Universal Margin Factors and Selection Criteria in Variable Environment

For nonautonomous dynamical systems, the principle of inheriting local properties by global Poincaré maps is developed. Using this method, a selection criterion for systems of close competitors is found: to gain competitive advantage, it suffices to outproduce other populations with a margin. The margin factor in question remains uniformly bounded as the number of competitors in the community grows.     

Synergistic effects in mixed wormlike micelles of dimeric and single-chain cationic surfactants at high ionic strength

The mixed micellization between the cationic gemini surfactant [ C₁₂H₂₅( CH₃)₂N⁺( C₂H₄) N⁺( CH₃)₂ C₁₂H₂₅•2Br^-] and the cationic cetyltrimethylammonium bromide (CTAB) in 150 mM KBr solutions has been investigated. The variation of the cmc of the mixtures, measured by surface tension experiments, with composition revealed synergism in micelle formation. T-Jump and light scattering experiments performed in the vicinity of the crossover volume fraction showed the existence of two micellar populations, possibly linear and toroidal micelles. Rheological and dynamic light scattering experiments allowed to fully characterize the linear viscoelasticity of the mixtures. These measurements revealed synergistic gains in viscoelastic properties with a maximum of the stress-relaxation time around the equimolar composition. These effects are ascribed to a progressive intermicellar crosslinking resulting from a continuous increase of the end-cap energy with the 12-2-12 content in the mixture. Received: 18 November 2002 / Accepted: 8 April 2003 / Published online: 27 May 2003 RID="a" ID="a"e-mail: candau@fresnel.u-strasbg.fr     

Dispersive flow of disks through a two-dimensional Galton board

We report here an experimental and numerical study of the flow properties of disks driven by gravity through a hexagonal lattice of obstacles, i.e. a Galton board. During the fall, particles experience dissipative collisions that scatter them in random directions. A driven-diffusion regime can be achieved under certain conditions. A characteristic length of the motion and its dependence on geometrical parameters of the system is analyzed in the steady regime. The influence of collective effects on the dispersion process is investigated by comparison between single- and many-particle flows. The characterization of the dynamics and the diffusive properties of the flow in a system like a Galton board can be expanded to other granular systems, particularly static solid particle mixers and will give some insight in understanding granular mixing.     

Kinematical separation of α-n final-state interaction at 50 MeV incident energy

A kinematically complete experiment has been performed to study the α-n final-state interaction (FSI) in the α + d ↦α + p + n break-up reaction at 50 MeV incident energy for the alpha-particles. For this, we have chosen four pairs of correlation angles for the outgoing alpha and protons. These are ( θ_α = 18^°, θ_p = 42^°), ( θ_α = 20^°, θ_p = 45^°), ( θ_α = 22^°, θ_p = 42^°) and ( θ_α = 22^°, θ_p = 47^°), selected kinematically where the allowed phase spaces are in favor of the α-n final-state interaction. Our experimental data show strong α-n FSI in all the selected configurations. Also, the FSI is found to be stronger at the lower alpha-particle energy when two FSI peaks appear in the same configuration. Received: 17 June 2002 / Accepted: 3 October 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: dey_s2001@yahoo.com; Present address: 445 Waupelani Drive, Apt. F3, State College, PA 16801, USA. Communicated by M. Gar?on     

The $-game

We propose a payoff function extending Minority Games (MG) that captures the competition between agents to make money. In contrast with previous MG, the best strategies are not always targeting the minority but are shifting opportunistically between the minority and the majority. The emergent properties of the price dynamics and of the wealth of agents are strikingly different from those found in MG. As the memory of agents is increased, we find a phase transition between a self-sustained speculative phase in which a “stubborn majority” of agents effectively collaborate to arbitrage a market-maker for their mutual benefit and a phase where the market-maker always arbitrages the agents. A subset of agents exhibit a sustained non-equilibrium risk-return profile. Received 5 June 2002 / Received in final form 21 November 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: sornette@unice.fr RID="b" ID="b"CNRS UMR7536 RID="c" ID="c"CNRS UMR6622     

Phase diagram of the spin extended model

The quantum phase transition in the ground state of the extended spin S = 1/2 XY model has been studied in detail. Using the exact solution of the model the low temperature thermodynamics, as well as the ground state phase diagram of the model in the presence of applied uniform and/or staggered magnetic field are discussed. Received 29 November 2002 / Received in final form 24 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: japa@iph.hepi.edu.ge     

Scaling behavior of a nonlinear oscillator with additive noise,white and colored

We study analytically and numerically the problem of a nonlinear mechanical oscillator with additive noise in the absence of damping. We show that the amplitude, the velocity and the energy of the oscillator grow algebraically with time. For Gaussian white noise, an analytical expression for the probability distribution function of the energy is obtained in the long-time limit. In the case of colored, Ornstein-Uhlenbeck noise, a self-consistent calculation leads to (different) anomalous diffusion exponents. Dimensional analysis yields the qualitative behavior of the prefactors (generalized diffusion constants) as a function of the correlation time. Received 10 October 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: mallick@spht.saclay.cea.fr     

Optical properties of the quasi-two-dimensional dichalcogenides 2H-TaSe and 2H-NbSe

We present a comprehensive analysis of the optical constants of the two-dimensional dichalcogenide materials 2 H - TaSe ₂ and 2 H - NbSe ₂ , in an attempt to address the physics of two-dimensional correlated systems. The title compounds were studied over several decades in frequency, from the far-infrared to the ultraviolet. Measurements with linearly polarized light have allowed us to obtain both the in-plane and out-of-plane components of the conductivity tensor. Although the electromagnetic response of dichalcogenides is strongly anisotropic, both the in-plane and out-of-plane components of the conductivity tensor share many common features, including the presence of a well-defined metallic component, as well as a “mid-infrared band”. We discuss the implications of these results in the context of the spectroscopic results of other classes of low-dimensional conductors such as the high-temperature superconducting cuprates. In particular, the analysis of the redistribution of the spectral weight as a function of temperature, as well as the behavior of the quasiparticles relaxation rate, points to significant distinctions between the charge dynamics of dichalcogenides and other classes of low dimensional conductors. Received 28 October 2002 / Received in final form 10 March 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: degiorgi@solid.phys.ethz.ch     

Nature of the Peierls- to Mott-insulator transition in 1D

In order to clarify the physics of the crossover from a Peierls band insulator to a correlated Mott-Hubbard insulator, we analyze ground-state and spectral properties of the one-dimensional half-filled Holstein-Hubbard model using quasi-exact numerical techniques. In the adiabatic limit the transition is connected to the band to Mott insulator transition of the ionic Hubbard model. Depending on the strengths of the electron-phonon coupling and the Hubbard interaction the transition is either first order or evolves continuously across a narrow intermediate phase with finite spin, charge, and optical excitation gaps. Received 7 July 2002 / Received in final form 21 October 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: holger.fehske@physik.uni-greifswald.de     

Carbon nanotube bundles and thin layers probed by micro-Raman spectroscopy

Raman spectra of single-wall carbon nanotubes (CNTs) either in the form of micrometer sized bundles or thin layers prepared by dilution and sonication of powders have been compared. We have been able to collect the Raman spectrum of nanotube bundles that are not in touch with the substrate, and therefore not affected by interactions with the substrate surface. This spectrum resulted to be similar to that of the precursor nanotube powders, whereas relevant changes in the Raman spectrum are detected when the diluted powders form very thin layers on either metallic or insulating surfaces, as probed by confocal microraman imaging on well defined areas of the CNTs layers. In the case of thin layers, the intensity of the Raman D band, detected between 1 320 and 1 340 cm^-1 and ascribed to disorder effects, is strongly enhanced. This enhancement occurs independently on the kind of substrate. Received 2 September 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: sangalet@dmf.bs.unicatt.it