8 research outputs found
Realization of a strongly interacting Bose-Fermi mixture from a two-component Fermi gas
We show the emergence of a strongly interacting Bose-Fermi mixture from a
two-component Fermi mixture with population imbalance. By analyzing in situ
density profiles of Li atoms in the BCS-BEC crossover regime we identify a
critical interaction strength, beyond which all minority atoms pair up with
majority atoms, and form a Bose condensate. This is the regime where the system
can be effectively described as a boson-fermion mixture. We determine the
dimer-fermion and dimer-dimer scattering lengths and beyond-mean-field
contributions. Our study realizes a Gedanken experiment of bosons immersed in a
Fermi sea of one of their constituents, revealing the composite nature of the
bosons.Comment: 5 pages, 5 figure
Determination of the Superfluid Gap in Atomic Fermi Gases by Quasiparticle Spectroscopy
We present majority and minority radiofrequency (RF) spectra of strongly
interacting imbalanced Fermi gases of Li-6. We observed a smooth evolution in
the nature of pairing correlations from pairing in the superfluid region to
polaron binding in the highly polarized normal region. The imbalance induces
quasiparticles in the superfluid region even at very low temperature. This
leads to a local bimodal spectral response, which allows us to determine the
superfluid gap \Delta and the Hartree energy U.Comment: 5 pages, 4 figures (appendix: 3 pages, 5 figures
Determination of the Fermion Pair Size in a Resonantly Interacting Superfluid
Fermionic superfluidity requires the formation of pairs. The actual size of
these fermion pairs varies by orders of magnitude from the femtometer scale in
neutron stars and nuclei to the micrometer range in conventional
superconductors. Many properties of the superfluid depend on the pair size
relative to the interparticle spacing. This is expressed in BCS-BEC crossover
theories, describing the crossover from a Bardeen-Cooper-Schrieffer (BCS) type
superfluid of loosely bound and large Cooper pairs to Bose-Einstein
condensation (BEC) of tightly bound molecules. Such a crossover superfluid has
been realized in ultracold atomic gases where high temperature superfluidity
has been observed. The microscopic properties of the fermion pairs can be
probed with radio-frequency (rf) spectroscopy. Previous work was difficult to
interpret due to strong and not well understood final state interactions. Here
we realize a new superfluid spin mixture where such interactions have
negligible influence and present fermion-pair dissociation spectra that reveal
the underlying pairing correlations. This allows us to determine the
spectroscopic pair size in the resonantly interacting gas to be 2.6(2)/kF (kF
is the Fermi wave number). The pairs are therefore smaller than the
interparticle spacing and the smallest pairs observed in fermionic superfluids.
This finding highlights the importance of small fermion pairs for superfluidity
at high critical temperatures. We have also identified transitions from fermion
pairs into bound molecular states and into many-body bound states in the case
of strong final state interactions.Comment: 8 pages, 7 figures; Figures updated; New Figures added; Updated
discussion of fit function
Direct Observation of the Superfluid Phase Transition in Ultracold Fermi Gases
Water freezes into ice, atomic spins spontaneously align in a magnet, liquid
helium becomes superfluid: Phase transitions are dramatic phenomena. However,
despite the drastic change in the system's behaviour, observing the transition
can sometimes be subtle. The hallmark of Bose-Einstein condensation (BEC) and
superfluidity in trapped, weakly interacting Bose gases is the sudden
appearance of a dense central core inside a thermal cloud. In strongly
interacting gases, such as the recently observed fermionic superfluids, this
clear separation between the superfluid and the normal parts of the cloud is no
longer given. Condensates of fermion pairs could be detected only using
magnetic field sweeps into the weakly interacting regime. The quantitative
description of these sweeps presents a major theoretical challenge. Here we
demonstrate that the superfluid phase transition can be directly observed by
sudden changes in the shape of the clouds, in complete analogy to the case of
weakly interacting Bose gases. By preparing unequal mixtures of the two spin
components involved in the pairing, we greatly enhance the contrast between the
superfluid core and the normal component. Furthermore, the non-interacting
wings of excess atoms serve as a direct and reliable thermometer. Even in the
normal state, strong interactions significantly deform the density profile of
the majority spin component. We show that it is these interactions which drive
the normal-to-superfluid transition at the critical population imbalance of
70(5)%.Comment: 16 pages (incl. Supplemental Material), 5 figure
Observation of Fermi Polarons in a Tunable Fermi Liquid of Ultracold Atoms
We have observed Fermi polarons, dressed spin-down impurities in a spin-up Fermi sea of ultracold atoms. The polaron manifests itself as a narrow peak in the impurities’ rf spectrum that emerges from a broad incoherent background. We determine the polaron energy and the quasiparticle residue for various interaction strengths around a Feshbach resonance. At a critical interaction, we observe the transition from polaronic to molecular binding. Here, the imbalanced Fermi liquid undergoes a phase transition into a Bose liquid, coexisting with a Fermi sea.National Science FoundationAlfred P. Sloan FoundationAFOSR-MUR
Management and prognostic markers in patients with autoimmune encephalitis requiring ICU treatment
Objective To assess intensive care unit (ICU) complications, their management, and prognostic factors associated with outcomes in a cohort of patients with autoimmune encephalitis (AE). Methods This study was an observational multicenter registry of consecutively included patients diagnosed with AE requiring Neuro-ICU treatment between 2004 and 2016 from 18 tertiary hospitals. Logistic regression models explored the influence of complications, their management, and diagnostic findings on the dichotomized (0-3 vs 4-6) modified Rankin Scale score at hospital discharge. Results Of 120 patients with AE (median age 43 years [interquartile range 24-62]; 70 females), 101 developed disorders of consciousness, 54 autonomic disturbances, 42 status epilepticus, and 39 severe sepsis. Sixty-eight patients were mechanically ventilated, 85 patients had detectable neuronal autoantibodies, and 35 patients were seronegative. Worse neurologic outcome at hospital discharge was associated with necessity of mechanical ventilation (sex- and age-adjusted OR 6.28; 95% CI, 2.71-15.61) tracheostomy (adjusted OR 6.26; 95% CI, 2.68-15.73), tumor (adjusted OR 3.73; 95% CI, 1.35-11.57), sepsis (adjusted OR 4.54; 95% CI, 1.99-10.43), or autonomic dysfunction (adjusted OR 2.91; 95% CI, 1.24-7.3). No significant association was observed with autoantibody type, inflammatory changes in CSF, or pathologic MRI. Conclusion In patients with AE, mechanical ventilation, sepsis, and autonomic dysregulation appear to indicate longer or incomplete convalescence. Classic ICU complications better serve as prognostic markers than the individual subtype of AE. Increased awareness and effective management of these AE-related complications are warranted, and further prospective studies are needed to confirm our findings and to develop specific strategies for outcome improvement
Phase diagram of a two-component Fermi gas with resonant interactions
The pairing of fermions is at the heart of superconductivity and
superfluidity. The recent experimental realization of strongly interacting
atomic Fermi gases has opened a new, controllable way to study novel forms of
pairing and superfluidity. A major controversial issue has been the stability
of superfluidity against an imbalance between the two spin components when the
fermions interact resonantly. Here we present the phase diagram of a
spin-polarized Fermi gas of Li atoms at unitarity, mapping out the
superfluid phase versus temperature and density imbalance. Using tomographic
techniques, we reveal spatial discontinuities in the spin polarization, the
signature of a first-order superfluid-to-normal phase transition, which
disappears at a tricritical point where the nature of the phase transition
changes from first-order to second-order. At zero temperature, there is a
quantum phase transition from a fully-paired superfluid to a
partially-polarized normal gas. These observations and the implementation of an
in situ ideal gas thermometer provide quantitative tests of theoretical
calculations on the stability of resonant superfluidity.Comment: 8 pages, 8 figures, reference added, typo correcte