1,721,052 research outputs found
Josephson-type oscillations of a driven Bose-Einstein condensate in an optical lattice
No full textCoherent transport behaviour has been demonstrated in experiments on atomic Bose-Einstein condensates driven by an external force through a quasi one-dimensional optical lattice. We exploit the Wannier-function representation of band states to elucidate the relationship of such coherent transport with the weak-link Josephson effect and test the observability of multimode behaviour in condensates by numerical solution of the Gross-Pitaevskii equation
Coherent and dissipative transport of a Bose-Einstein condensate inside an optical lattice
No full textBose-Einstein condensates of alkali atoms inside an optical lattice provide a unique opportunity to investigate such issues as coherent transport of matter, Josephson-type phenomena, and dissipation of superfluidity under highly controllable experimental conditions. We give a short review of theoretical progress in evaluating the collective excitations of a periodic condensate and the coherent transport of matter in different configurations, which has been based on an adaptation of the Wannier function representation of quasi-particle states in periodic potentials and on the use of advanced numerical methods for the solution of the Gross-Pitaevskii equation. We present various methods by which the band structure of the elementary excitations may be probed and show that in the superfluid regime the combination of a constant force and a harmonic force in the presence of the optical lattice drives Josephson-type oscillations of the condensate, leading to observable resonances and multimode behaviour. Finally, we discuss decoherence of the condensate and dissipation via sound wave emission above a local velocity threshold
Output from Bose condensates in tunnel arrays: the role of mean-field interactions and of transverse confinement
No full textWe present numerical studies of atomic transport in 3D and 1D models for 3 mode-locked, pulsed atom laser as realized by Anderson and Kasevich [Science 281 (1998) 1686] using an elongated Bose condensate of Rb-87 atoms poured into 3 vertical optical lattice. From our 3D results we ascertain in a quantitative manner the role of mean-field interactions in determining the shape and the size of the pulses in the case of Gaussian transverse confinement. By comparison with 1D simulations we single out a best-performing 1D reduction of the mean-field interactions, which yields quantitatively useful predictions for all main features of the matter output. (C) 2000 Elsevier Science B.V. All rights reserved
Generalized quantum hydrodynamics of a trapped dilute Bose gas
No full textQuantal kinetic equations for particle and current densities of condensate and non-condensate in a confined Bose-condensed fluid are set up by expansion of the one-body density matrix about its diagonal. A microscopic Landau equation for superfluid flow in the inhomogeneous system is derived. Current-density functional theory in the local (long-wavelength) approximation is then used to propose a unified treatment of various damping mechanisms. (C) 1997 Elsevier Science B.V
Coherent transport in a Bose-Einstein condensate inside an optical lattice
No full textExperiments on atomic Bose-Einstein condensates inside quasi-one-dimensional optical lattices and related developments in the realization of atom lasers are currently at the frontiers in atomic physics. We give a short review of theoretical progress in evaluating the coherent transport of matter in such configurations, which has been based on an adaptation of the Wannier function representation of quasi-particle states in periodic potentials and on the use of advanced numerical methods for the solution of the time-dependent Gross-Pitaevskii equation. We present various methods by which the band structure of the elementary excitations of a periodic condensate may be probed and describe in terms of Bloch oscillations the coherent emission of matter pulses from a condensate inside an optical lattice under the force of gravity. A harmonic force can be applied to a condensate inside a magnetic trap by a rapid displacement of the center of the trap and a transition of the condensate from superfluid to dissipative behavior is driven by superposing an optical lattice. Finally, in the superfluid regime the combination of a constant force and a harmonic force in the presence of an optical lattice drives Josephson-type oscillations of the condensate, leading to observable resonances and multimode behavior
Output coupling of Bose condensates from atomic tunnel arrays: a numerical study
No full textWe present an extensive numerical study of the transport properties of an interacting Bose-Einstein condensate subject to the forces induced by a periodic potential and by gravity. This system models a mode-locked, pulsed laser of matter waves, which was recently realized by Anderson and Kasevich [Science 781 (1998) 1686] with a condensate of Rb-87 atoms confined in an optical lattice. We solve the time-dependent Gross-Pitaevskii equation for this model by a novel fast algorithm and examine in detail the role of the atomic interactions in determining the existence, the shape, the period of formation and the size of the pulses. The transport behaviour of the system as a function of its governing parameters is summarized in a simple two-dimensional diagram, (C) 1999 Published by Elsevier Science B.V. All rights reserved
Matter wave dynamics in an optical lattice: decoherence of Josephson-type oscillations from the Gross-Pitaevskii equation
No full textWe consider an atomic Bose-Einstein condensate described by the nonlinear Gross-Pitaevskii equation (GPE), which is driven by a harmonic force to move through a spatially periodic potential representing a quasi-one-dimensional (ID) optical lattice. For moderate values of the centre-of-mass displacement and of the potential barrier height, the condensate executes undamped oscillations and the alternating matter current can be mapped into the superfluid current passing through a Josephson junction in an AC field. By solving numerically a quasi-ID reduction of the GPE, we study how this coherent transport behaviour breaks down as (i) the strength of the harmonic force is increased, and (ii) the barrier height of the lattice potential is raised towards an extreme tight-binding limit where phase coherence between atomic clouds in neighbouring potential wells is lost. The emergence of decoherence is followed in both coordinate and momentum space, to trace the region of experimentally accessible parameters in which localization from nonlinearity may be observable in a measurement of the momentum distribution. (C) 2002 Elsevier Science B.V. All rights reserved
Time-dependent linear response of an inhomogeneous Bose superfluid: microscopic theory and connection to current-density functional theory
No full textThe dynamics of a confined fluid of Bose atoms is treated within the linear response regime, with a view to establishing a current-density functional formalism for an inhomogeneous superfluid state. After evaluating in full detail a simplified case of an external coupling to the density and phase of the condensate, the theory is extended to include the coupling to the total current density. The Kohn-Sham response functions of the condensate and all the exchange-correlation kernels for the superfluid are introduced from the microscopic equations of motion and are expressed in a physically transparent way through functional derivatives of correlation functions. A microscopic formula for the superfluid density is derived and used to introduce a generalized hydrodynamic approach for a weakly inhomogeneous two-fluid model in isothermal conditions. Local-density expressions are thereby derived for the velocities of first and second sound in the weakly inhomogeneous superfluid and for visco-elastic functions describing the transition from the hydrodynamic to the collisionless regime. Landau's hydrodynamic theory and known results in Green's functions language are recovered in the limiting case of a homogeneous superfluid. (C) 1998 Elsevier Science B.V. All rights reserved
Model of r space Boson-Fermion mixture and its relevance to high-T-c cuprates
No full textMotivated by a correlation between experimentally measured in-plane resistivity R and nuclear spin-lattice relaxation time T-1 for Cu nuclei in an underdoped high-T-c cuprate, a model is set up and solved for the equilibrium between (e) Fermion monomers and (2e) composite r space Bosons. Quantum statistics is fully included and of special interest for the R-T-1 correlation are the numbers of Fermion monomers in equilibrium with these composite Bosons for T > T-c. The model is shown to give the gist of the explanation of a pronounced minimum in a plot of the product RT1 vs T for the underdoped cuprate. Some contact is also made with transverse plasmon measurements, which are related to the composite Boson density in the condensate below T-c. Refinements of the simple model used here will eventually need to treat the finite lifetime of the composite Bosons and the screening of the charged particles, especially in the normal state. These can be expected to reduce the temperature range over which, in the normal state, the composite Boson number density is an appreciable fraction of the Fermion monomer density
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