Matter X waves

We predict that an ultra-cold Bose gas in an optical lattice can give rise to a new form of condensation, namely matter X waves. These are non-spreading 3D wave-packets which reflect the symmetry of the Laplacian with a negative effective mass along the lattice direction, and are allowed to exist in the absence of any trapping potential even in the limit of non-interacting atoms. This result has also strong implications for optical propagation in periodic structuresComment: 5 pages, 2 figure

Regular spatial structures in arrays of Bose-Einstein condensates induced by modulational instability

We show that the phenomenon of modulational instability in arrays of Bose-Einstein condensates confined to optical lattices gives rise to coherent spatial structures of localized excitations. These excitations represent thin disks in 1D, narrow tubes in 2D, and small hollows in 3D arrays, filled in with condensed atoms of much greater density compared to surrounding array sites. Aspects of the developed pattern depend on the initial distribution function of the condensate over the optical lattice, corresponding to particular points of the Brillouin zone. The long-time behavior of the spatial structures emerging due to modulational instability is characterized by the periodic recurrence to the initial low-density state in a finite optical lattice. We propose a simple way to retain the localized spatial structures with high atomic concentration, which may be of interest for applications. Theoretical model, based on the multiple scale expansion, describes the basic features of the phenomenon. Results of numerical simulations confirm the analytical predictions.Comment: 17 pages, 13 figure

Raman coupler for a trapped two-component quantum-degenerate Fermi gas

We investigate theoretically the Raman coupling between two internal states of a trapped low-density quantum-degenerate Fermi gas. In general, the trap frequencies associated with the two internal states can be different, leading to the onset of collapses and revivals in the population difference of the two internal states. This behavior can be changed drastically by two-body collisions. In particular, we show that under appropriate conditions they can suppress the dephasing leading to the collapse of the population difference, and restore almost full Rabi oscillations between the two internal states. These results are compared and contrasted to those for a quantum-degenerate bosonic gas.Comment: 7 pages incl. 7 PostScript figures (.eps), LaTeX using RevTeX4, submitted to Phys. Rev. A, modified versio

Creation of gap solitons in Bose-Einstein condensates

We discuss a method to launch gap soliton-like structures in atomic Bose-Einstein condensates confined in optical traps. Bright vector solitons consisting of a superposition of two hyperfine Zeeman sublevels can be created for both attractive and repulsive interactions between the atoms. Their formation relies on the dynamics of the atomic internal ground states in two far-off resonant counterpropagating sigma^+ sigma^- polarized laser beams which form the optical trap. Numerical simulations show that these solitons can be prepared from a one-component state provided with an initial velocity.Comment: 6 pages, 3 figure

Nonlinear atom optics and bright gap soliton generation in finite optical lattices

We theoretically investigate the transmission dynamics of coherent matter wave pulses across finite optical lattices in both the linear and the nonlinear regimes. The shape and the intensity of the transmitted pulse are found to strongly depend on the parameters of the incident pulse, in particular its velocity and density: a clear physical picture for the main features observed in the numerical simulations is given in terms of the atomic band dispersion in the periodic potential of the optical lattice. Signatures of nonlinear effects due the atom-atom interaction are discussed in detail, such as atom optical limiting and atom optical bistability. For positive scattering lengths, matter waves propagating close to the top of the valence band are shown to be subject to modulational instability. A new scheme for the experimental generation of narrow bright gap solitons from a wide Bose-Einstein condensate is proposed: the modulational instability is seeded in a controlled way starting from the strongly modulated density profile of a standing matter wave and the solitonic nature of the generated pulses is checked from their shape and their collisional properties

A simulation of two-dimensional Ostwald ripening on silver electrodes

Ostwald ripening of metal islands has been investigated by kinetic Monte Carlo simulations using the results of quantum-chemical calculations and of the embedded atom method as input data. On Au(1 0 0) Ostwald ripening was found to be kinetically hindered at ambient temperatures. In contrast, small islands on Ag(1 0 0) decayed readily at uncharged and at positively charged surfaces. The rate of ripening increases both with temperature and with the surface charge. The latter effect is caused by the interaction of local dipole moments with the double-layer field. This work confirms and extends recent investigations of the effect of the field on the surface mobility of metal electrodes