Magnetic moment and radius of the nucleon in a nonlocal model of meson-nucleon interaction

Magnetic moment and radius of the nucleon are calculated in a nonlocal extension of the chiral linear σ-model. Properties of the nonlocal model under the vector and axial transformations are considered. The conserved electromagnetic and vector currents, and partially conserved axial vector current are obtained. In the calculation of the nucleon electromagnetic vertex the π- and σ-loop diagrams are included. Contribution from vector mesons is added in the vector meson dominance model with a gauge-invariant photon-meson coupling. The nonlocality parameter associated with the πN interaction is fixed from the experimental magnetic moment of the neutron. Other parameters (nonlocality parameter for the σN interaction and the mass of the σ-meson) are constrained by the magnetic moment of the proton. The calculated electric and magnetic mean-square radii of the proton and neutron are in satisfactory agreement with experiment. Received: 12 February 2001 / Accepted: 4 September 2001     

Random quantum magnets with broad disorder distribution

We study the critical behavior of Ising quantum magnets with broadly distributed random couplings (J), such that P(ln J) ∼ | ln J|^{-1 - α}, α > 1, for large | ln J| (Lévy flight statistics). For sufficiently broad distributions, α < , the critical behavior is controlled by a line of fixed points, where the critical exponents vary with the Lévy index, α. In one dimension, with = 2, we obtained several exact results through a mapping to surviving Riemann walks. In two dimensions the varying critical exponents have been calculated by a numerical implementation of the Ma-Dasgupta-Hu renormalization group method leading to ≈ 4.5. Thus in the region 2 < α < , where the central limit theorem holds for | ln J| the broadness of the distribution is relevant for the 2d quantum Ising model. Received 6 December 2000 and Received in final form 22 January 2001     

Heated granular fluids: The random restitution coefficient approach

We introduce the model of inelastic hard spheres with random restitution coefficient α, in order to account for the fact that, in a vertically shaken granular system interacting elastically with the vibrating boundary, the energy injected vertically is transferred to the horizontal degrees of freedom through collisions only, which leads to heating through collisions, i.e. to inelastic horizontal collisions with an effective restitution coefficient that can be larger than 1. This allows the system to reach a non-equilibrium steady state, where we focus, in particular, on the single-particle velocity distribution f (v) in the horizontal plane, and on its deviation from a Maxwellian. Molecular Dynamics simulations and Direct Simulation Monte Carlo (DSMC) show that, depending on the distribution of α, different shapes of f (v) can be obtained, with very different high-energy tails. Moreover, the fourth cumulant of the velocity distribution quantifying the deviations from Gaussian statistics is obtained analytically from the Boltzmann equation and successfully tested against the simulations. Received 24 November 2000 and Received in final form 8 February 2001     

X-ray reflectivity of solid-supported, multilamellar membranes

We have investigated the structure of solid-supported, multilamellar membranes by X-ray reflectivity. The density profile is obtained by fitting the full q-range to a model using the bilayer Fourier coefficients as fitting parameters. The effect of hydration and the substrate boundary condition are discussed in view of the well-known Landau-Peierls effect and its implications for structure determination. The resulting bilayer density profile agrees remarkably well with previously published data of a molecular dynamics (MD) simulation for 1,2-oleoyl-palmitoyl-sn-glycero-3-phosphocholine (OPPC). Received 1 October 2001 and Received in final form 21 December 2001     

Ionization of iridium ions in the Dresden EBIT studied by X-ray spectroscopy of direct excitation and radiative recombination processes

Ir^q+ ( 41≤q≤64) ions with open-shell configurations have been produced in the electron beam of the room-temperature Dresden Electron Beam Ion Trap (Dresden EBIT) at electron excitation energies from 2 keV to 13 keV. X-ray emission from direct excitation processes and radiative capture in krypton-like to aluminium-like iridium ions is measured with an energy dispersive Si(Li) detector. The detected X-ray lines are analyzed and compared with results from multiconfigurational Dirac-Fock (MCDF) atomic structure calculations. This allows to determine dominant produced ion charge states at different electron energies. The analysis shows that at the realized working gas pressure of 5×10^-9mbar for higher charged ions the maximum ion charge state is not preferently determined by the chosen electron beam energy needed for ionization of certain atomic substates, but by the balance between ionization and charge state reducing processes as charge exchange and radiative recombination. This behaviour is also discussed on the basis of model calculations for the resulting ion charge state distribution. Received 12 July 2001 and Received in final form 10 September 2001     

Nanoparticles superficial density of charge in electric double-layered magnetic fluid: A conductimetric and potentiometric approach

We analyze potentiometric and conductimetric measurements simultaneously performed on Electric Double-Layer Magnetic Fluid based on cobalt ferrite nanoparticles, in order to obtain the pH-dependence of the particle surface charge density. We propose a mechanism for the charging of the particle surface. This model considers the ferrofluid solution as a mixture of strong and weak diprotic acids. We show how an exact analytical treatment involving proton transfer between the particle surface and the bulk solution allows the construction of a speciation diagram of the charged superficial sites. The saturation value of the superficial density of charge is found to be equal to 0.326 ± 0.065 C m^-2. Received 9 May 2001 and Received in final form 17 July 2001     

Thick skin in neutron/proton-rich sodium isotopes

Nucleon (both neutron and proton) density distributions of the chain of sodium isotopes are calculated using a semi-phenomenological model of nuclear density which incorporates correctly the asymptotic behaviour and the behaviour near the centre. The experimental charge root-mean-square radii and the single neutron and proton separation energies, required as input, are used. The calculated interaction cross-sections using these densities in the Glauber model agree well with the experiment. The calculated neutron rms radii r _n and the nuclear skin thickness ( r _n - r _p) closely agree with the corresponding experimental values and also are consistent with the Relativistic Hartree-Bogoliubov (RHB) calculations. Received: 24 April 2001 / Accepted: 28 June 2001     

Advanced memory effects in the aging of a polymer glass

A new kind of memory effect on low frequency dielectric measurements on plexiglass (PMMA) is described. These measurements show that cooling and heating the sample at constant rate give an hysteretic dependence on temperature of the dielectric constant ε. A temporary stop of cooling produces a downward relaxation of ε. Two main features are observed (i) when cooling is resumed ε goes back to the values obtained without the cooling stop (i.e. the low temperature state is independent of the cooling history) (ii) upon reheating ε keeps the memory of all the cooling stops (Advanced memory). The dependence of this effect on frequency and on the cooling rate is analyzed. The memory deletion is studied too. Finally the results are compared with those of similar experiments done in spin glasses and with the famous experiments of Kovacs. Received 24 September 2001 and Received in final form 20 November 2001     

Isomorphous phase transition and orientational freezing in hexagonal mixed crystal C 70(1 - x) C60x. A pseudospin model

A four-state pseudospin model is constructed for the isomorphous phase transition hcp-2↦hcp-1 in pure C₇₀ and in C₇₀-rich mixed crystal C _{70(1 - x)} C_60x. With the specific anisotropic pseudospin interactions adapted to the C₇₀ crystal the model is equivalent to a two-state Ising model in a temperature-dependent field. Replica symmetric state of the model is shown to approach the critical point when the width of distribution of random fields and/or of random bonds increases. The temperature of the phase transition and the phase equilibrium temperature then are practically constant, whereas the experiment shows their strong decrease with x. The main effect of dilution resides in an x-dependence of the model parameters. Dilatometric data on the hexagonal C _{70(1 - x)} C_60x are used to fit these parameters. A metastable disordered phase subsisting below the phase transition is discovered in a range of the model parameters and is shown to be responsible for the macroscopic behaviour of the system. A good agreement with experimental data is obtained for the spontaneous strain and for the x-dependence of the hysteresis. Received 20 April 2001 and Received in final form 26 September 2001     

Eigenvalues and eigenfunctions of a clover plate

We report a numerical study of the flexural modes of a plate using semi-classical analysis developed in the context of quantum systems. We first introduce the Clover billiard as a paradigm for a system inside which rays exhibit stable and chaotic trajectories. The resulting phase space explored by the ray trajectories is illustrated using the Poincare surface of section, and shows that it has both integrable and chaotic regions. Examples of the stable and the unstable periodic orbits in the geometry are presented. We numerically solve the biharmonic equation for the flexural vibrations of the Clover shaped plate with clamped boundary conditions. The first few hundred eigenvalues and the eigenfunctions are obtained using a boundary elements method. The Fourier transform of the eigenvalues show strong peaks which correspond to ray periodic orbits. However, the peaks corresponding to the shortest stable periodic orbits are not stronger than the peaks associated with unstable periodic orbits. We also perform statistics on the obtained eigenvalues and the eigenfunctions. The eigenvalue spacing distribution P(s) shows a strong peak and therefore deviates from both the Poisson and the Wigner distribution of random matrix theory at small spacings because of the C_4v symmetry of the Clover geometry. The density distribution of the eigenfunctions is observed to agree with the Porter-Thomas distribution of random matrix theory. Received 12 February 2001 and Received in final form 17 April 2001     

An analytic colllslonai-radlatlve model Incorporating non-LTE and optical depth effects

In this paper we analyze the variations in line intensities ratios due to a non-equilibrium situation and to optical depth effects. A four level model is proposed and the two particular situations for the possible transitions are considered. Electron density and temperature as well as the source thickness are used as independent parameters to find out in which way and extent they modify the ratios of levels populations compared with the ideal case of an equilibrium state and optically thin source. Accordingly with the ion of interest, electron temperatures ranging from I/20 to I/7 eV (I being the ionization energy), whereas electron densities in the interval from 10¹⁴ to 10²⁰ cm^-3 will be considered. These ranges are of special interest for diverse applications such as LIBS and measurement of transition probabilities. Some results are presented for real ions and a new expression for the escape factor is also proposed for general plasma conditions. Received 12 June 2001 and Received in final form 24 October 2001     

Stretched exponential relaxation in the Coulomb glass

The relaxation of the specific heat and the entropy to their equilibrium values is investigated numerically for the three-dimensional Coulomb glass at very low temperatures. The long time relaxation follows a stretched exponential function, f (t) = f ₀exp - (t/τ)^β , with the exponent β increasing with the temperature. The relaxation time diverges as an Arrhenius law when T→ 0. Received 24 May 2001 and Received in final form 12 September 2001     

Coupled aggregation and sedimentation processes: Three-dimensional off-lattice simulations

Coupled aggregation and sedimentation processes were studied by means of three-dimensional computer simulations. For this purpose, a large prism with no periodic boundary conditions for the sedimentation direction was considered. Furthermore, three equally sized and mutually excluded regions were defined inside the prism, a top, a middle and a bottom region. This allows to study the time evolution of the cluster size distribution and the cluster structure separately for each region. The mass distribution profile and the center-of-mass position were also accessed as a function of time. For the bottom region, the effects of the sediment formation on the kinetics of growth and on the cluster structure were clearly observed. The obtained results not only agree with the experimental data obtained by Allain et al. (C. Allain, M. Cloitre, M. Wafra, Phys. Rev. Lett. 74, 1478 (1995)) and with the simulations made by Gonzalez (A.E. Gonzalez, Phys. Rev. Lett. 86, 1243 (2001)), but also allow to gain further insight into the details. Received 7 November 2001     

Mobility in thin polymer films ranging from local segmental motion, Rouse modes to whole chain motion: A coupling model consideration

Large increases of mobility of local segmental relaxation observed in polymer films as the film thickness is decreased, as evidenced by decreases of the glass temperature, are not found for relaxation mechanisms that have longer length scales including the Rouse relaxation modes and the diffusion of entire polymer chains. We show that the coupling model predictions, when extended to consider polymer thin films, are consistent with a large increase of the mobility of the local segmental motions and the lack of such a change for the Rouse modes and the diffusion of entire polymer chains. There are two effects that can reduce the coupling parameter of the local segmental relaxation in thin films. One is the chain orientation that is induced parallel to the surface when the film thickness h becomes smaller than the end-to-end distance of the chains and the other is a finite-size effect when h is no longer large compared to the cooperative length scale. Extremely thin ( ≈ 1.5 nm) films obtained by intercalating a polymer into layered silicates have thickness significantly less than the cooperative length scale near the bulk polymer glass transition temperature. As a result, the coupling parameter of the local segmental relaxation in such thin films is reduced almost to zero. With this plausible assumption, we show the coupling model can explain quantitatively the large decrease of the local segmental relaxation time found experimentally. Received 1 August 2001 and Received in final form 1 December 2001     

Delocalization and wave-packet dynamics in one-dimensional diluted Anderson models

We study the nature of one-electron eigen-states in a one-dimensional diluted Anderson model where every Anderson impurity is diluted by a periodic function f(l). Using renormalization group and transfer matrix techniques, we provide accurate estimates of the extended states which appear in this model, whose number depends on the symmetry of the diluting function f(l). The density of states (DOS) for this model is also numerically obtained and its main features are related to the symmetries of the diluting function f(l). Further, we show that the emergence of extended states promotes a sub-diffusive spread of an initially localized wave-packet.Received: 7 July 2003, Published online: 19 November 2003PACS: 63.50. + x Vibrational states in disordered systems - 63.22. + m Phonons or vibrational states in low-dimensional structures and nanoscale materials - 62.30. + d Mechanical and elastic waves; vibrations     

Magnetic reconstructions at the surface of the B2 FeV alloy

We present an ab initio study of the magnetic surface reconstructions of the B2 FeV alloy using a self-consistent tight-binding linearized muffin tin orbital method developed in the atomic spheres approximation. For (001) and (111), the surface reconstruction stabilizes configurations unstable in the bulk alloy. When Fe is at the (001) surface, a c(2×2) in-plane antiferromagnetic order is found to be the ground state with magnetic moments of -2.32 and 2.27. A p(1×1) ↓ ferromagnetic order is displayed in case of V toplayer with a magnetic moment of -1.83. At the (111) surface, we obtain for Fe toplayer two solutions p(1×1)↑ and p(2×1). The configuration p(1×1)↑ is found to be the ground state with a magnetic moment per atom of 2.34. For V toplayer, only the p(1×1) ↓ solution is obtained with a moment of -0.84. In all cases, the Fe-V coupling is always antiparallel like in the bulk. Our results are discussed and compared to experiments. Received 11 August 2000 and Received in final form 8 June 2001     

Multifractal properties of aperiodic Ising model on hierarchical lattices: role of the geometric fluctuations

The role of the geometric fluctuations on the multifractal properties of the local magnetization of aperiodic ferromagnetic Ising models on hierarchical lattices is investigated. The geometric fluctuations are introduced by generalized Fibonacci sequences. The local magnetization is evaluated via an exact recurrent procedure encompassing real space renormalization group decimation. The symmetries of the local magnetization patterns induced by the aperiodic couplings is found to be strongly (weakly) different, with respect to the ones of the corresponding homogeneous systems, when the geometric fluctuations are relevant (irrelevant) to change the critical properties of the system. At the criticality, the measure defined by the local magnetization is found to exhibit a non-trivial F(α) spectra being shifted to higher values of α when relevant geometric fluctuations are considered. The critical exponents are found to be related with some special points of the F(α) function and agree with previous results obtained by the quite distinct transfer matrix approach. Received 2 April 2001 and Received in final form 14 August 2001     

On N-wave type systems and their gauge equivalent

The class of nonlinear evolution equations (NLEE) - gauge equivalent to the N-wave equations related to the simple Lie algebra are derived and analyzed. They are written in terms of (x, t) ∈ satisfying r = rank nonlinear constraints. The corresponding Lax pairs and the time evolution of the scattering data are found. The Zakharov-Shabat dressing method is appropriately modified to construct their soliton solutions. Received 20 October 2001 / Received in final form 30 April 2002 Published online 2 October 2002 RID="a" ID="a"e-mail: gerjikov@inrne.bas.bg     

A simple model of DNA denaturation and mutually avoiding walks statistics

Recently Garel, Monthus and Orland [Europhys. Lett. 55, 132 (2001)] considered a model of DNA denaturation in which excluded volume effects within each strand are neglected, while mutual avoidance is included. Using an approximate scheme they found a first order denaturation. We show that a first order transition for this model follows from exact results for the statistics of two mutually avoiding random walks, whose reunion exponent is c > 2, both in two and three dimensions. Analytical estimates of c due to the interactions with other denaturated loops, as well as numerical calculations, indicate that the transition is even sharper than in models where excluded volume effects are fully incorporated. The probability distribution of distances between homologous base pairs decays as a power law at the transition. Received 8 July 2002 / Received in final form 25 July 2002 Published online 17 September 2002     

Charge-independent effective interactions for shell-model studies in the 0g9/2-1p1/2 space

The matrix elements of the effective Hamiltonian in the 0g _9/2-1p _1/2 space are determined by a least-square fit to the energies of 477 levels of nuclei with 38≤Z≤50 and 47≤N≤50. The results of the calculation are found to be in better agreement with experiment than those obtained with previously determined interactions. Received: 31 May 2001 / Accepted: 14 June 2001