10 research outputs found
Dynamic Kosterlitz-Thouless transition in 2D Bose mixtures of ultra-cold atoms
We propose a realistic experiment to demonstrate a dynamic
Kosterlitz-Thouless transition in ultra-cold atomic gases in two dimensions.
With a numerical implementation of the Truncated Wigner Approximation we
simulate the time evolution of several correlation functions, which can be
measured via matter wave interference. We demonstrate that the relaxational
dynamics is well-described by a real-time renormalization group approach, and
argue that these experiments can guide the development of a theoretical
framework for the understanding of critical dynamics.Comment: 5 pages, 6 figure
Optical Flux Lattices for Two-Photon Dressed States
We describe a simple scheme by which "optical flux lattices" can be
implemented in ultracold atomic gases using two-photon dressed states. This
scheme can be applied, for example, to the ground state hyperfine levels of
commonly used atomic species. The resulting flux lattices simulate a magnetic
field with high mean flux density, and have low energy bands analogous to the
lowest Landau level. We show that in practical cases the atomic motion
significantly deviates from the adiabatic following of one dressed state, and
that this can lead to significant interactions even for fermions occupying a
single band. Our scheme allows experiments on cold atomic gases to explore
strong correlation phenomena related to the fractional quantum Hall effect,
both for fermions and bosons.Comment: 6 page
Vortex Formation in Two-Dimensional Bose Gas
We discuss the stability of a homogeneous two-dimensional Bose gas at finite
temperature against formation of isolated vortices. We consider a patch of
several healing lengths in size and compute its free energy using the Euclidean
formalism. Since we deal with an open system, which is able to exchange
particles and angular momentum with the rest of the condensate, we use the
symmetry-breaking (as opposed to the particle number conserving) formalism, and
include configurations with all values of angular momenta in the partition
function. At finite temperature, there appear sphaleron configurations
associated to isolated vortices. The contribution from these configurations to
the free energy is computed in the dilute gas approximation. We show that the
Euclidean action of linearized perturbations of a vortex is not positive
definite. As a consequence the free energy of the 2D Bose gas acquires an
imaginary part. This signals the instability of the gas. This instability may
be identified with the Berezinskii, Kosterlitz and Thouless (BKT) transition.Comment: RevTeX, 13 pages, 3 figure
The trapped two-dimensional Bose gas: from Bose-Einstein condensation to Berezinskii-Kosterlitz-Thouless physics
We analyze the results of a recent experiment with bosonic rubidium atoms
harmonically confined in a quasi-two-dimensional geometry. In this experiment a
well defined critical point was identified, which separates the
high-temperature normal state characterized by a single component density
distribution, and the low-temperature state characterized by a bimodal density
distribution and the emergence of high-contrast interference between
independent two-dimensional clouds. We first show that this transition cannot
be explained in terms of conventional Bose-Einstein condensation of the trapped
ideal Bose gas. Using the local density approximation, we then combine the
mean-field (MF) Hartree-Fock theory with the prediction for the
Berezinskii-Kosterlitz-Thouless transition in an infinite uniform system. If
the gas is treated as a strictly 2D system, the MF predictions for the spatial
density profiles significantly deviate from those of a recent Quantum
Monte-Carlo (QMC) analysis. However when the residual thermal excitation of the
strongly confined degree of freedom is taken into account, an excellent
agreement is reached between the MF and the QMC approaches. For the interaction
strength corresponding to the experiment, we predict a strong correction to the
critical atom number with respect to the ideal gas theory (factor ). A
quantitative agreement between theory and experiment is reached concerning the
critical atom number if the predicted density profiles are used for temperature
calibration.Comment: 23 pages, 7 figures, accepted for publication in New Journal of
Physics. v3: Typos and acknowledgment section correcte
Solid-state laser system for laser cooling of Sodium
We demonstrate a frequency-stabilized, all-solid laser source at 589 nm with
up to 800 mW output power. The laser relies on sum-frequency generation from
two laser sources at 1064 nm and 1319 nm through a PPKTP crystal in a
doubly-resonant cavity. We obtain conversion efficiency as high as 2 W/W^2
after optimization of the cavity parameters. The output wavelength is tunable
over 60 GHz, which is sufficient to lock on the Sodium D2 line. The robustness,
beam quality, spectral narrowness and tunability of our source make it an
alternative to dye lasers for atomic physics experiments with Sodium atoms
Resonant-light diffusion in a disordered atomic layer
Light scattering in dense media is a fundamental problem of many-body
physics, which is also relevant for the development of optical devices. In this
work we investigate experimentally light propagation in a dense sample of
randomly positioned resonant scatterers confined in a layer of sub-wavelength
thickness. We locally illuminate the atomic cloud and monitor
spatially-resolved fluorescence away from the excitation region. We show that
light spreading is well described by a diffusion process, involving many
scattering events in the dense regime. For light detuned from resonance we find
evidence that the atomic layer behaves as a graded-index planar waveguide.
These features are reproduced by a simple geometrical model and numerical
simulations of coupled dipoles