4,419 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
QCD sum rule analysis of the field strength correlator
The gauge invariant two-point correlator for the gluon field strength tensor
is analysed by means of the QCD sum rule method. To this end, we make use of a
relation of this correlator to a two-point function for a quark-gluon hybrid in
the limit of the quark mass going to infinity. From the sum rules a relation
between the gluon correlation length and the gluon condensate is obtained. We
briefly compare our results to recent determinations of the field strength
correlator on the lattice.Comment: 16 pages, 1 figur
Dependence of the BEC transition temperature on interaction strength: a perturbative analysis
We compute the critical temperature T_c of a weakly interacting uniform Bose
gas in the canonical ensemble, extending the criterion of condensation provided
by the counting statistics for the uniform ideal gas. Using ordinary
perturbation theory, we find in first order , where T_c^0 is the transition temperature of the corresponding
ideal Bose gas, a is the scattering length, and is the particle number
density.Comment: 14 pages (RevTeX
The Structure of the Vortex Liquid at the Surface of a Layered Superconductor
A density-functional approach is used to calculate the inhomogeneous vortex
density distribution in the flux liquid phase at the planar surface of a
layered superconductor, where the external magnetic field is perpendicular to
the superconducting layers and parallel to the surface. The interactions with
image vortices are treated within a mean field approximation as a functional of
the vortex density. Near the freezing transition strong vortex density
fluctuations are found to persist far into the bulk liquid. We also calculate
the height of the Bean-Livingston surface barrier.Comment: 8 pages, RevTeX, 2 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
Atomtronics: ultracold atom analogs of electronic devices
Atomtronics focuses on atom analogs of electronic materials, devices and
circuits. A strongly interacting ultracold Bose gas in a lattice potential is
analogous to electrons in solid-state crystalline media. As a consequence of
the band structure, cold atoms in a lattice can exhibit insulator or conductor
properties. P-type and N-type material analogs can be created by introducing
impurity sites into the lattice. Current through an atomtronic wire is
generated by connecting the wire to an atomtronic battery which maintains the
two contacts at different chemical potentials. The design of an atomtronic
diode with a strongly asymmetric current-voltage curve exploits the existence
of superfluid and insulating regimes in the phase diagram. The atomtronic
analog of a bipolar junction transistor exhibits large negative gain. Our
results provide the building blocks for more advanced atomtronic devices and
circuits such as amplifiers, oscillators and fundamental logic gates
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
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