1,410 research outputs found
Superfluid current disruption in a chain of weakly coupled Bose-Einstein Condensates
We report the experimental observation of the disruption of the superfluid
atomic current flowing through an array of weakly linked Bose-Einstein
condensates. The condensates are trapped in an optical lattice superimposed on
a harmonic magnetic potential. The dynamical response of the system to a change
of the magnetic potential minimum along the optical lattice axis goes from a
coherent oscillation (superfluid regime) to a localization of the condensates
in the harmonic trap ("classical" insulator regime). The localization occurs
when the initial displacement is larger than a critical value or, equivalently,
when the velocity of the wavepacket's center of mass is larger than a critical
velocity dependent on the tunnelling rate between adjacent sites.Comment: 8 pages, 4 figure
Disorder-enhanced phase coherence in trapped bosons on optical lattices
The consequences of disorder on interacting bosons trapped in optical
lattices are investigated by quantum Monte Carlo simulations. At small to
moderate strengths of potential disorder a unique effect is observed: if there
is a Mott plateau at the center of the trap in the clean limit, phase coherence
{\it increases} as a result of disorder. The localization effects due to
correlation and disorder compete against each other, resulting in a partial
delocalization of the particles in the Mott region, which in turn leads to
increased phase coherence. In the absence of a Mott plateau, this effect is
absent. A detailed analysis of the uniform system without a trap shows that the
disordered states participate in a Bose glass phase.Comment: 4 pages, 4 figure
Fractional Statistics in Three Dimensions: Compact Maxwell-Higgs System
We show that a (3+1)-dimensional system composed of an open magnetic vortex
and an electrical point charge exhibits the phenomenon of Fermi-Bose
transmutation. In order to provide the physical realization of this system we
focus on the lattice compact scalar electrodynamics whose topological
excitations are open Nielsen-Olesen strings with magnetic monopoles attached at
their ends.Comment: 8 page
Superfluid and Dissipative Dynamics of a Bose-Einstein Condensate in a Periodic Optical Potential
We create Bose-Einstein condensates of 87-rubidium in a static magnetic trap
with a superimposed blue-detuned 1D optical lattice. By displacing the magnetic
trap center we are able to control the condensate evolution. We observe a
change in the frequency of the center-of-mass oscillation in the harmonic
trapping potential, in analogy with an increase in effective mass. For fluid
velocities greater than a local speed of sound, we observe the onset of
dissipative processes up to full removal of the superfluid component. A
parallel simulation study visualizes the dynamics of the BEC and accounts for
the main features of the observed behavior.Comment: 4 pages, including figure
Analysis of Localization Phenomena in Weakly Interacting Disordered Lattice Gases
Disorder plays a crucial role in many systems particularly in solid state
physics. However, the disorder in a particular system can usually not be chosen
or controlled. We show that the unique control available for ultracold atomic
gases may be used for the production and observation of disordered quantum
degenerate gases. A detailed analysis of localization effects for two possible
realizations of a disordered potential is presented. In a theoretical analysis
clear localization effects are observed when a superlattice is used to provide
a quasiperiodic disorder. The effects of localization are analyzed by
investigating the superfluid fraction and the localization length within the
system. The theoretical analysis in this paper paves a clear path for the
future observation of Anderson-like localization in disordered quantum gases.Comment: 9 pages, 13 figure
Loss and revival of phase coherence in a Bose-Einstein condensate moving through an optical lattice
We investigate the phase coherence of a trapped Bose-Einstein condensate that
undergoes a dynamical superfluid-insulator transition in the presence of a
one-dimensional optical lattice. We study the evolution of the condensate after
a sudden displacement of the harmonic trapping potential by solving the
Gross-Pitaevskii equation, and comparing the results with the prediction of two
effective 1D models. We show that, owing to the 3D nature of the system, the
breakdown of the superfluid current above a critical displacement is not
associated to a sharp transition, but there exists a range of displacements for
which the condensate can recover a certain degree of coherence. We also discuss
the implications on the interference pattern after the ballistic expansion as
measured in recent experiments at LENS.Comment: 7 pages, 9 figure
On the Logarithmic Triviality of Scalar Quantum Electrodynamics
Using finite size scaling and histogram methods we obtain numerical results
from lattice simulations indicating the logarithmic triviality of scalar
quantum electrodynamics, even when the bare gauge coupling is chosen large.
Simulations of the non-compact formulation of the lattice abelian Higgs model
with fixed length scalar fields on lattices with ranging from
through indicate a line of second order critical points.
Fluctuation-induced first order transitions are ruled out. Runs of over ten
million sweeps for each produce specific heat peaks which grow
logarithmically with and whose critical couplings shift with picking
out a correlation length exponent of consistent with mean field
theory. This behavior is qualitatively similar to that found in pure
.Comment: 9 page
Excitations of Bose-Einstein condensates in a one-dimensional periodic potential
We report on the experimental investigation of the response of a
three-dimensional Bose-Einstein condensate (BEC) in the presence of a
one-dimensional (1D) optical lattice. By means of Bragg spectroscopy we probe
the band structure of the excitation spectrum in the presence of the periodic
potential. We selectively induce elementary excitations of the BEC choosing the
transferred momentum and we observe different resonances in the energy
transfer, corresponding to the transitions to different bands. The frequency,
the width and the strength of these resonances are investigated as a function
of the amplitude of the 1D optical lattice.Comment: 5 pages, 4 figure
Collective oscillations of two colliding Bose-Einstein condensates
Two 87Rb condensates (F=2, m_f=2 and m_f=1) are produced in highly displaced
harmonic traps and the collective dynamical behaviour is investigated. The
mutual interaction between the two condensates is evidenced in the
center-of-mass oscillations as a frequency shift of 6.4(3)%. Calculations based
on a mean-field theory well describe the observed effects of periodical
collisions both on the center-of-mass motion and on the shape oscillations.Comment: 5 pages, 3 figures, revtex - revised versio
Bosons in Disordered Optical Potentials
In this work we systematically investigate the condensate properties,
superfluid properties and quantum phase transitions in interacting Bose gases
trapped in disordered optical potentials. We numerically solve the Bose-Hubbard
Hamiltonian exactly for different: (a) types of disorder, (b) disorder
strengths, and (c) interatomic interactions. The three types of disorder
studied are: quasiperiodic disorder, uniform random disorder and random
speckle-type disorder. We find that the Bose glass, as identified by Fisher et
al [Phys. Rev. B {\bf 40}, 546 (1989)], contains a normal condensate component
and we show how the three different factors listed above affect it.Comment: 4 pages, 4 figures (low res) v2 Title,Abstract,Introduction: changes;
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