413 research outputs found
Excitation spectrum of Mott shells in optical lattices
We theoretically study the excitation spectrum of confined macroscopic
optical lattices in the Mott-insulating limit. For large systems, a fast
numerical method is proposed to calculate the ground state filling and
excitation energies. We introduce many-particle on-site energies capturing
multi-band effects and discuss tunnelling on a perturbative level using an
effectively restricted Hilbert space. Results for small one-dimensional
lattices obtained by this method are in good agreement with the exact
multi-band diagonalization of the Hamiltonian. Spectral properties associated
with the formation of regions with constant filling, so-called Mott shells, are
investigated and interfaces between the shells with strong particle
fluctuations are characterized by gapless local excitations
Endoscopic imaging of quantum gases through a fiber bundle
We use a coherent fiber bundle to demonstrate the endoscopic absorption
imaging of quantum gases. We show that the fiber bundle introduces spurious
noise in the picture mainly due to the strong core-to-core coupling. By direct
comparison with free-space pictures, we observe that there is a maximum column
density that can be reliably measured using our fiber bundle, and we derive a
simple criterion to estimate it. We demonstrate that taking care of not
exceeding such maximum, we can retrieve exact quantitative information about
the atomic system, making this technique appealing for systems requiring
isolation form the environment
Localization and delocalization of ultracold bosonic atoms in finite optical lattices
We study bosonic atoms in small optical lattices by exact diagonalization and
observe a striking similarity to the superfluid to Mott insulator transition in
macroscopic systems. The momentum distribution, the formation of an energy gap,
and the pair correlation function show only a weak size dependence. For
noncommensurate filling we reveal in deep lattices a mixture of localized and
delocalized particles, which is sensitive to lattice imperfections. Breaking
the lattice symmetry causes a Bose-glass-like behavior. We discuss the nature
of excited states and orbital effects by using an exact diagonalization
technique that includes higher bands.Comment: 8 pages, 10 figures. Published versio
Magnetically tuned spin dynamics resonance
We present the experimental observation of a magnetically tuned resonance
phenomenon resulting from spin mixing dynamics of ultracold atomic gases. In
particular we study the magnetic field dependence of spin conversion in F=2
87Rb spinor condensates in the crossover from interaction dominated to
quadratic Zeeman dominated dynamics. We discuss the observed phenomenon in the
framework of spin dynamics as well as matter wave four wave mixing. Furthermore
we show that the validity range of the single mode approximation for spin
dynamics is significantly extended in the regime of high magnetic field
Spontaneous pattern formation in an anti-ferromagnetic quantum gas
Spontaneous pattern formation is a phenomenon ubiquitous in nature, examples
ranging from Rayleigh-Benard convection to the emergence of complex organisms
from a single cell. In physical systems, pattern formation is generally
associated with the spontaneous breaking of translation symmetry and is closely
related to other symmetry-breaking phenomena, of which (anti-)ferromagnetism is
a prominent example. Indeed, magnetic pattern formation has been studied
extensively in both solid-state materials and classical liquids. Here, we
report on the spontaneous formation of wave-like magnetic patterns in a spinor
Bose-Einstein condensate, extending those studies into the domain of quantum
gases. We observe characteristic modes across a broad range of the magnetic
field acting as a control parameter. Our measurements link pattern formation in
these quantum systems to specific unstable modes obtainable from linear
stability analysis. These investigations open new prospects for controlled
studies of symmetry breaking and the appearance of structures in the quantum
domain
Evolution from a Bose-Einstein condensate to a Tonks-Girardeau gas: An exact diagonalization study
We study ground state properties of spinless, quasi one-dimensional bosons
which are confined in a harmonic trap and interact via repulsive
delta-potentials. We use the exact diagonalization method to analyze the pair
correlation function, as well as the density, the momentum distribution,
different contributions to the energy and the population of single-particle
orbitals in the whole interaction regime. In particular, we are able to trace
the fascinating transition from bosonic to fermi-like behavior in
characteristic features of the momentum distribution which is accessible to
experiments. Our calculations yield quantitative measures for the interaction
strength limiting the mean-field regime on one side and the Tonks-Girardeau
regime on the other side of an intermediate regime.Comment: 5 pages, 5 figure
Gap and screening in Raman scattering of a Bose condensed gas
We propose different spectroscopic methods to explore the nature of the
thermal excitations of a trapped Bose condensed gas: 1) a four photon process
to probe the uniform region in the trap center: 2) a stimulated Raman process
in order to analyze the influence of a momentum transfer in the resulting
scattered atom momentum distribution. We apply these methods to address
specifically the energy spectrum and the scattering amplitude of these
excitations in a transition between two hyperfine levels of the gas atoms. In
particular, we exemplify the potential offered by these proposed techniques by
contrasting the spectrum expected, from the {\it non conserving} Bogoliubov
approximation valid for weak depletion, to the spectrum of the finite
temperature extensions like the {\it conserving} generalized random phase
approximation (GRPA). Both predict the existence of the Bogoliubov collective
excitations but the GRPA approximation distinguishes them from the single atom
excitations with a gapped and parabolic dispersion relation and accounts for
the dynamical screening of any external perturbation applied to the gas. We
propose two feasible experiments, one concerns the observation of the gap
associated to this second branch of excitations and the other deals with this
screening effect.Comment: 6 pages, 5 figure
Measurement of a Mixed Spin Channel Feshbach Resonance in Rubidium 87
We report on the observation of a mixed spin channel Feshbach resonance at
the low magnetic field value of (9.09 +/- 0.01) G for a mixture of |2,-1> and
|1,+1> states in 87Rb. This mixture is important for applications of
multi-component BECs of 87Rb, e.g. in spin mixture physics and for quantum
entanglement. Values for position, height and width of the resonance are
reported and compared to a recent theoretical calculation of this resonance.Comment: 4 pages, 3 figures minor changes, actualized citation
No Anomalous Fluctuations Exist in Stable Equilibrium Systems
An equilibrium statistical system is known to be stable if the fluctuations
of global observables are normal, when their dispersions are proportional to
the number of particles, or to the system volume. A general theorem is
rigorously proved for the case, when an observable is a sum of linearly
independent terms: The dispersion of a global observable is normal if and only
if all partial dispersions of its terms are normal, and it is anomalous if and
only if at least one of the partial dispersions is anomalous. This theorem, in
particular, rules out the possibility that in a stable system with
Bose-Einstein condensate some fluctuations of either condensed or noncondensed
particles could be anomalous. The conclusion is valid for arbitrary systems,
whether uniform or nonuniform, interacting weakly or strongly. The origin of
fictitious fluctuation anomalies, arising in some calculations, is elucidated.Comment: Latex file, 9 page
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