Intermittent granular flow and clogging with internal avalanches

The dynamics of intermittent granular flow through an orifice at the bottom of a granular bin and the associated clogging due to formation of arches blocking the outlet, is studied numerically in two dimensions. When the hole size is less than the grain diameter, only a single grain is removed from the system so that the system self-organizes to a steady state and the distribution of the grain displacements decays as power laws. On the other hand, when hole sizes are within few times of the grain diameter, the outflow distributions are also observed to follow a power law. Received 21 July 1999 and Received in final form 17 September 1999     

Semiclassical steady-state analysis of a degenerate two-photon laser

The steady-state analysis of a single-mode two-photon laser are treated semiclassically by using the Maxwell-Bloch equations. The theory is applied to a ring-laser model. We find similarities and significant differences between the one- and two-photon polarizations of the medium, population inversion and mode-pulling formula. The population inversion and the longitudinal variation of the steady-state modulus of the field are studied numerically. Received: 1 March 1999 / Received in final form: 8 April 1999     

Spontaneous alloying of tin atoms into nanometer-sized gold clusters and phase stability in the resultant alloy clusters

Alloying behavior and phase stability has been studied in situ by transmission electron microscopy using clusters in the Au-Sn system. When tin atoms are vapor-deposited onto nm-sized gold clusters, rapid dissolution of tin atoms into gold clusters takes place and as a result Au-rich solid solution, amorphous-like Au-Sn alloy and AuSn compound clusters are formed depending upon the concentration of tin. The remarkable enhancement of solubility has been observed in Au-rich solid solution and AuSn compound. It becomes more difficult to form two phases in the interior of individual clusters even if the composition of alloy clusters falls in the two-phase region in the phase diagram for the bulk alloy and as a result amorphous-like phase is stabilized in nm-sized Au-Sn alloy clusters. Received 2 August 1999 and Received in final form 8 November 1999     

Hydrodynamic interactions in the structural fluctuation of a ternary amphiphilic system C12E5/water/ n-octane

Dynamical fluctuations of microemulsion and lamellar structures in a ternary amphiphilic system C₁₂E ₅/water/n-octane are studied by means of neutron spin echo spectrometry. The decay rate of the time correlation of the concentration was analyzed in terms of a theory (M. Nonomura, T. Ohta, J. Chem. Phys. 110, 7516 (1999)), in which both van Hove and hydrodynamic interactions are considered. The result shows that the time correlation function is expressed mostly by a single exponential determined exclusively by hydrodynamic interactions. Received 1 December 1999 and Received in final form 12 March 2001     

Random phase approximation of the spin-wave excitation for quasi-one-dimensional magnetic polymer

Based on a theoretical model proposed for quasi-one-dimensional organic polymer ferromagnets, the ground state and low-lying magnetic excitation are studied. Within Hartree-Fock approximation, the ground state of the system is shown to be a stable ferromagnetic state due to the electron-electron correlation and topological structure of the system. The random-phase approximation is employed to explore the magnon excitation and the excitation spectrum is obtained, including an acoustic mode and four optical modes. It is found that the acoustic mode possesses the characteristic of the ferromagnetic magnon. Received 9 July 1999     

Anharmonic oscillators energies via artificial perturbation method

A new pseudoperturbative (artificial in nature) methodical proposal [#!ref15!#] is used to solve for Schr?dinger equation with a class of phenomenologically useful and methodically challenging anharmonic oscillator potentials . The effect of the [#!ref4!#,#!ref5!#] Padé approximant on the leading eigenenergy term is studied. Comparison with results from numerical (exact) and several eligible (approximation) methods is made. Received 2 July 1999 and Received in final form 18 November 1999     

Green function approximation and path integral numerical methods

We evaluate the short time Green function by a series expansion of the exponent. The resulting expression allows the computation of the energy eigenvalues and eigenvectors of a quantum system with a very high precision. As an example, we show the comparison between the numerical results and the analytical solutions for harmonic and multi-well potentials both in one and two dimensions. Received 1st March 1999 and Received in final form 20 September 1999     

Phase transitions in a bistable system driven by two colored noises

The colored noise problem is studied from the point of view of consistent Markovian approximations through extending unified colored-noise approximation to the case of two-colored-noise driving systems. A bistable system simultaneously driven by multiplicative and additive colored noise is investigated by means of the extended unified colored-noise approximation. It is found that, for weak strength and color of the additive noise, the form of the stationary probability distribution changes from a unimodal to a bimodal structure via a three modal one as the correlation time of the multiplicative colored noise increases, showing the system undergoes a first order phase transition from a monostable to a bistable state. Numerical simulations support our results. Received 10 August 1998 and Received in final form 23 April 1999     

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     

Finite-size effects in the self-organized critical forest-fire model

We study finite-size effects in the self-organized critical forest-fire model by numerically evaluating the tree density and the fire size distribution. The results show that this model does not display the finite-size scaling seen in conventional critical systems. Rather, the system is composed of relatively homogeneous patches of different tree densities, leading to two qualitatively different types of fires: those that span an entire patch and those that do not. As the system size becomes smaller, the system contains less patches, and finally becomes homogeneous, with large density fluctuations in time. Received 24 April 1999 and Received in final form 26 October 1999     

The -relaxation dynamics of a simple liquid

We present a detailed analysis of the -relaxation dynamics of a simple glass former, a binary Lennard-Jones system with a stochastic dynamics. By testing the various predictions of mode-coupling theory, including the recently proposed corrections to the asymptotic scaling laws, we come to the conclusion that in this time regime the dynamics is described very well by this theory. Received 5 February 1999 and Received in final form 7 June 1999     

Thermoelastic behaviour of polyvinylacetate monolayers at the air-water interface: Evidences for liquid-solid phase transition

The thermoelastic behaviour of polyvinylacetate monolayers spread on an aqueous subphase has been studied using rheological data previously published (Monroy et al., Phys. Rev. E 58, 7629 (1998)). The results show fluid-like viscoelastic behaviour well above a transition temperature , while at lower temperatures a soft solid-like behaviour emerges. The correlation between thermodynamic and elastic properties below can be described in terms of scaling laws. Received 12 January 1999 and Received in final form 11 June 1999     

Symmetry breaking induced by a magnetic field in a quasi-two-dimensional d-wave superconductor

A model of quasi-two-dimensional d-wave superconductor, with strong nesting properties of the Fermi surface is considered. The orbital effect of a moderate magnetic field applied perpendicularly to the conducting planes is studied in the mean field approximation. It is shown that the field can induce a time reversal symmetry breaking SDW order coexisting with the superconducting order and can open a gap over the whole Fermi surface. The anomalies recently observed in the heat conductivity in might be ascribed to this effect. Received 7 May 1999 and Received in final form 13 August 1999     

Application of electron paramagnetic resonance to the study of Cu2+ ions in Cu-Na ion-exchanged glasses

The copper ion-exchange process in two different silicate glasses is experimentally studied by means of electron paramagnetic resonance analysis. Differences in the environment of divalent copper ions are found depending on the substrate. The Cu²⁺ concentration in-depth profile is determined and compared to the total copper diffusion profile as determined from secondary ion mass spectrometry. Received: 12 February 1998 / Accepted: 5 January 1999 / Published online: 7 April 1999     

Stochastic resonance in ferromagnetic domain motion

A model for the motion of a single ferromagnetic domain is studied numerically and analytically. A single strip in two dimensions and pinned at two inhomogeneities is considered. We suppose two stable configurations (positively or negatively curved with pinned ends) due to the action of a bistable potential. Further, it is assumed that the domain is driven externally by periodic and noisy magnetic fields. The noise makes the domain able to flip between the two configurations. The small temporally periodic fields synchronize these flippings and the phenomenon of stochastic resonance is observed. The signal to noise ratio of the output is investigated and shows a maximum for a nonvanishing intensity of the applied noise. Its dependency on the stiffness of the domain is studied. Received 14 May 1999 and Received in final form 14 October 1999     

Experimental observation of noise-induced sensitivity to small signals in a system with on-off intermittency

An experimental (electronic circuit) realization and analytic studies of overdamped Kramers oscillator with an exponential nonlinearity under combined effect of a large multiplicative noise and a small periodic signal were performed. Under certain conditions, when the system exhibits on-off intermittency, it becomes sensitive to very small periodic signals, amplifying them greatly. Received 21 May 1999 and Received in final form 28 December 1999     

Noise-induced bifurcations and chaos in the average motion of globally-coupled oscillators

A system of coupled master equations simplified from a model of noise-driven globally coupled bistable oscillators under periodic forcing is investigated. In the thermodynamic limit, the system is reduced to a set of two coupled differential equations. Rich bifurcations to subharmonics and chaotic motions are found. This behavior can be found only for certain intermediate noise intensities. Noise with intensities which are too small or too large will certainly spoil the bifurcations. In a system with large though finite size, the bifurcations to chaos induced by noise can still be detected to a certain degree. Received 6 April 1999 and Received in final form 1 November 1999     

Proton and electron impact on molecular and atomic oxygen: II. Emission cross-sections for ionisation and excitation of atomic oxygen

Ionisation and excitation of atomic oxygen in collisions with protons and electrons were studied experimentally in the intermediate velocity range applying the method of fluorescence spectroscopy. Absolute emission cross-sections for 2s-electron ionisation, excited ion production and excitation of atomic oxygen are presented and for electron impact compared to available data. For proton impact no emission cross-sections have been published so far. Received 9 September 1999 and Received in final form 15 December 1999     

Efficient microwave-induced optical frequency conversion

Frequency conversion process is studied in a medium of atoms with a configuration of levels, where transition between two lower states is driven by a microwave field. In this system, conversion efficiency can be very high by virtue of the effect of electromagnetically induced transparency (EIT). Depending on intensity of the microwave field, two regimes of EIT are realized: “dark-state” EIT for the weak field, and Autler-Townes-type EIT for the strong one. We study both cases via analytical and numerical solution and find optimum conditions for the conversion. Received 13 December 1999 and Received in final form 6 March 2000     

Temperature-dependent ionization potential of sodium clusters

The ionization potential of sodium clusters () at a finite temperature is studied using density functional theory and ab initio molecular dynamics. The threshold regions of the photoionization efficiency curves are deduced from the integrated IP distributions, which are obtained from the energy eigenvalues of the highest occupied Kohn-Sham states during molecular dynamics by applying a theoretically well-defined shift. The calculated ionization potentials are directly compared to the experimental values. The energetically best geometry of Na₅₅ is found to be a slightly distorted icosahedron. Received 16 April 1999 and Received in final form 6 July 1999