93 research outputs found
Condensation Energy of a Spin-1/2 Strongly Interacting Fermi Gas
We report a measurement of the condensation energy of a two-component Fermi
gas with tunable interactions. From the equation of state of the gas, we infer
the properties of the normal phase in the zero-temperature limit. By comparing
the pressure of the normal phase at T=0 to that of the low-temperature
superfluid phase, we deduce the condensation energy, i.e. the energy gain of
the system in being in the superfluid rather than normal state. We compare our
measurements to a ladder approximation description of the normal phase, and to
a fixed node Monte-Carlo approach, finding excellent agreement. We discuss the
relationship between condensation energy and pairing gap in the BEC-BCS
crossover.Comment: 4 figure
Experimental realization of strong effective magnetic fields in an optical lattice
We use Raman-assisted tunneling in an optical superlattice to generate large
tunable effective magnetic fields for ultracold atoms. When hopping in the
lattice, the accumulated phase shift by an atom is equivalent to the
Aharonov-Bohm phase of a charged particle exposed to a staggered magnetic field
of large magnitude, on the order of one flux quantum per plaquette. We study
the ground state of this system and observe that the frustration induced by the
magnetic field can lead to a degenerate ground state for non-interacting
particles. We provide a measurement of the local phase acquired from
Raman-induced tunneling, demonstrating time-reversal symmetry breaking of the
underlying Hamiltonian. Furthermore, the quantum cyclotron orbit of single
atoms in the lattice exposed to the magnetic field is directly revealed.Comment: 6 pages, 5 figure
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
Experimental realization of plaquette resonating valence bond states with ultracold atoms in optical superlattices
The concept of valence bond resonance plays a fundamental role in the theory
of the chemical bond and is believed to lie at the heart of many-body quantum
physical phenomena. Here we show direct experimental evidence of a
time-resolved valence bond quantum resonance with ultracold bosonic atoms in an
optical lattice. By means of a superlattice structure we create a
three-dimensional array of independent four-site plaquettes, which we can fully
control and manipulate in parallel. Moreover, we show how small-scale plaquette
resonating valence bond states with s- and d-wave symmetry can be created and
characterized. We anticipate our findings to open the path towards the creation
and analysis of many-body RVB states in ultracold atomic gases.Comment: 7 page, 4 figures in main text, 3 figures in appendi
Collective Oscillations of an Imbalanced Fermi Gas: Axial Compression Modes and Polaron Effective Mass
We investigate the low-lying compression modes of a unitary Fermi gas with
imbalanced spin populations. For low polarization, the strong coupling between
the two spin components leads to a hydrodynamic behavior of the cloud. For
large population imbalance we observe a decoupling of the oscillations of the
two spin components, giving access to the effective mass of the Fermi polaron,
a quasi-particle composed of an impurity dressed by particle-hole pair
excitations in a surrounding Fermi sea. We find , in agreement
with the most recent theoretical predictions.Comment: 4 pages, 4 figures, submitted to PR
Controlling Correlated Tunneling and Superexchange Interactions with AC-Driven Optical Lattices
The dynamical control of tunneling processes of single particles plays a
major role in science ranging from Shapiro steps in Josephson junctions to the
control of chemical reactions via light in molecules. Here we show how such
control can be extended to the regime of strongly interacting particles.
Through a weak modulation of a biased tunnel contact, we have been able to
coherently control single particle and correlated two-particle hopping
processes. We have furthermore been able to extend this control to
superexchange spin interactions in the presence of a magnetic-field gradient.
We show how such photon assisted superexchange processes constitute a novel
approach to realize arbitrary XXZ spin models in ultracold quantum gases, where
transverse and Ising type spin couplings can be fully controlled in magnitude
and sign.Comment: 10 pages, 9 figure
Transmission of near-resonant light through a dense slab of cold atoms
The optical properties of randomly positioned, resonant scatterers is a
fundamentally difficult problem to address across a wide range of densities and
geometries. We investigate it experimentally using a dense cloud of rubidium
atoms probed with near-resonant light. The atoms are confined in a slab
geometry with a sub-wavelength thickness. We probe the optical response of the
cloud as its density and hence the strength of the light-induced dipole-dipole
interactions are increased. We also describe a theoretical study based on a
coupled dipole simulation which is further complemented by a perturbative
approach. This model reproduces qualitatively the experimental observation of a
saturation of the optical depth, a broadening of the transition and a blue
shift of the resonance
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