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Delmenhorst, Germany

Winter C.,University of Bremen | Bartholoma A.,Senckenberg Institute | Capperucci R.,Senckenberg Institute | Callies U.,Helmholtz Center Geesthacht | And 7 more authors.
Wasser und Abfall | Year: 2014

Statements on the state and the development of coastal waters can only be made through long-term observations and model approaches. In the joint project WIMO (Scientific Monitoring Concepts for the German Bight), concepts and methods of measurement are developed and assessed that deal with basic scientific issues as well as meet the requirements of regulatory monitoring in the framework of the European legislation.

Lima A.R.P.,Free University of Berlin | Pelster A.,Hanse Wissenschaftskolleg | Pelster A.,University of Kaiserslautern
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2012

We theoretically investigate various beyond mean-field effects on Bose gases at zero temperature featuring the anisotropic and long-range dipole-dipole interaction in addition to the isotropic and short-range contact interaction. Within the realm of the Bogoliubov-de Gennes theory, we consider static properties and low-lying excitations of both homogeneous and harmonically trapped dipolar bosonic gases. For the homogeneous system, the condensate depletion, the ground-state energy, the equation of state, and the speed of sound are discussed in detail. Making use of the local density approximation, we extend these results in order to study the properties of a dipolar Bose gas in a harmonic trap and in the regime of large particle numbers. After deriving the equations of motion for the general case of a triaxial trap, we analyze the influence of quantum fluctuations on important properties of the gas, such as the equilibrium configuration and the low-lying excitations in the case of a cylinder-symmetric trap. In addition to the monopole and quadrupole oscillation modes, we also discuss the radial quadrupole mode. We find that the latter acquires a quantum correction exclusively due to the dipole-dipole interaction. As a result, we identify the radial quadrupole as a reasonably accessible source for the signature of dipolar many-body effects and stress the enhancing character that dipolar interactions have for quantum fluctuations in the other oscillation modes. © 2012 American Physical Society.

Hinrichs D.,Carl von Ossietzky University | Pelster A.,Hanse Wissenschaftskolleg | Pelster A.,University of Kaiserslautern | Holthaus M.,Carl von Ossietzky University
Applied Physics B: Lasers and Optics | Year: 2013

We develop a strategy for calculating critical exponents for the Mott insulator-to-superfluid transition shown by the Bose-Hubbard model. Our approach is based on the field-theoretic concept of the effective potential, which provides a natural extension of the Landau theory of phase transitions to quantum critical phenomena. The coefficients of the Landau expansion of that effective potential are obtained by high-order perturbation theory. We counteract the divergency of the weak-coupling perturbation series by including the seldom considered Landau coefficient a 6 into our analysis. Our preliminary results indicate that the critical exponents for both the condensate density and the superfluid density, as derived from the two-dimensional Bose-Hubbard model, deviate by less than 1 % from the best known estimates computed so far for the three-dimensional XY universality class. © 2013 Springer-Verlag Berlin Heidelberg.

Al-Jibbouri H.,Free University of Berlin | Vidanovic I.,University of Belgrade | Vidanovic I.,Goethe University Frankfurt | Balaz A.,University of Belgrade | And 2 more authors.
Journal of Physics B: Atomic, Molecular and Optical Physics | Year: 2013

We investigate geometric resonances in Bose-Einstein condensates by solving the underlying time-dependent Gross-Pitaevskii equation for systems with two- and three-body interactions in an axially symmetric harmonic trap. To this end, we use a recently developed analytical method (Vidanović et al 2011 Phys. Rev. A 84 013618), based on both a perturbative expansion and a Poincaré-Lindstedt analysis of a Gaussian variational approach, as well as a detailed numerical study of a set of ordinary differential equations for variational parameters. By changing the anisotropy of the confining potential, we numerically observe and analytically describe strong nonlinear effects: shifts in the frequencies and mode coupling of collective modes, as well as resonances. Furthermore, we discuss in detail the stability of a Bose-Einstein condensate in the presence of an attractive two-body interaction and a repulsive three-body interaction. In particular, we show that a small repulsive three-body interaction is able to significantly extend the stability region of the condensate. © 2013 IOP Publishing Ltd.

Nikolic B.,Free University of Berlin | Nikolic B.,University of Belgrade | Balaz A.,University of Belgrade | Pelster A.,Hanse Wissenschaftskolleg | Pelster A.,University of Kaiserslautern
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

Here we study properties of a homogeneous dipolar Bose-Einstein condensate in a weak anisotropic random potential with Lorentzian correlation at zero temperature. To this end we solve perturbatively the Gross-Pitaevskii equation to second order in the random potential strength and obtain analytic results for the disorder ensemble averages of both the condensate and the superfluid depletion, the equation of state, and the sound velocity. For a pure contact interaction and a vanishing correlation length, we reproduce the seminal results of Huang and Meng, which were originally derived within a Bogoliubov theory around a disorder-averaged background field. For dipolar interaction and isotropic Lorentzian-correlated disorder, we obtain results which are qualitatively similar to the case of an isotropic Gaussian-correlated disorder. In the case of an anisotropic disorder, the physical observables show characteristic anisotropies which arise from the formation of fragmented dipolar condensates in the local minima of the disorder potential. © 2013 American Physical Society.

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