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arXiv:1105.5015 · Physical Review A

Stability of a dipolar Bose-Einstein condensate in a one-dimensional lattice

S. Müller, Juliette Billy, E. A. L. Henn, Holger Kadau, Axel Griesmaier, M. Jona-Lasinio, Luis Santos, Tilman Pfau

Published Nov 2, 2011• 85 citations
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Abstract

We show that in contrast with contact interacting gases, an optical lattice changes drastically the stability properties of a dipolar condensate, inducing a crossover from dipolar destabilization to dipolar stabilization for increasing lattice depths. Performing stability measurements on a ${}^{52}$Cr Bose-Einstein condensate in an interaction-dominated regime, repulsive dipolar interaction balances negative scattering lengths down to $\ensuremath{-}17$ Bohr radii. Our findings are in excellent agreement with mean-field calculations, revealing the important destabilizing role played by intersite dipolar interactions in deep lattices.

DOI
10.1103/physreva.84.053601

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We show that in contrast with contact interacting gases, an optical lattice changes drastically the stability properties of a dipolar condensate, inducing a crossover from dipolar destabilization to dipolar stabilization for increasing lattice depths. Performing stability measurements on a 52{}^{52}52Cr Bose-Einstein condensate in an interaction-dominated regime, repulsive dipolar interaction balances negative scattering lengths down to \ensuremath−17\ensuremath{-}17\ensuremath−17 Bohr radii. Our findings are in excellent agreement with mean-field calculations, revealing the important destabilizing role played by intersite dipolar interactions in deep lattices.
We study the stabilization properties of dipolar Bose–Einstein condensate in a deep one-dimensional optical lattice with an additional external parametrically modulated harmonic trap potential. Through both analytical and numerical methods, we solve a dimensionless nonlocal nonlinear discrete Gross–Pitaevskii equation with both the short-range contact interaction and the long-range dipole–dipole interaction. It is shown that, the stability of dipolar condensate in modulated deep optical lattice can be controled by coupled effects of the contact interaction, the dipolar interaction and the external modulation. The system can be stabilized when the dipolar interaction, the contact interaction, the average strength of potential and the ratio of amplitude to frequency of the modulation satisfy a critical condition. In addition, the breather state, the diffused state and the attractive-interaction-induced-trapped state are predicted. The dipolar interaction and the external modulation of the lattice play important roles in stabilizing the condensate.
We report on the observation of the collapse of a dipolar Bose-Einstein condensate (BEC) upon release from a one-dimensional optical lattice. In strong contrast with the typical behavior of quantum gases under time of flight (TOF), we show here that a strongly dipolar BEC, stabilized in the lattice, may become unstable during the TOF dynamics due to the combined effect of the anisotropy of the dipolar interaction and intersite coherence. Such TOF-triggered collapse is a peculiar case of a collapse induced by the deconfinement of a dipolar many-body system at a fixed two-body interaction strength.
The subject of this thesis is the experimental investigation of a dipolar 52-Chromium Bose-Einstein condensate (BEC) in a one-dimensional optical lattice potential. At first, the influence of the periodic landscape on the stability of the quantum gas is investigated. Based on these measurements, the collapse dynamics of the system, i.e. its time evolution after passing the threshold to the unstable regime, is observed and analyzed. Both the investigations aim to contribute to the basic understanding of dipolar quantum gases. In addition, they may provide a basis for further experiments with dipolar BECs in optical lattices. Amongst others, the observation of self-organized density structures is one of the biggest goals in this field of research.
Ultracold dilute atomic gases are highly controllable systems whose properties are fundamentally determined by the interatomic interactions.In typical experiments on ultra cold gases particles interact dominantly via a short-range isotropic potential.However, during the last years a new generation of experiments is starting to explore systems, where an additional interaction, the long-range and anisotropic dipole-dipole interaction plays a significant or possibly dominant role.The effects induced by the anisotropy of the dipole-dipole interaction are crucially enhanced if the dipolar gas is confined in a deep optical lattice.In this thesis we study novel phenomena in such systems of dipolar gases in deep optical lattices.The nonlocal nonlinearity introduced by the dipole-dipole interaction plays a crucial role in the physics of dipolar Bose-Einstein condensates.In particular, it may distort significantly the stability of straight vortex lines.Remarkably, in the presence of a periodic potential along the vortex line, the spectrum of transverse modes shows a roton-like minimum, which eventually destabilizes the straight vortex when the BEC as a whole is still stable, opening the possibility for new scenarios for vortex line configurations in dipolar gases.We have analyzed this instability, and showed that it leads to a second-order-like phase transition from a straight vortex line into novel helical or snake-like configurations, depending on the dipole orientation.Strong 1D lattices usually lead to unconnected two-dimensional gases.The long-range character of the dipole-dipole interactions leads to a novel scenario where non-overlapping gases at different sites may interact significantly.We show that the excitations of non-overlapping condensates in 1D optical lattices acquire a band-like character, being collectively shared by different sites.In particular, the hybridization of the modes significantly enhances the rotonization of the excitations, and may induce roton-instability.We discuss the observability of this effect in on-going experiments.Fermionic polar molecules in deep 1D optical lattices may form selfassembled filaments when the electric dipoles are oriented along the lattice axis.These composites are bosons or fermions depending on the number of molecules per chain, leading to a peculiar and complex Bose-Fermi mixture, vi which we discuss in detail for the simplest case of a three-well potential.We show that the interplay between filament binding energy, transverse filament modes, and trimer Fermi energy leads to a rich variety of possible scenarios ranging from a degenerate Fermi gas of trimers to a binary mixture of two different types of bosonic dimers.We study the intriguing zero temperature and finite temperature physics of these composites for the particular case of an ideal filament gas loaded in 1D sites, and discuss possible methods to probe these chain mixtures.
Dipolar interactions support the formation of intersite soliton molecules in a stack of quasi-one-dimensional (quasi-1D) traps. We show that the stability and properties of individual solitons and soliton molecules in such a geometry crucially depend on the interplay between contact and dipolar interactions. In particular, two different quasi-1D soliton regimes are possible: a one-dimensional (1D) soliton characterized by purely repulsive dipole-dipole interactions (DDI) and a three-dimensional (3D) soliton for which a sufficiently large dipole moment renders the DDI attractive. Furthermore, we find that in contrast to the dimers of polar molecules the soliton dimers exhibit a nontrivial behavior of the elementary excitations that stems from the competition between onsite and intersite DDI. Finally, we prove the existence of soliton trimers in a regime where molecular trimers do not occur. We demonstrate that the soliton molecules that we report are well feasible under realistic experimental conditions.
We investigate the vortex formation properties of a three-dimensional (3D) dipolar Bose-Einstein condensate (BEC) of 52 Cr atoms in an optical lattice potential through numerical simulations of the time-dependent Gross-Pitaevskii equation with a dissipation term. The mechanisms that influence the dipole-dipole interaction and the repulsive contact interaction between atoms during vortex formation are analyzed. We find that the stability of the vortices weakens with increasing attractive dipolar strength and that the repulsive contact interaction between atoms can increase the number of vortices.