The critical number of atoms in an attractive Bose-Einstein condensate on optical plus harmonic traps

The stability of an attractive Bose-Einstein condensate on a joint one-dimensional optical lattice and an axially symmetrical harmonic trap is studied using the numerical solution of the time-dependent mean-field Gross-Pitaevskii equation and the critical number of atoms for a stable condensate is calculated. We also calculate this critical number of atoms in a double-well potential which is always greater than that in an axially symmetrical harmonic trap. The critical number of atoms in an optical trap can be made smaller or larger than the corresponding number in the absence of the optical trap by moving a node of the optical lattice potential in the axial direction of the harmonic trap. This variation of the critical number of atoms can be observed experimentally and compared with the present calculations.

Numerical solution of the two-dimensional Gross-Pitaevskii equation for trapped interacting atoms

We present a numerical scheme for solving the time-independent nonlinear Gross-Pitaevskii equation in two dimensions describing the Bose-Einstein condensate of trapped interacting neutral atoms at zero temperature. The trap potential is taken to be of the harmonic-oscillator type and the interaction both attractive and repulsive. The Gross-Pitaevskii equation is numerically integrated consistent with the correct boundary conditions at the origin and in the asymptotic region. Rapid convergence is obtained in all cases studied. In the attractive case there is a limit Co the maximum number of atoms in the condensate. (C) 2000 Published by Elsevier B.V. B.V. All rights reserved.

Positronium scattering by atoms and molecules at low energies

The recent theoretical and experimental activities in positronium (Ps) scattering by atoms and molecules are reviewed with special emphasis at low energies. We critically compare the results of different groups - theoretical and experimental. The theoretical approaches considered include the R-matrix and close-coupling methods applied to Ps-H, Ps-He and Ps-Li scattering, and a coupled-channel approach with a nonlocal model potential for Ps scattering by H, He, H-2, Ne, Ar, Li, Na, K, Rb, Cs and Ps and for pickoff quenching in Ps-He scattering. Results for scattering lengths, partial. total and differential cross-sections as well as resonance and binding energies in different systems are discussed. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Collision of positronium with atoms, molecules and ions using a modified coupled-channel framework

Scattering of positronium (Ps) from atoms (H, He, Ne, Ar), molecule (H(2)) and ion (He(+)) have been investigated using a coupled-channel (CC) formalism with a regularised non-local exchange potential. The advantage of using such a regularized exchange potential in the close-coupling formalism and the normalizability aspect of the solution at low energies with a minimum effective coupling are discussed. Results for the elastic and total scattering cross-sections, resonance and binding energies in Ps-H, and pick-off annihilation results in Ps-He are found to be in excellent agreement with measurements and variational predictions. (C) 2000 Elsevier B.V. B.V. All rights reserved.

Stability and collapse of a hybrid Bose-Einstein condensate of atoms and molecules

The dynamics of stability and collapse of a trapped atomic Bose-Einstein condensate (BEC) coupled to a molecular one is studied using the time-dependent Gross-Pitaevskii (GP) equation including a nonlinear interaction term which can transform two atoms into a molecule and vice versa. We find an interesting oscillation of the number of atoms and molecules for a BEC of fixed mass. This oscillation is a consequence of continuous transformation in the condensate of two atoms into a molecule and vice versa. For the study of collapse an absorptive contact interaction and an imaginary quartic three-body recombination term are included in the GP equation. It is possible to have a collapse of one or both components when one or more of the nonlinear terms in the GP equation are attractive in nature, respectively.

Three helium atoms and the scaling limit

We show that a scaling limit approach, previously applied in three-body low-energy nuclear physics, is realized for the first excited state of He-4 trimer. The present result suggests that such approach has a wider application.

Study of a degenerate dipolar Fermi gas of Dy-161 atoms

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Radiative association of C and P, and Si and P atoms

The rate coefficients for the formation of carbon monophosphide (CP) and silicon monophosphide (SiP) by radiative association are estimated for temperatures ranging from 300 to 14 100 K. In this temperature range, the radiative association rate coefficients are found to vary from 1.14 x 10(-18) to 1.62 x 10(-18) cm(3) s(-1) and from 3.73 x 10(-20) to 7.03 x 10(-20) cm(3) s(-1) for CP and SiP, respectively. In both cases, rate coefficients increase slowly with the increase in temperature.

Radiative association of Ti and O atoms

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

The interaction between atoms of Au and Cu with clean Si(111) surface: A study combining synchrotron radiation grazing incidence X-ray fluorescence analysis and theoretical calculations

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

An ab initio study of oxygen vacancies and doping process of Nb and Cr atoms on TiO2 (110) surface models

We theoretically investigated how the formation of oxygen vacancies and the addition of niobium and chromium atoms as dopants modify the varistor properties of TiO2. The calculations were carried out at the HF level using a contracted basis set, developed by Huzinaga et al.. to represent the atomic centers on the (110) surface for the large (TiO2)(15) cluster model. The change of the values for the net atomic charges and band gap after oxygen vacancy formation and the presence of dopants in the lattice are analyzed and discussed. It is shown that the formation of oxygen vacancies decreases the band gap while an opposite effect is found when dopants are located in the reduced surface. The theoretical results are compared with available experimental data. A plausible explanation of the varistor behavior of this system is proposed. (C) 1997 John Wiley & Sons, Inc.

THAUMATOMYRMEX STRIPS MILLIPEDES FOR PREY - A NOVEL PREDATORY BEHAVIOR IN ANTS, AND THE 1ST CASE OF SYMPATRY IN THE GENUS (HYMENOPTERA, FORMICIDAE)

We describe, for the first time, the predatory behaviour of Thaumatomyrmex ants on millipedes of the family Polyxenidae, based on field observations of T. atrox and a field and laboratory study of T. contumax. The capture of the prey and the removal process of its body-covering setae by the ants before they eat the millipede are described. This specialized behaviour in at least two species of the genus, belonging to two distinct groups of species, indicates a general trend in Thaumatomyrmex. We coupled this study with a comparative morphological analysis of the mouthparts and digestive tube of these and other Thaumatomyrmex species. Also, we report the first case of sympatry in the genus, which suggests that Thaumatomyrmex includes several species. and not only one highly variable taxon, as hypothesized earlier.

Row-column response surface designs

Factorial experiments are widely used in industry to investigate the effects of process factors on quality response variables. Many food processes, for example, are not only subject to variation between days, but also between different times of the day. Removing this variation using blocking factors leads to row-column designs. In this paper, an algorithm is described for constructing factorial row-column designs when the factors are quantitative, and the data are to be analysed by fitting a polynomial model. The row-column designs are constructed using an iterative interchange search, where interchanges that result in an improvement in the weighted mean of the efficiency factors corresponding to the parameters of interest are accepted. Some examples illustrating the performance of the algorithm are given.