Collective diffusion of O/W(1 1 0) at high coverages: Monte Carlo simulations

Following our previously developed twin-spin model description of the equilibrium properties of the O/W(110) system we study its collective diffusion by means of a Monte Carlo dynamics. It is shown that the collective diffusion coefficient for our model has a different density dependence than this following from other models known in the subject literature. The essential feature of the model, responsible for its different dynamics, are the state dependent interactions. Further assumptions concerning interaction dependent barriers make our results comparable to the experimental data.

Probing the classical field approximation-thermodynamics and decaying vortices

Harry Schmidt, Krzysztof Góra, Filip Floegel, Mariusz Gajda and Kazimierz Rzazewski

We review our version of the classical field approximation to the dynamics of a finite-temperature Bose gas. In the case of a periodic box potential, we investigate the role of the high-momentum cut-off, essential in the method. In particular, we show that the cut-off going to the infinity limit describes the particle number going to infinity with the scattering length going to zero. In this weak-interaction limit, the relative population of the condensate tends to unity. We also show that the cross-over energy, at which the probability distribution of the condensate occupation changes its character, grows with a growing scattering length. In the more physical case of the condensate in the harmonic trap we investigate the dissipative dynamics of a vortex. We compare the decay time and the velocities of the vortex with the available analytic estimates.

J. Opt. B: Quantum Semiclass. Opt. vol. 5, S1-S7 (2003)

Scattering of sulfur ions carbon: Classical-trajectory Monte Carlo results

We analyze classically the scattering of sulfur ions by carbon using the classical-trajectory Monte Carlo method. It is assumed that the scatterer and scattered nuclei are coupled to each other as well as to all electrons, but there is no coupling between electrons themselves. To initialize the state of both atoms, quasiexact energies are used that are obtained from the Dirac-Fock method. Effective charges are used to partially take into account the intra-atomic interactions between electrons. We concentrate on the cross sections for production of vacancies in the K and L shells and capture of electrons to K, L, and M shells of the sulfur ions. The dependence of these cross sections on the energy of the projectile sulfur ions and on the initial charge states of these ions is analyzed. Our results will be helpful in the interpretation of x-ray spectra from highly ionized fast sulfur projectiles passing through a carbon foil.

Phys. Rev. A vol. 67, 012713-1-10 (2003) Slabkowska, Katarzyna, Polasik Marek, Janowicz Maciej

Mesoscopic models of oxygen migration on the Ru(001) surface

Phys. Rev. B 66, 165418 (2002)

Magdalena A. Załuska-Kotur, Zbigniew W. Gortel, and Robert Teshima

Atomic migration within the organized oxygen overlayers on the basal plane of Ru(001) is modeled within phenomenological mesoscopic model accounting for all possible atomic jumps within the adsorbate and capable of accessing several time scales in the migration kinetics. Experimentally observed diffusion and trapping of the isolated oxygen vacancy within the 3O(2×2) overlayer at coverage θ=3/4 are accounted for. The comparison with experiment is used to propose values of parameters for interactions between an atom adsorbed at a hcp site and another one temporarily at some of the neighboring bridge sites. The modeling is extended to the O(2×2) overlayer at θ=1/4 and it is shown that for this structure a competing diffusion channel exists which breaks down the equivalence of the atomic migration behavior between the organized θ=1/4 and θ=3/4 overlayers. For both overlayers simplified effective models are formulated, free of unobservable metastable configurations and unobservable time scales. Considerations for the ideal O(2×1) rowlike overlayer at θ=1/2 confirm that occasional jumps of the oxygen atoms to one side of the rows should be observable in the STM images for this overlayer in contrast to the θ=1/4 and 3/4 overlayers for which an excess atom or a vacancy, respectively, are necessary to initiate observable atomic migration processes.

Graphical examples of geometrical and wave optics

Władysław Żakowicz

A simple method of the description of optical pulses of finite dimensions and finite duration time is presented. The pulses are given by superpositions of the known analytical as well as numerical solutions of the Maxwell equations. The examples show optical properties of scattering in detail. In the general case the wide pulses split in the scattering processes. The description of these phenomena requires a full wave approach. The pulses (or their fragments) that do not split but move smoothly through the optical system (or its fragment) may be described with the help of the geometrical optics theory.

Acta Phys. Polon. B 33, 2059 (2002)

Collective diffusion in a twin-spin model of O/W (110)

Following our previously developed twin-spin model description of the equilibrium properties of the O/W(110) system we study its collective diffusion by means of a Monte Carlo dynamics. It is shown that the collective diffusion coefficient for our model has a different density dependence than this following from other models known in the subject literature. The essential feature of the model, responsible for its different dynamics, are the state dependent interactions. Further assumptions concerning interaction dependent barriers make our results comparable to the experimental data.

Phys. Rev. B vol. 67, 155406-1-7 (2003) Magdalena A. Zaluska-Kotur, Stanislaw Krukowski, Lukasz A. Turski

Diffusion and desorption processes in ordered (2x2) lattice gas phase

Surf. Sc. 507, 150 (2002)

ZAŁUSKA-KOTUR M. A., ŁUSAKOWSKI A., KRUKOWSKI S.,TURSKI Ł. A.

We study two-dimensional lattice gas with interaction dependent rates of the particle diffusion and desorption. We run Monte Carlo dynamics and study decay of harmonic perturbation of the profile. In the close to equilibrium, long-wavelength limit dynamics separate into desorption process, independent of the perturbation wave vector k, and proportional to k2 collective diffusion. In most cases both processes can be analyzed separately. We show that in some highly ordered systems, like (2 x 2) ordered structure of repulsive square lattice gas, one process can change characteristics of the other. We observe that the diffusion coefficient becomes slower when the desorption process is more rapid. This effect is even more visible in higher temperatures. As a result the measured activation energy of the diffusion process can be changed by the presence of the rapid desorption and adsorption processes even when system is close to equilibrium

Thermodynamics of an interacting trapped Bose-Einstein gas in the classical field approximation

Phys. Rev. A 66, 051602 (2002)

Krzysztof Góral, Mariusz Gajda and Kazimierz Rzążewski

We present a convenient technique describing the condensate in dynamical equilibrium with the thermal cloud, at temperatures close to the critical one. We show that the whole isolated system may be viewed as a single classical field undergoing nonlinear dynamics leading to a steady state. In our procedure it is the observation process and the finite detection time that allow for splitting the system into the condensate and the thermal cloud.

Center-of-mass motion in the many-body theory of Bose-Einstein condensates

Iwo Białynicki-Birula and Zofia Białynicka-Birula

The method of generating a family of new solutions starting from any wave function satisfying the nonlinear Schrödinger equation in a harmonic potential proposed recently [J. J. García-Ripoll, V. M. Pérez-García, and V. Vekslerchik, Phys. Rev. E 64, 056602 (2001)] is extended to the many-body theory of mutually interacting particles. Our method is based on a generalization of the displacement operator known in quantum optics, and results in the separation of the center-of-mass motion from the internal dynamics of many-body systems. The center-of-mass motion is analyzed for an anisotropic rotating trap and a region of instability for intermediate rotational velocities is predicted.

Phys. Rev. A 65, 063606 (2002)