3,694 research outputs found
Parametric excitation of a Bose-Einstein condensate in a 1D optical lattice
We study the response of a Bose-Einstein condensate to a periodic modulation
of the depth of an optical lattice. Using Gross-Pitaevskii theory, we show that
a modulation at frequency Omega drives the parametric excitation of Bogoliubov
modes with frequency Omega/2. The ensuing nonlinear dynamics leads to a rapid
broadening of the momentum distribution and a consequent large increase of the
condensate size after free expansion. We show that this process does not
require the presence of a large condensate depletion. Our results reproduce the
main features of the spectrum measured in the superfluid phase by Stoeferle et
al., Phys. Rev. Lett. 92, 130403 (2004).Comment: 4 pages, 4 figures, more results added, to appear in PRA Rapid
Communication
Free-electron Model for Mesoscopic Force Fluctuations in Nanowires
When two metal electrodes are separated, a nanometer sized wire (nanowire) is
formed just before the contact breaks. The electrical conduction measured
during this retraction process shows signs of quantized conductance in units of
G_0=2e^2/h. Recent experiments show that the force acting on the wire during
separation fluctuates, which has been interpreted as being due to atomic
rearrangements. In this report we use a simple free electron model, for two
simple geometries, and show that the electronic contribution to the force
fluctuations is comparable to the experimentally found values, about 2 nN.Comment: 4 pages, 3 figures, reference correcte
Quantized Vortex States of Strongly Interacting Bosons in a Rotating Optical Lattice
Bose gases in rotating optical lattices combine two important topics in
quantum physics: superfluid rotation and strong correlations. In this paper, we
examine square two-dimensional systems at zero temperature comprised of
strongly repulsive bosons with filling factors of less than one atom per
lattice site. The entry of vortices into the system is characterized by jumps
of 2 pi in the phase winding of the condensate wavefunction. A lattice of size
L X L can have at most L-1 quantized vortices in the lowest Bloch band. In
contrast to homogeneous systems, angular momentum is not a good quantum number
since the continuous rotational symmetry is broken by the lattice. Instead, a
quasi-angular momentum captures the discrete rotational symmetry of the system.
Energy level crossings indicative of quantum phase transitions are observed
when the quasi-angular momentum of the ground-state changes.Comment: 12 Pages, 13 Figures, Version
Propagation of sound in a Bose Einstein condensate in an optical lattice
We study the propagation of sound waves in a Bose-Einstein condensate trapped
in a one-dimensional optical lattice. We find that the velocity of propagation
of sound wavepackets decreases with increasing optical lattice depth, as
predicted by the Bogoliubov theory. The strong interplay between nonlinearities
and the periodicity of the external potential raise new phenomena which are not
present in the uniform case. Shock waves, for instance, can propagate slower
than sound waves, due to the negative curvature of the dispersion relation.
Moreover, nonlinear corrections to the Bogoliubov theory appear to be important
even with very small density perturbations, inducing a saturation on the
amplitude of the sound signal
Complementarity of Resonant Scalar, Vector-Like Quark and Superpartner Searches in Elucidating New Phenomena
The elucidation of the nature of new phenomena requires a multi-pronged
approach to understand the essential physics that underlies it. As an example,
we study the simplified model containing a new scalar singlet accompanied by
vector-like quarks, as motivated by the recent diphoton excess at the LHC. To
be specific, we investigate three models with -doublet, vector-like
quarks with Yukawa couplings to a new scalar singlet and which also couple
off-diagonally to corresponding Standard Model fermions of the first or third
generation through the usual Higgs boson. We demonstrate that three classes of
searches can play important and complementary roles in constraining this model.
In particular, we find that missing energy searches designed for superparticle
production, supply superior sensitivity for vector-like quarks than the
dedicated new quark searches themselves.Comment: References added; small bug found in model and analysis
implementation, numerical results slightly modified, conclusions unchange
Rotation of an atomic Bose-Einstein condensate with and without a quantized vortex
We theoretically examine the rotation of an atomic Bose-Einstein condensate
in an elliptical trap, both in the absence and presence of a quantized vortex.
Two methods of introducing the rotating potential are considered -
adiabatically increasing the rotation frequency at fixed ellipticity, and
adiabatically increasing the trap ellipticity at fixed rotation frequency.
Extensive simulations of the Gross-Pitaevskii equation are employed to map out
the points where the condensate becomes unstable and ultimately forms a vortex
lattice. We highlight the key features of having a quantized vortex in the
initial condensate. In particular, we find that the presence of the vortex
causes the instabilities to shift to lower or higher rotation frequencies,
depending on the direction of the vortex relative to the trap rotation.Comment: 15 pages, 8 figure
Observation of plaquette fluctuations in the spin-1/2 honeycomb lattice
Quantum spin liquids are materials that feature quantum entangled spin
correlations and avoid magnetic long-range order at T = 0 K. Particularly
interesting are two-dimensional honeycomb spin lattices where a plethora of
exotic quantum spin liquids have been predicted. Here, we experimentally study
an effective S=1/2 Heisenberg honeycomb lattice with competing nearest and
next-nearest neighbor interactions. We demonstrate that YbBr avoids order
down to at least T=100 mK and features a dynamic spin-spin correlation function
with broad continuum scattering typical of quantum spin liquids near a quantum
critical point. The continuum in the spin spectrum is consistent with plaquette
type fluctuations predicted by theory. Our study is the experimental
demonstration that strong quantum fluctuations can exist on the honeycomb
lattice even in the absence of Kitaev-type interactions, and opens a new
perspective on quantum spin liquids.Comment: 32 pages, 7 Figure
Quantum transport and momentum conserving dephasing
We study numerically the influence of momentum-conserving dephasing on the
transport in a disordered chain of scatterers. Loss of phase memory is caused
by coupling the transport channels to dephasing reservoirs. In contrast to
previously used models, the dephasing reservoirs are linked to the transport
channels between the scatterers, and momentum conserving dephasing can be
investigated. Our setup provides a model for nanosystems exhibiting conductance
quantization at higher temperatures in spite of the presence of phononic
interaction. We are able to confirm numerically some theoretical predictions.Comment: 7 pages, 4 figure
Production of J/psi Mesons at HERA
Inelastic and diffractive production of J/psi mesons at HERA is reviewed. The
data on inelastic photoproduction are described well within errors by the
Colour Singlet Model in next-to-leading order. A search for colour octet
processes predicted within the NRQCD/factorisation approach is conducted in
many regions of phase space. No unambiguous evidence has been found to date.
Diffractive elastic production of J/psi mesons has been measured in the limit
of photoproduction up to the highest photon proton center of mass energies. The
increase of the cross section is described by pQCD models. At larger Q^2, the W
dependence is found to be similar to that observed in photoproduction. First
analyses of data at high t yield a powerlike dependence on |t|. A LO BFKL
calculation gives a good description of the data.Comment: 14 pages, 12 figures, contribution to Ringberg 200
Hall effects in Bose-Einstein condensates in a rotating optical lattice
Using the Kubo formalism, we demonstrate fractional quantum Hall features in
a rotating Bose-Einstein condensate in a co-rotating two-dimensional optical
lattice. The co-rotating lattice and trap potential allow for an effective
magnetic field and compensation of the centrifugal potential. Fractional
quantum Hall features are seen for the single-particle system and for few
strongly interacting many-particle systems.Comment: 11 pages, 13 figure
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