2,438 research outputs found
Bragg spectroscopy of a strongly interacting Fermi gas
We present a comprehensive study of the Bose-Einstein condensate to
Bardeen-Cooper-Schrieffer (BEC-BCS) crossover in fermionic Li using Bragg
spectroscopy. A smooth transition from molecular to atomic spectra is observed
with a clear signature of pairing at and above unitarity. These spectra probe
the dynamic and static structure factors of the gas and provide a direct link
to two-body correlations. We have characterised these correlations and measured
their density dependence across the broad Feshbach resonance at 834 G.Comment: Replaced with published versio
Contact and sum-rules in a near-uniform Fermi gas at unitarity
We present an experimental study of the high-energy excitation spectra of
unitary Fermi gases. Using focussed beam Bragg spectroscopy, we locally probe
atoms in the central region of a harmonically trapped cloud where the density
is nearly uniform, enabling measurements of the dynamic structure factor for a
range of temperatures both below and above the superfluid transition. Applying
sum-rules to the measured Bragg spectra, we resolve the characteristic
behaviour of the universal contact parameter, , across the superfluid
transition. We also employ a recent theoretical result for the kinetic
(second-moment) sum-rule to obtain the internal energy of gases at unitarity.Comment: 5 pages, 4 figure
Direct Evidence for a Magnetic f-electron Mediated Cooper Pairing Mechanism of Heavy Fermion Superconductivity in CeCoIn5
To identify the microscopic mechanism of heavy-fermion Cooper pairing is an
unresolved challenge in quantum matter studies; it may also relate closely to
finding the pairing mechanism of high temperature superconductivity.
Magnetically mediated Cooper pairing has long been the conjectured basis of
heavy-fermion superconductivity but no direct verification of this hypothesis
was achievable. Here, we use a novel approach based on precision measurements
of the heavy-fermion band structure using quasiparticle interference (QPI)
imaging, to reveal quantitatively the momentum-space (k-space) structure of the
f-electron magnetic interactions of CeCoIn5. Then, by solving the
superconducting gap equations on the two heavy-fermion bands
with these magnetic interactions as mediators of the
Cooper pairing, we derive a series of quantitative predictions about the
superconductive state. The agreement found between these diverse predictions
and the measured characteristics of superconducting CeCoIn5, then provides
direct evidence that the heavy-fermion Cooper pairing is indeed mediated by the
f-electron magnetism.Comment: 19 pages, 4 figures, Supplementary Information: 31 pages, 5 figure
Thermodynamics of an attractive 2D Fermi gas
Thermodynamic properties of matter are conveniently expressed as functional
relations between variables known as equations of state. Here we experimentally
determine the compressibility, density and pressure equations of state for an
attractive 2D Fermi gas in the normal phase as a function of temperature and
interaction strength. In 2D, interacting gases exhibit qualitatively different
features to those found in 3D. This is evident in the normalized density
equation of state, which peaks at intermediate densities corresponding to the
crossover from classical to quantum behaviour.Comment: Contains minor revision
Crossover from 2D to 3D in a weakly interacting Fermi gas
We have studied the transition from two to three dimensions in a low
temperature weakly interacting Li Fermi gas. Below a critical atom number,
, only the lowest transverse vibrational state of a highly anisotropic
oblate trapping potential is occupied and the gas is two-dimensional. Above
the Fermi gas enters the quasi-2D regime where shell structure
associated with the filling of individual transverse oscillator states is
apparent. This dimensional crossover is demonstrated through measurements of
the cloud size and aspect ratio versus atom number.Comment: Replaced with published manuscrip
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Developing a scalable training model in global mental health: pilot study of a video-assisted training Program for Generalist Clinicians in Rural Nepal.
BackgroundIn low- and middle-income countries, mental health training often includes sending few generalist clinicians to specialist-led programs for several weeks. Our objective is to develop and test a video-assisted training model addressing the shortcomings of traditional programs that affect scalability: failing to train all clinicians, disrupting clinical services, and depending on specialists.MethodsWe implemented the program -video lectures and on-site skills training- for all clinicians at a rural Nepali hospital. We used Wilcoxon signed-rank tests to evaluate pre- and post-test change in knowledge (diagnostic criteria, differential diagnosis, and appropriate treatment). We used a series of 'Yes' or 'No' questions to assess attitudes about mental illness, and utilized exact McNemar's test to analyze the proportions of participants who held a specific belief before and after the training. We assessed acceptability and feasibility through key informant interviews and structured feedback.ResultsFor each topic except depression, there was a statistically significant increase (Δ) in median scores on knowledge questionnaires: Acute Stress Reaction (Δ = 20, p = 0.03), Depression (Δ = 11, p = 0.12), Grief (Δ = 40, p < 0.01), Psychosis (Δ = 22, p = 0.01), and post-traumatic stress disorder (Δ = 20, p = 0.01). The training received high ratings; key informants shared examples and views about the training's positive impact and complementary nature of the program's components.ConclusionVideo lectures and on-site skills training can address the limitations of a conventional training model while being acceptable, feasible, and impactful toward improving knowledge and attitudes of the participants
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Neuroimaging Studies of Essential Tremor: How Well Do These Studies Support/Refute the Neurodegenerative Hypothesis?
Background: Tissue‐based research has recently led to a new patho‐mechanistic model of essential tremor (ET)—the cerebellar degenerative model. We are not aware of a study that has reviewed the current neuroimaging evidence, focusing on whether the studies support or refute the neurodegenerative hypothesis of ET. This was our aim.
Methods: References for this review were identified by searches of PubMed (1966 to February 2014).
Results: Several neuroimaging methods have been used to study ET, most of them based on magnetic resonance imaging (MRI). The methods most specific to address the question of neurodegeneration are MRI‐based volumetry, magnetic resonance spectroscopy, and diffusion‐weighted imaging. Studies using each of these methods provide support for the presence of cerebellar degeneration in ET, finding reduced cerebellar brain volumes, consistent decreases in cerebellar N‐acetylaspartate, and increased mean diffusivity. Other neuroimaging techniques, such as functional MRI and positron emission tomography (PET) are less specific, but still sensitive to potential neurodegeneration. These techniques are used for measuring a variety of brain functions and their impairment. Studies using these modalities also largely support cerebellar neuronal impairment. In particular, changes in 11C‐flumazenil binding in PET studies and changes in iron deposition in an MRI study provide evidence along these lines. The composite data point to neuronal impairment and likely neuronal degeneration in ET.
Discussion: Recent years have seen a marked increase in the number of imaging studies of ET. As a whole, the combined data provide support for the presence of cerebellar neuronal degeneration in this disease
A note on leapfrogging vortex rings
In this paper we provide examples, by numerical simulation using the Navier-Stokes equations for axisymmetric laminar flow, of the 'leapfrogging' motion of two, initially identical, vortex rings which share a common axis of symmetry. We show that the number of clear passes that each ring makes through the other increases with Reynolds number, and that as long as the configuration remains stable the two rings ultimately merge to form a single vortex ring
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