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 $m^*/m=1.17(10)$, in agreement with the most recent theoretical predictions.Comment: 4 pages, 4 figures, submitted to PR

Multi-component gap solitons in spinor Bose-Einstein condensates

We model the nonlinear behaviour of spin-1 Bose-Einstein condensates (BECs) with repulsive spin-independent interactions and either ferromagnetic or anti-ferromagnetic (polar) spin-dependent interactions, loaded into a one-dimensional optical lattice potential. We show that both types of BECs exhibit dynamical instabilities and may form spatially localized multi-component structures. The localized states of the spinor matter waves take the form of vector gap solitons and self-trapped waves that exist only within gaps of the linear Bloch-wave band-gap spectrum. Of special interest are the nonlinear localized states that do not exhibit a common spatial density profile shared by all condensate components, and consequently cannot be described by the single mode approximation (SMA), frequently employed within the framework of the mean-field treatment. We show that the non-SMA states can exhibits Josephson-like internal oscillations and self-magnetisation, i.e. intrinsic precession of the local spin. Finally, we demonstrate that non-stationary states of a spinor BEC in a lattice exhibit coherent undamped spin-mixing dynamics, and that their controlled conversion into a stationary state can be achieved by the application of an external magnetic field.Comment: 12 pages, 13 figure

The Farmer Life School: experience from an innovative approach to HIV education

The Farmer Life School (FLS) is an innovative approach to integrating HIV education into life skills and technical training for farmers. This study aims to gain insight into the strengths and weaknesses of this relatively new approach, through the implementation of an adapted version in South Africa. The results are presented of a pilot with three groups of community gardeners, predominantly women, attending weekly sessions. Impact was assessed in terms of three key elements: participation, learning, and empowerment. Data were collected through extensive session reports, follow-up interviews, and reflection exercises with facilitators and participating groups and individuals. The results suggest that a group-based discovery learning approach such as the FLS has great potential to improve food security and wellbeing, while allowing participants to explore issues around HIV/AIDS. However, the analysis also shows that HIV/AIDS-related illness and death, and the factors that drive the epidemic and its impact, undermine farmers\' ability to participate, the safety and trust required for learning, and the empowerment process. Participatory approaches such as the FLS require a thorough understanding of and adaptation to the context. Keywords: Farmer Life School, HIV/AIDS, participation, learning, empowerment.SAHARA J Vol. 5 (2) 2008: pp. 56-6

Preferences for health outcomes associated with Group A Streptococcal disease and vaccination

<p>Abstract</p> <p>Background</p> <p>A 26-valent Group A Streptococcus (GAS) vaccine candidate has been developed that may provide protection against pharyngitis, invasive disease and rheumatic fever. However, recommendations for the use of a new vaccine must be informed by a range of considerations, including parents' preferences for different relevant health outcomes. Our objectives were to: (1) describe parent preferences for GAS disease and vaccination using willingness-to-pay (WTP) and time trade-off (TTO) methods; and (2) understand how parents' implied WTP for a quality-adjusted life year (QALY) gained might vary depending on the particular health outcome considered (e.g. averted GAS disease vs. vaccine adverse events).</p> <p>Methods</p> <p>Telephone interviews were conducted with parents of children diagnosed with GAS pharyngitis at 2 pediatric practice sites in the Boston metropolitan area. WTP and TTO (trading parental longevity for child's health) questions for 2 vaccine and 4 disease-associated health states were asked using a randomly selected opening bid, followed by a 2^ndbid and a final open-ended question about the amount willing to pay or trade. Descriptive analyses included medians and interquartile ranges for WTP and TTO estimates. The Wilcoxon signed-rank test was used to assess differences in WTP/QALY values for vaccine adverse events vs. disease states.</p> <p>Results</p> <p>Of 119 respondents, 100 (84%) and 96 (81%) provided a complete set of responses for WTP and TTO questions, respectively. The median WTP and discounted (at 3% per year) TTO values to avoid each health state were as follows: local reaction, $30, 0.12 days; systemic reaction,$50, 0.22 days; impetigo, $75, 1.25 days; strep throat,$75, 2.5 days; septic arthritis, $1,000, 6.6 days; and toxic shock syndrome,$3,000, 31.0 days. The median WTP/QALY was significantly higher for vaccine adverse events (~$60,000/QALY) compared to disease states ($18,000 to $36,000/QALY).</p> <p>Conclusions</p> <p>Parents strongly prefer to prevent GAS disease in children compared to vaccine adverse events. However, implied WTP/QALY ratios were higher for the prevention of vaccine adverse events. Regret for errors of commission vs. omission may differ and should be considered by vaccine policymakers.</p

How do electronic carriers cross Si-bound alkyl monolayers?

Electron transport through Si-C bound alkyl chains, sandwiched between n-Si and Hg, is characterized by two distinct types of barriers, each dominating in a different voltage range. At low voltage, current depends strongly on temperature but not on molecular length, suggesting transport by thermionic emission over a barrier in the Si. At higher voltage, the current decreases exponentially with molecular length, suggesting tunneling through the molecules. The tunnel barrier is estimated, from transport and photoemission data, to be ~1.5 eV with a 0.25me effective mass.Comment: 13 pages, 3 figure

Flexible epidemiological model for estimates and short-term projections in generalised HIV/AIDS epidemics

International audienceObjective UNAIDS and country analysts use a simple infectious disease model, embedded in the Estimation and Projection Package (EPP), to generate annual updates on the global HIV/AIDS epidemic. Our objective was to develop modifications to the current model that improve fit to recently observed prevalence trends across countries. Methods Our proposed alternative to the current EPP approach simplifies the model structure and explicitly models changes in average infection risk over time, operationalised using penalised B-splines in a Bayesian framework. We also present an alternative approach to initiating the epidemic that improves standardisation and efficiency, and add an informative prior distribution for changes in infection risk beyond the last data point that enhances the plausibility of short-term extrapolations. Results The spline-based model produces better fits than the current model to observed prevalence trends in settings that have recently experienced levelling or rising prevalence following a steep decline, such as Uganda and urban Rwanda. The model also predicts a deceleration of the decline in prevalence for countries with recent experience of steady declines, such as Kenya and Zimbabwe. Estimates and projections from our alternative model are comparable to those from the current model where the latter performs well. Conclusions A more flexible epidemiological model that accommodates changing infection risk over time can provide better estimates and short-term projections of HIV/AIDS incidence, prevalence and mortality than the current EPP model. The alternative model specification can be incorporated easily into existing analytical tools that are used to produce updates on the global HIV/AIDS epidemic

Expansion of a lithium gas in the BEC-BCS crossover

We report on experiments in $^6$Li Fermi gases near Feshbach resonances. A broad s-wave resonance is used to form a Bose-Einstein condensate of weakly bound $^6$Li$_2$ molecules in a crossed optical trap. The measured molecule-molecule scattering length of $170^{+100}_{-60}$ 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 $^6$Li- $^7$Li mixture.Comment: 10 pages, 3 figures, Proceedings of ICAP 200

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

Measurement of interaction energy near a Feshbach resonance in a 6Li Fermi gas

We investigate the strongly interacting regime in an optically trapped $^6$Li Fermi mixture near a Feshbach resonance. The resonance is found at $800(40)$G in good agreement with theory. Anisotropic expansion of the gas is interpreted by collisional hydrodynamics. We observe an unexpected and large shift ($80$G) between the resonance peak and both the maximum of atom loss and the change of sign of the interaction energy.Comment: 4 pages, 4 figure