895 research outputs found
Exploring the Kibble-Zurek mechanism with homogeneous Bose gases
Out-of-equilibrium phenomena is a subject of considerable interest in many
fields of physics. Ultracold quantum gases, which are extremely clean,
well-isolated and highly controllable systems, offer ideal platforms to
investigate this topic. The recent progress in tailoring trapping potentials
now allows the experimental production of homogeneous samples in custom
geometries, which is a key advance for studies of the emergence of coherence in
interacting quantum systems. Here we review recent experiments in which
temperature quenches have been performed across the Bose-Einstein condensation
(BEC) phase transition in an annular geometry and in homogeneous 3D and
quasi-2D gases. Combined, these experiments give a comprehensive picture of the
Kibble-Zurek (KZ) scenario through complementary measurements of correlation
functions and topological defects density. They also allow the measurement of
KZ scaling laws, the direct confirmation of the "freeze-out" hypothesis that
underlies the KZ theory, and the extraction of critical exponents of the
Bose-Einstein condensation transition.Comment: 11 pages, 6 figures; topical revie
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
The Equation of State of a Low-Temperature Fermi Gas with Tunable Interactions
Interacting fermions are ubiquitous in nature and understanding their
thermodynamics is an important problem. We measure the equation of state of a
two-component ultracold Fermi gas for a wide range of interaction strengths at
low temperature. A detailed comparison with theories including Monte-Carlo
calculations and the Lee-Huang-Yang corrections for low-density bosonic and
fermionic superfluids is presented. The low-temperature phase diagram of the
spin imbalanced gas reveals Fermi liquid behavior of the partially polarized
normal phase for all but the weakest interactions. Our results provide a
benchmark for many-body theories and are relevant to other fermionic systems
such as the crust of neutron stars.Comment: 28 pages, 7 figure
Observation of Weak Collapse in a Bose-Einstein Condensate
We study the collapse of an attractive atomic Bose-Einstein condensate prepared in the uniform potential of an optical-box trap. We characterize the critical point for collapse and the collapse dynamics, observing universal behavior in agreement with theoretical expectations. Most importantly, we observe a clear experimental signature of the counterintuitive weak collapse, namely, that making the system more unstable can result in a smaller particle loss. We experimentally determine the scaling laws that govern the weak-collapse atom loss, providing a benchmark for the general theories of nonlinear wave phenomena.The GeForce GTX TITAN X used for the numerical simulations was donated by the NVIDIA Corporation. This work was supported by the Royal Society, EPSRC (Grant No. EP/ N011759/1), ERC (QBox), AFOSR, and ARO. A. L. G. and N. N. acknowledge support from Trinity College, Cambridge
Gastrointestinal colonization by KPC-producing Klebsiella pneumoniae following hospital discharge: duration of carriage and risk factors for persistent carriage
AbstractThe natural history of KPC-producing Klebsiella pneumoniae (KPC KP) carriage is unknown. We aimed to examine the duration of KPC KP carriage following hospital discharge and to study the risk factors for persistent carriage. A cohort of 125 KPC KP carriers was followed monthly for between 3 and 6 months after discharge from an acute-care hospital. Rectal swabs and data were collected at baseline and at each visit. KPC KP was detected by culture and direct blaKPC PCR. Acquisition time was regarded as the earliest date of KPC KP isolation. Resolution of carriage was defined as a negative KPC KP test in at least two consecutive samples. Analyses were separated for recent (<4 months) (REC, 75 patients) and remote (≥4 months) (REM, 50 patients) acquisition groups. Risk factors for persistent carriage were examined by survival analyses for the REC group and by prevalence methods for the REM group. The mean age of patients was 67.5 years and 49.6% were male. Forty-six (61%) patients in the REC group and 14 (28%) in the REM group were persistent carriers (p < 0.001). A significant risk factor for persistent carriage identified in both the REC and REM groups was the presence of any catheter (p < 0.05). Unique risk factor groups included long-term care facility (LTCF) residence (p < 0.01) and a low functional status as measured by the Barthel’s index (p < 0.05) in the REC group and high Charlson’s score in the REM group (p < 0.05). Out of the entire 100 patients who had at least one negative sample, only 65 remained negative on subsequent cultures. In conclusion, persistent carriage of KPC KP is associated with catheter use and a low functional status; it is more common in patients with recent acquisition and is related to LTCF stay. A single negative KPC KP test is insufficient to exclude persistent carriage
Two- and three-body contacts in the unitary Bose gas
In many-body systems governed by pairwise contact interactions, a wide range of observables is linked by a single parameter, the two-body contact, which quantifies two-particle correlations. This profound insight has transformed our understanding of strongly interacting Fermi gases. Using Ramsey interferometry, we studied coherent evolution of the resonantly interacting Bose gas, and we show here that it cannot be explained by only pairwise correlations. Our experiments reveal the crucial role of three-body correlations arising from Efimov physics and provide a direct measurement of the associated three-body contact.This work was supported by EPSRC [Grant No. EP/N011759/1], ERC (QBox), ARO and AFOSR. N.N. ac- knowledges support from Trinity College, Cambridge, R.P.S. from the Royal Society and R.L. from the E.U. Marie-Curie program [Grant No. MSCA-IF-2015 704832]
The equation of state of ultracold Bose and Fermi gases: a few examples
We describe a powerful method for determining the equation of state of an
ultracold gas from in situ images. The method provides a measurement of the
local pressure of an harmonically trapped gas and we give several applications
to Bose and Fermi gases. We obtain the grand-canonical equation of state of a
spin-balanced Fermi gas with resonant interactions as a function of
temperature. We compare our equation of state with an equation of state
measured by the Tokyo group, that reveals a significant difference in the
high-temperature regime. The normal phase, at low temperature, is well
described by a Landau Fermi liquid model, and we observe a clear thermodynamic
signature of the superfluid transition. In a second part we apply the same
procedure to Bose gases. From a single image of a quasi ideal Bose gas we
determine the equation of state from the classical to the condensed regime.
Finally the method is applied to a Bose gas in a 3D optical lattice in the Mott
insulator regime. Our equation of state directly reveals the Mott insulator
behavior and is suited to investigate finite-temperature effects.Comment: 14 pages, 6 figure
Metastability in spin polarised Fermi gases and quasiparticle decays
We investigate the metastability associated with the first order transition from normal to superfluid phases in the phase diagram of two-component polarised Fermi gases.We begin by detailing the dominant decay processes of single quasiparticles.Having determined the momentum thresholds of each process and calculated their rates, we apply this understanding to a Fermi sea of polarons by linking its metastability to the stability of individual polarons, and predicting a region of metastability for the normal partially polarised phase. In the limit of a single impurity, this region extends from the interaction strength at which a polarised phase of molecules becomes the groundstate, to the one at which the single quasiparticle groundstate changes character from polaronic to molecular. Our argument in terms of a Fermi sea of polarons naturally suggests their use as an experimental probe. We propose experiments to observe the threshold of the predicted region of metastability, the interaction strength at which the quasiparticle groundstate changes character, and the decay rate of polarons
Elliptic flow in a strongly interacting normal Bose gas
We study the anisotropic, elliptic expansion of a thermal atomic Bose gas
released from an anisotropic trapping potential, for a wide range of
interaction strengths across a Feshbach resonance. We show that in our system
this hydrodynamic phenomenon is for all interaction strengths fully described
by a microscopic kinetic model with no free parameters. The success of this
description crucially relies on taking into account the reduced thermalising
power of elastic collisions in a strongly interacting gas, for which we derive
an analytical theory. We also perform time-resolved measurements that directly
reveal the dynamics of the energy transfer between the different expansion
axes
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