Landau and dynamical instabilities of Bose-Einstein condensates with superfluid flow in a Kronig-Penney potential

We study the elementary excitations of Bose-Einstein condensates in a one-dimensional periodic potential and discuss the stability of superfluid flow based on the Kronig-Penney model. We analytically solve the Bogoliubov equations and calculate the excitation spectrum. The Landau and dynamical instabilities occur in the first condensate band when the superfluid velocity exceeds certain critical values, which agrees with the result of condensates in a sinusoidal potential. It is found that the onset of the Landau instability coincides with the point where the perfect transmission of low-energy excitations is forbidden, while the dynamical instability occurs when the effective mass is negative. It is well known that the condensate band has a peculiar structure called swallowtail when the periodic potential is shallow compared to the mean field energy. We find that the upper side of the swallowtail is dynamically unstable although the excitations have the linear dispersion reflecting the positive effective mass.Comment: 6 pages, 2 figures, Proceedings of the International Symposium on Quantum Fluids and Solids (QFS2006

The unseen face of humanitarian crisis in Eastern Democratic Republic of Congo: was nutritional relief properly targeted?

STUDY OBJECTIVE—Comparison of children's nutritional status in refugee populations with that of local host populations, one year after outbreak refugee crisis in the North Kivu region of Democratic Republic of Congo.
DESIGN—Cross sectional surveys.
SETTING—Temporary and other settlements, in the town of Goma and surrounding rural areas.
SUBJECTS—Anthropometric indicators of nutritional status and presence or absence of oedema were measured among 5121 children aged 6( )to 59 months recruited by cluster sampling with probability proportional to size, between June and August 1995.
RESULTS—Children in all locations demonstrated a typical pattern of growth deficit relative to international reference. Prevalence of acute malnutrition (wt/ht < −2 Z score) was higher among children in the rural non-refugee populations (3.8 and 5.8%) than among those in the urban non-refugee populations (1.4%) or in the refugee population living in temporary settlements (1.7%). Presence of oedema was scarcely noticed in camps (0.4%) while it was a common observation at least in the most remote rural areas (10.1%). As compared with baseline data collected in 1989, there is evidence that nutritional status was worsening in rural non-refugee populations.
CONCLUSIONS—Children living in the main town or in the refugee camps benefited the most from nutritional relief while those in the rural non-refugee areas were ignored. This is a worrying case of inequity in nutritional relief.


Keywords: malnutrition; disasters; equit

International Asteroid Warning Network Timing Campaign: 2019 XS

As part of the International Asteroid Warning Network's observational exercises, we conducted a campaign to observe near-Earth asteroid 2019 XS around its close approach to Earth on 2021 November 9. The goal of the campaign was to characterize errors in the observation times reported to the Minor Planet Center, which become an increasingly important consideration as astrometric accuracy improves and more fast-moving asteroids are observed. As part of the exercise, a total of 957 astrometric observations of 2019 XS during the encounter were reported and subsequently were analyzed to obtain the corresponding residuals. While the timing errors are typically smaller than 1 s, the reported times appear to be negatively biased, i.e., they are generally earlier than they should be. We also compared the observer-provided position uncertainty with the cross-track residuals, which are independent of timing errors. A large fraction of the estimated uncertainties appear to be optimistic, especially when <0 2. We compiled individual reports for each observer to help identify and remove the root cause of any possible timing error and improve the uncertainty quantification process. We suggest possible sources of timing errors and describe a simple procedure to derive reliable, conservative position uncertainties. © 2022. The Author(s). Published by the American Astronomical Society.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]