Research highlights

Effective long-range interactions in confined curved dimensions

In ultracold atomic physics the three dimensional motion of neutral atoms or charged particles can be confined to a lower dimensional typically one- or two-dimensional trap by employing electromagnetic forces. If these traps are of curved character which is possible e.g. in evanescent fields surrounding optical nanofibers, they confine the motion to a curved low-dimensional manifold. This opens the perspective of designing novel effective finite-range and possibly also long-range interactions since the dynamics is constrained to the curved geometry but the interaction takes places via the dynamically forbidden dimensions. The corresponding forces can now become oscillating with increasing distance between the particles and are widely tunable via the parameters of the confining curved manifold. Exploring as a prototype example the one-dimensional helix, it can be immediately shown that a plethora of local equilibrium configurations and consequently bound states emerge already for two particles even if the particles were repelling each other in free space. With an increasing number of interacting particles an ever increasing wealth of symmetry-adapted and symmetry-distorted configurations create a very complex energy landscape exhibiting a dense spectrum of local equilibria. It can be anticipated that the thermodynamical properties and quantum physics of the many-body interacting helical chain show novel structural properties such as enriched phase diagrams as well as an intriguing dynamical behaviour.

phase space portrait of the effective long-range interactions

Europhysics Letters 95, 50005 (2011)

 

Dark-bright ring solitons in Bose-Einstein condensates

In a recent Fast Track Communication in Journal of Physics B we investigate dark-bright ring solitons in two-component Rubidium Bose-Einstein condensates. The figure shows a typical density profile of such a rotationally symmetric two-dimensional generalization of the more familiar 1D dark-bright solitons. In the limit of large densities of the dark component, we describe the soliton dynamics by means of an equation of motion for the ring radius. The presence of the bright, "filling" species is demonstrated to have a stabilizing effect on the ring dark soliton. Near the linear limit, we discuss the symmetry-breaking bifurcations of dark-bright soliton stripes and vortex-bright soliton clusters from the dark-bright ring and relate the stabilizing effect of filling to changes in the bifurcation diagram. Finally, we show that stabilization by means of a second component is not limited to the radially symmetric structures, but can also be observed in a cross-like dark-bright soliton configuration.

Journal of Physics B Fast Track 44, 191003 (2011)

 

Particle trapping mechanisms

In a recent issue of Europhysics Letters we report on a new mechanism for particle trapping, which can be exploited for the selective loading of a driven (optical) lattice with cold atoms, thus permitting local addressability. The working principle is based on the breaking of the spatio-temporal translation symmetry, which is responsible for the equivalence of all lattice sites, by applying modulated phase shifts to the lattice sites. The patterned trapping of the particles occurs in confined chaotic seas, created via the ramping of the height of the lattice potential. The trapping mechanism is schematically illustrated in the figure

i.e. (a) shows the lattice before ramping the potential, after ramping (b) and at the end of the simulation (c). Blue particles are trapped, red ones are diffusive. Possible applications of this trapping mechanism are atom lithography or quantum information processing.

Europhysics Letters 94, 40001 (2011)

 

Formation of density waves

In the subgroup "Quantum and Classical Dynamics of Driven Billiards and Lattices" we have recently demonstrated a mechanism for the controlled conversion of ballistic to diffusive motion and vice versa. This process takes place at the interfaces of domains with different time-dependent forces in lattices of laterally oscillating barrier potentials. As a consequence, long-time transient oscillations of the particle density are formed, which can be converted to permanent density waves by an appropriate tuning of the driving forces.

In the figure the particle density as a function of the position and time is shown. In the course of the dynamics regions of constant particle density arise in the system.

Europhysics Letters 95, 30005 (2011)

 

Stabilizing vortex clusters by means of anisotropic confinement

In our recent work published in Europhysics Letters we shed new light on the stability of vortex clusters in Bose-Einstein condensates when rotational symmetry of the trapping potential is broken, as can easily be done in present-day experiments. We investigate the effects of such anisotropic confinement on the stability and dynamics of a number of well-known vortex cluster states, in particular the popular dipole and tripole configurations. Sufficiently strong anisotropies are shown to stabilize states with arbitrary numbers of vortices that are highly unstable in the isotropic limit. Conversely, anisotropy can be used to destabilize states which are stable in the isotropic limit. Near the linear limit, we identify the bifurcations of vortex states including their emergence from linear eigenstates, once again highlighting the intimate connection between dark soliton stripes and vortex clusters. In the strongly nonlinear limit, a particle-like description of the dynamics of the vortices in the anisotropic trap is developed. Both are in very good agreement with numerical results. The figure shows two snapshots of a collective mode in a stabilized 17 vortex cluster, exhibiting string-like oscillatory behaviour.

 Europhysics Letters 93, 20008 (2011)

 

Bridging the gap from individual atoms to a gas

 

A long lasting question in physics is the emergence of the properties of many-body systems while pursuing their assembly from a few-body system atom by atom. What is the route from 'few' to 'many' and how much of few-body characteristics is there in many-body physics? These questions have been addressed recently for a simple but yet fundamental prototype system: short-range interacting ultracold atoms in a harmonic trap.The idea is to start from the exact wave function describing a pair of atoms, and generalize it to more atoms according to a well-defined construction principle. It turns out that the properties of the thus obtained many-body wave function agree very well with results of numerical approaches, where no alternative analytical treatment exists for this system. For very strong and very weak interactions the agreement is excellent, with a minor increase of the deviations for intermediate strengths of the repulsion and a larger number of atoms.We show that at least for this system, 'more' is still a bit different from few. Nevertheless we can learn a lot from the successes and failures of such an immediate guess: taking two and constructing many.

Physical Review Letters 108, 045301 (2012)

 

Ultra-long-range giant dipole molecules in crossed electric and magnetic fields

In this work we show the existence of ultra-long-range giant dipole Rydberg molecules formed by
a neutral alkali ground state atom that is bound to the decentered electronic wave function of
a giant dipole atom. The adiabatic potential surfaces emerging from the interaction of the ground
state atom with the giant dipole electron possess a rich topology depending on the degree of
electronic excitation. Binding energies and the vibrational motion in the energetically lowest
surfaces are analyzed by means of perturbation theory and exact diagonalization techniques.
The resulting molecules are truly giant with internuclear distances up to several $\mu m$.
Finally, we demonstrate the existence of intersection manifolds of excited electronic states
that lead to a vibrational decay of the ground state atom dynamics.

 

Europhysics Letters 97, 43001 (2012)

 

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