2,175 research outputs found
Dynamics and phase evolution of Bose-Einstein condensates in one-dimensional optical lattices
We report experimental results on the dynamics and phase evolution of
Bose-Einstein condensates in 1D optical lattices. The dynamical behaviour is
studied by adiabatically loading the condensate into the lattice and
subsequently switching off the magnetic trap. In this case, the condensate is
free to expand inside the periodic structure of the optical lattice. The phase
evolution of the condensate, on the other hand, can be studied by
non-adiabatically switching on the periodic potential. We observe decays and
revivals of the interference pattern after a time-of-flight.Comment: 6 pages, 5 figures; submitted to the Proceedings of the 11th Laser
Physics Workshop, Bratislava 200
Nonlinear effects for Bose Einstein condensates in optical lattices
We present our experimental investigations on the subject of dynamical
nonlinearity-induced instabilities and of nonlinear Landau-Zener tunneling
between two energy bands in a Rubidium Bose-Einstein condensate in an
accelerated periodic potential. These two effects may be considered two
different regimes (for small and large acceleration) of the same physical
system and studied with the same experimental protocol. Nonlinearity introduces
an asymmetry in Landau-Zener tunneling; as a result, tunneling from the ground
state to the excited state is enhanced whereas in the opposite direction it is
suppressed. When the acceleration is lowered, the condensate exhibits an
unstable behaviour due to nonlinearity. We also carried out a full numerical
simulation of both regimes integrating the full Gross-Pitaevskii equation; for
the Landau-Zener effect we also used a simple two-level model. In both cases we
found good agreement with the experimental results.Comment: 9 pages, 7 figures. Submitted to Laser Physic
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Shear design of HSC beams with combination of links and horizontal web steel
The existing recommendations in Eurocode 2 and the British Code of Practice for the shear design of beams are derived from research conducted essentially on normal-strength concrete (NSC) with cube strengths up to 50 MPa, and it was found that the shear strengths of high-strength concrete (HSC) members made with limestone aggregate are below the characteristic resistances of identical NSC members. Previous experimental tests have also shown that significant differences exist in the angle of crack of shear failure of NSC and HSC. This paper presents data from five beam tests, which demonstrate that HSC with limestone aggregate has a reduced shear strength compared with NSC made with gravel and thus shows a gap in knowledge in the design approach to shear resistance of HSC beams. Previous investigations have suggested that horizontal web steels can contribute to the overall shear resistance of a reinforced concrete member in conjunction with the other constituents, concrete, tension and shear steel. The paper also presents data from tests on 11 beam tests and shows that the shear resistance of HSC beams is highly dependent on dowel action resulting from horizontal web bars positioned at the centre of the depth of the beam. Past attempts to quantify this dowel action are investigated and an improved design rule is proposed
Expansion of matter waves in static and driven periodic potentials
We study the non-equilibrium dynamics of cold atoms held in an optical
lattice potential. The expansion of an initially confined atom cloud occurs in
two phases: an initial quadratic expansion followed by a ballistic behaviour at
long times. Accounting for this gives a good description of recent experimental
results, and provides a robust method to extract the effective intersite
tunneling from time-of-flight measurements.Comment: 4 pages, 3 eps figure
Instabilities of a Bose-Einstein condensate in a periodic potential: an experimental investigation
By accelerating a Bose-Einstein condensate in a controlled way across the
edge of the Brillouin zone of a 1D optical lattice, we investigate the
stability of the condensate in the vicinity of the zone edge. Through an
analysis of the visibility of the interference pattern after a time-of-flight
and the widths of the interference peaks, we characterize the onset of
instability as the acceleration of the lattice is decreased. We briefly discuss
the significance of our results with respect to recent theoretical work.Comment: 7 pages, 3 figures; submitted to Optics Express (Focus Issue on Cold
Atomic Gases in Optical Lattices
Observation of St\"{u}ckelberg oscillations in accelerated optical lattices
We report the experimental observation of St\"{u}ckelberg oscillations of
matter waves in optical lattices. Extending previous work on Landau-Zener
tunneling of Bose-Einstein condensates in optical lattices, we study the
effects of the accumulated phase between two successive crossings of the
Brillouin zone edge. Our results agree well with a simple model for multiple
Landau-Zener tunneling events taking into account the band structure of the
optical lattice.Comment: 4 pages, 4 figure
Manipulation of ultracold atomic mixtures using microwave techniques
We used microwave radiation to evaporatively cool a mixture of of 133Cs and
87Rb atoms in a magnetic trap. A mixture composed of an equal number (around
10^4) of Rb and Cs atoms in their doubly polarized states at ultracold
temperatures was prepared. We also used microwaves to selectively evaporate
atoms in different Zeeman states.Comment: 9 pages, 6 figure
Rydberg excitation of a Bose-Einstein condensate
We have performed two-photon excitation via the 6P3/2 state to n=50-80 S or D
Rydberg state in Bose-Einstein condensates of rubidium atoms. The Rydberg
excitation was performed in a quartz cell, where electric fields generated by
plates external to the cell created electric charges on the cell walls.
Avoiding accumulation of the charges and realizing good control over the
applied electric field was obtained when the fields were applied only for a
short time, typically a few microseconds. Rydberg excitations of the
Bose-Einstein condensates loaded into quasi one-dimensional traps and in
optical lattices have been investigated. The results for condensates expanded
to different sizes in the one-dimensional trap agree well with the intuitive
picture of a chain of Rydberg excitations controlled by the dipole-dipole
interaction. The optical lattice applied along the one-dimensional geometry
produces localized, collective Rydberg excitations controlled by the
nearest-neighbour blockade.Comment: 7 pages, 7 figures, Laser Physics in press. arXiv admin note: text
overlap with arXiv:1103.423
Resonantly enhanced tunneling of Bose-Einstein condensates in periodic potentials
We report on measurements of resonantly enhanced tunneling of Bose-Einstein
condensates loaded into an optical lattice. By controlling the initial
conditions of our system we were able to observe resonant tunneling in the
ground and the first two excited states of the lattice wells. We also
investigated the effect of the intrinsic nonlinearity of the condensate on the
tunneling resonances.Comment: accepted for publication in Phys. Rev. Letter
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