79 research outputs found
Faraday waves in elongated superfluid fermionic clouds
We use hydrodynamic equations to study the formation of Faraday waves in a
superfluid Fermi gas at zero temperature confined in a strongly elongated
cigar-shaped trap. First, we treat the role of the radial density profile in
the limit of an infinite cylindrical geometry and analytically evaluate the
wavelength of the Faraday pattern. The effect of the axial confinement is fully
taken into account in the numerical solution of hydrodynamic equations and
shows that the infinite cylinder geometry provides a very good description of
the phenomena.Comment: 6 pages, 7 figures. Figures 4 and 6 in high resolution on reques
Many-body interferometry of magnetic polaron dynamics
The physics of quantum impurities coupled to a many-body environment is among
the most important paradigms of condensed matter physics. In particular, the
formation of polarons, quasiparticles dressed by the polarization cloud, is key
to the understanding of transport, optical response, and induced interactions
in a variety of materials. Despite recent remarkable developments in ultracold
atoms and solid-state materials, the direct measurement of their ultimate
building block, the polaron cloud, has remained a fundamental challenge. We
propose and anlalyze a unique platform to probe time-resolved dynamics of
polaron-cloud formation with an interferometric protocol. We consider an
impurity atom immersed in a two-component Bose-Einstein condensate, where the
impurity generates spin-wave excitations that can be directly measured by the
Ramsey interference of surrounding atoms. The dressing by spin waves leads to
the formation of magnetic polarons and reveals a unique interplay between few-
and many-body physics that is signified by single- and multi-frequency
oscillatory dynamics corresponding to the formation of many-body bound states.
Finally, we discuss concrete experimental implementations in ultracold atoms.Comment: 6+6 pages, 3+2 figures. See also Research highlight
[doi:10.1038/s41567-018-0088-x] in Nature Physic
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
Expansion of a lithium gas in the BEC-BCS crossover
We report on experiments in Li Fermi gases near Feshbach resonances. A
broad s-wave resonance is used to form a Bose-Einstein condensate of weakly
bound Li molecules in a crossed optical trap. The measured
molecule-molecule scattering length of nm at 770 G is found
in good agreement with theory. The expansion energy of the cloud in the BEC-BCS
crossover region is measured. Finally we discuss the properties of p-wave
Feshbach resonances observed near 200 Gauss and new s-wave resonances in the
heteronuclear Li- Li mixture.Comment: 10 pages, 3 figures, Proceedings of ICAP 200
Interaction control and bright solitons in coherently-coupled Bose-Einstein condensates
We demonstrate fast control of the interatomic interactions in a
Bose-Einstein condensate by coherently coupling two atomic states with intra-
and inter-state scattering lengths of opposite signs. We measure the elastic
and inelastic scattering properties of the system and find good agreement with
a theoretical model describing the interactions between dressed states. In the
attractive regime, we observe the formation of bright solitons formed by
dressed-state atoms. Finally, we study the response of the system to an
interaction quench from repulsive to attractive values, and observe how the
resulting modulational instability develops into a bright soliton train.Comment: 6 pages, 4 figure
Experimental Study of the BEC-BCS Crossover Region in Lithium 6
We report Bose-Einstein condensation of weakly bound Limolecules in a
crossed optical trap near a Feshbach resonance. We measure a molecule-molecule
scattering length of nm at 770 G, in good agreement with
theory.We study the expansion of the cloud in the BEC-BCS crossoverregion.Comment: 4 pages, 3 figures, submitted to PR
Quantum liquid droplets in a mixture of Bose-Einstein condensates
Bose-Einstein condensatesquantum mixturesquantum liquidsquantum fluctuationsQuantum droplets are small clusters of atoms self-bound by the balance of attractive and repulsive
forces. Here we report on the observation of a novel type of droplets, solely stabilized by contact
interactions in a mixture of two Bose-Einstein condensates. We demonstrate that they are several
orders of magnitude more dilute than liquid helium by directly measuring their size and density
via in situ imaging. Moreover, by comparison to a single-component condensate, we show that
quantum many-body effects stabilize them against collapse. We observe that droplets require a
minimum atom number to be stable. Below, quantum pressure drives a liquid-to-gas transition that
we map out as a function of interaction strength. These ultra-dilute isotropic liquids remain weakly
interacting and constitute an ideal platform to benchmark quantum many-body theories.Peer ReviewedPreprin
Quantitative Determination of Temperature in the Approach to Magnetic Order of Ultracold Fermions in an Optical Lattice
We perform a quantitative simulation of the repulsive Fermi-Hubbard model using an ultracold gas trapped in an optical lattice. The entropy of the system is determined by comparing accurate measurements of the equilibrium double occupancy with theoretical calculations over a wide range of parameters. We demonstrate the applicability of both high-temperature series and dynamical mean-field theory to obtain quantitative agreement with the experimental data. The reliability of the entropy determination is confirmed by a comprehensive analysis of all systematic errors. In the center of the Mott insulating cloud we obtain an entropy per atom as low as 0.77k(B) which is about twice as large as the entropy at the Neel transition. The corresponding temperature depends on the atom number and for small fillings reaches values on the order of the tunneling energy
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