dynamics of a free four-level system in the -V configuration. Quantum interference strongly manifests itself in this system, as can be seen by looking at the combined spectral distribution of the two emitted photons and at the time evolution of the intermediate-level populations, whose effective lifetimes can become very long under certain conditions for the atomic parameters. This effect is attributable to a population transfer mechanism induced in the time evolution equations by the Fano terms, also responsible for the strong modifications of the spectral correlation between the emitted photons which we analyze in detail. Finally, population trapping can also occur when the two intermediate levels are degenerate. Received: 20 October 1998     

Bose-einstein correlations and color reconnection in W-pair production

We propose a systematic study of Bose-Einstein correlations between identical hadrons coming from different W decays. Experimentally accessible signatures of these correlations as well as of possible color reconnection effects are discussed on the basis of two-particle inclusive densities. Received: 7 July 1998 / / Published online: 19 November 1998     

Coulomb blockade of the persistent current in a one-dimensional ballistic Luttinger liquid ring

The effect of an electrostatic energy (in the geometrical capacitance approach) on a persistent current is considered. It is shown that at high temperatures the current amplitude shows periodic dips as a function of the potential difference between a ring and a reservoir. These dips correspond to a lift of the Coulomb blockade. In a minimum of a dips a current is periodic in a magnetic flux with a period at any temperatures. Received: 6 April 1998 / Revised: 5 June 1998 / Accepted: 21 July 1998     

Laser influence on positron-antiproton radiative capture collision

The laser-assisted radiative capture between a positron and an antiproton is studied in detail. The theoretical results show that the cross-section for antihydrogen formation is significantly reduced with the application of a laser background. This effect is most marked when the laser polarization is parallelto the incident direction. Received: 13 November 1998 / Received in final form: 16 March 1999     

Stability and vibrations of catenoid-shaped smectic films

Catenoid-shaped smectic films are spanned between two coaxial circular frames separated by a distance H. It is shown that there exists a critical height H^* such that below it two shapes of the catenoid are possible. The stability of these two shapes is analysed in terms of their vibrations. The spectrum of eigenfrequencies is calculated as a function of the catenoid height. It is shown that the frequency of the fundamental mode is real for the stable shape and imaginary for the other shape. Experimental study of vibrational eigenmodes performed on stable SCE4 films confirms this theoretical prediction. Received: 25 November 1997 / Received in final form: 30 January 1998 / Accepted: 9 March 1998