18 research outputs found
Fit-free determination of scale invariant equations of state: application to the 2D Bose gas across the Berezinksii-Kosterlitz-Thouless transition
We present a general "fit-free" method for measuring the equation of state
(EoS) of a scale-invariant gas. This method, which is inspired from the
procedure introduced by Ku et al. [Science 335, 563 (2012)] for the unitary
three-dimensional Fermi gas, provides a general formalism which can be readily
applied to any quantum gas in a known trapping potential, in the frame of the
local density approximation. We implement this method on a weakly-interacting
two-dimensional Bose gas in the vicinity of the Berezinskii-Kosterlitz-Thouless
transition, and determine its EoS with unprecedented accuracy in the critical
region. Our measurements provide an important experimental benchmark for
classical field approaches which are believed to accurately describe quantum
systems in the weakly interacting but non-perturbative regime.Comment: 5 pages, 5 figure
Emergence of coherence in a uniform quasi-two-dimensional Bose gas
Phase transitions are ubiquitous in our three-dimensional world. By contrast
most conventional transitions do not occur in infinite uniform two-dimensional
systems because of the increased role of thermal fluctuations. Here we explore
the dimensional crossover of Bose-Einstein condensation (BEC) for a weakly
interacting atomic gas confined in a novel quasi-two-dimensional geometry, with
a flat in-plane trap bottom. We detect the onset of an extended phase
coherence, using velocity distribution measurements and matter-wave
interferometry. We relate this coherence to the transverse condensation
phenomenon, in which a significant fraction of atoms accumulate in the ground
state of the motion perpendicular to the atom plane. We also investigate the
dynamical aspects of the transition through the detection of topological
defects that are nucleated in a quench cooling of the gas, and we compare our
results to the predictions of the Kibble-Zurek theory for the conventional BEC
second-order phase transition.Comment: main text = 24 pages, 6 figures + supplementary material = 10 pages,
5 figure
A two-dimensional magneto-optical trap of dysprosium atoms as a compact source for efficient loading of a narrow-line three-dimensional magneto-optical trap
We report on a scheme for loading dysprosium atoms into a narrow-line
three-dimensional magneto-optical trap (3D MOT). Our innovative approach
replaces the conventional Zeeman slower with a 2D MOT operating on the broad
421-nm line to create a high-flux beam of slow atoms. Even in the absence of a
push beam, we demonstrate efficient loading of the 3D MOT, which operates on
the narrower 626-nm intercombination line. Adding push beams working at either
421 nm or 626 nm, significant enhancement of the loading rate is achieved. We
reach the best performance, with an enhancement factor of , using a push
beam red-detuned to the 626-nm line. With loading rates greater than
atoms/s achieved at a moderate oven reservoir temperature of 800\,^{\circ}C,
our method offers similar or greater performance than Zeeman-slower-based
systems. Our 2D-MOT-based approach constitutes a promising first step for
state-of-the-art quantum gas experiments with several advantages over the
Zeeman-slower-based setup and is readily adaptable to other open-shell
lanthanides
Superfluid behaviour of a two-dimensional Bose gas
Two-dimensional (2D) systems play a special role in many-body physics.
Because of thermal fluctuations, they cannot undergo a conventional phase
transition associated to the breaking of a continuous symmetry. Nevertheless
they may exhibit a phase transition to a state with quasi-long range order via
the Berezinskii-Kosterlitz-Thouless (BKT) mechanism. A paradigm example is the
2D Bose fluid, such as a liquid helium film, which cannot Bose-condense at
non-zero temperature although it becomes superfluid above a critical phase
space density. Ultracold atomic gases constitute versatile systems in which the
2D quasi-long range coherence and the microscopic nature of the BKT transition
were recently explored. However, a direct observation of superfluidity in terms
of frictionless flow is still missing for these systems. Here we probe the
superfluidity of a 2D trapped Bose gas with a moving obstacle formed by a
micron-sized laser beam. We find a dramatic variation of the response of the
fluid, depending on its degree of degeneracy at the obstacle location. In
particular we do not observe any significant heating in the central, highly
degenerate region if the velocity of the obstacle is below a critical value.Comment: 5 pages, 3 figure