377 research outputs found
Adiabatically changing the phase-space density of a trapped Bose gas
We show that the degeneracy parameter of a trapped Bose gas can be changed
adiabatically in a reversible way, both in the Boltzmann regime and in the
degenerate Bose regime. We have performed measurements on spin-polarized atomic
hydrogen in the Boltzmann regime demonstrating reversible changes of the
degeneracy parameter (phase-space density) by more than a factor of two. This
result is in perfect agreement with theory. By extending our theoretical
analysis to the quantum degenerate regime we predict that, starting close
enough to the Bose-Einstein phase transition, one can cross the transition by
an adiabatic change of the trap shape.Comment: 4 pages, 3 figures, Latex, submitted to PR
Soliton Lattices in the Incommensurate Spin-Peierls Phase: Local Distortions and Magnetizations
It is shown that nonadiabatic fluctuations of the soliton lattice in the
spin-Peierls system CuGeO_3 lead to an important reduction of the NMR line
widths. These fluctuations are the zero-point motion of the massless phasonic
excitations. Furthermore, we show that the discrepancy of X-ray and NMR soliton
widths can be understood as the difference between a distortive and a magnetic
width. Their ratio is controlled by the frustration of the spin system. By this
work, theoretical and experimental results can be reconciled in two important
points.Comment: 9 pages, 5 figures included, Revtex submitted to Physical Review
Modulated Phases in Spin-Peierls Systems
Lattice modulations in the high magnetic field phase and close to impurities
in spin-Peierls systems are considered and compared to experiment. Necessary
extensions of existing theories are proposed. The influence of zero-point
fluctuations on magnetic amplitudes is shown.Comment: 10 pages, 4 figures included, to appear in Advances in Solid State
Physics/Festkoerperprobleme Spring Conference 1999 of the DP
Mean Field Calculations of Bose-Einstein Condensation of 7Li Atoms In a Harmonic Trap
A self-consistent mean-field theory for bosons for T>0 is used to reconcile
predictions of collapse with recent observations of Bose-Einstein condensation
of 7Li. Eigenfunctions of a (non-separable) Hamiltonian that includes the
anisotropic external trap field and atom-atom interactions are obtained by an
iteration process. A sum over the Bose distribution, and the ``alternating
direction implicit'' algorithm are used. Near Tc, the ensemble exhibits a
localized condensate composed of atoms in the few lowest states. For lower T,
numerical instability indicates collapse to a more dense phase.Comment: 11 pages + 4 figure
Theory of Bose-Einstein condensation in trapped gases
The phenomenon of Bose-Einstein condensation of dilute gases in traps is
reviewed from a theoretical perspective. Mean-field theory provides a framework
to understand the main features of the condensation and the role of
interactions between particles. Various properties of these systems are
discussed, including the density profiles and the energy of the ground state
configurations, the collective oscillations and the dynamics of the expansion,
the condensate fraction and the thermodynamic functions. The thermodynamic
limit exhibits a scaling behavior in the relevant length and energy scales.
Despite the dilute nature of the gases, interactions profoundly modify the
static as well as the dynamic properties of the system; the predictions of
mean-field theory are in excellent agreement with available experimental
results. Effects of superfluidity including the existence of quantized vortices
and the reduction of the moment of inertia are discussed, as well as the
consequences of coherence such as the Josephson effect and interference
phenomena. The review also assesses the accuracy and limitations of the
mean-field approach.Comment: revtex, 69 pages, 38 eps figures, new version with more references,
new figures, various changes and corrections, for publ. in Rev. Mod. Phys.,
available also at http://www-phys.science.unitn.it/bec/BEC.htm
Cold adaptation drives population genomic divergence in the ecological specialist, Drosophila montana
Funding: UK Natural Environment Research Council (Grant Number(s): NE/L501852/1, NE/P000592/1); Academy of Finland (GrantNumber(s): 267244, 268214, 322980), Ella ja Georg Ehrnroothin SÀÀtiö.Detecting signatures of ecological adaptation in comparative genomics is challenging, but analysing population samples with characterised geographic distributions, such as clinal variation, can help identify genes showing covariation with important ecological variation. Here, we analysed patterns of geographic variation in the cold-adapted species Drosophila montana across phenotypes, genotypes and environmental conditions and tested for signatures of cold adaptation in population genomic divergence. We first derived the climatic variables associated with the geographic distribution of 24 populations across two continents to trace the scale of environmental variation experienced by the species, and measured variation in the cold tolerance of the flies of six populations from different geographic contexts. We then performed pooled whole genome sequencing of these six populations, and used Bayesian methods to identify SNPs where genetic differentiation is associated with both climatic variables and the population phenotypic measurements, while controlling for effects of demography and population structure. The top candidate SNPs were enriched on the X and fourth chromosomes, and they also lay near genes implicated in other studies of cold tolerance and population divergence in this species and its close relatives. We conclude that ecological adaptation has contributed to the divergence of D. montana populations throughout the genome and in particular on the X and fourth chromosomes, which also showed highest interpopulation FST. This study demonstrates that ecological selection can drive genomic divergence at different scales, from candidate genes to chromosome-wide effects.Publisher PDFPeer reviewe
Macro-Climatic Distribution Limits Show Both Niche Expansion and Niche Specialization among C4 Panicoids
Grasses are ancestrally tropical understory species whose current dominance in warm open habitats is linked to the evolution of C4 photosynthesis. C4 grasses maintain high rates of photosynthesis in warm and water stressed environments, and the syndrome is considered to induce niche shifts into these habitats while adaptation to cold ones may be compromised. Global biogeographic analyses of C4 grasses have, however, concentrated on diversity patterns, while paying little attention to distributional limits. Using phylogenetic contrast analyses, we compared macro-climatic distribution limits among ~1300 grasses from the subfamily Panicoideae, which includes 4/5 of the known photosynthetic transitions in grasses. We explored whether evolution of C4 photosynthesis correlates with niche expansions, niche changes, or stasis at subfamily level and within the two tribes Paniceae and Paspaleae. We compared the climatic extremes of growing season temperatures, aridity, and mean temperatures of the coldest months. We found support for all the known biogeographic distribution patterns of C4 species, these patterns were, however, formed both by niche expansion and niche changes. The only ubiquitous response to a change in the photosynthetic pathway within Panicoideae was a niche expansion of the C4 species into regions with higher growing season temperatures, but without a withdrawal from the inherited climate niche. Other patterns varied among the tribes, as macro-climatic niche evolution in the American tribe Paspaleae differed from the pattern supported in the globally distributed tribe Paniceae and at family level.Fil: Aagesen, Lone. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Instituto de BotĂĄnica Darwinion. Academia Nacional de Ciencias Exactas, FĂsicas y Naturales. Instituto de BotĂĄnica Darwinion; ArgentinaFil: Biganzoli, Fernando. Universidad de Buenos Aires. Facultad de AgronomĂa. Departamento de MĂ©todos Cuantitativos y Sistemas de InformaciĂłn; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Bena, MarĂa Julia. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Instituto de BotĂĄnica Darwinion. Academia Nacional de Ciencias Exactas, FĂsicas y Naturales. Instituto de BotĂĄnica Darwinion; ArgentinaFil: Godoy BĂŒrki, Ana Carolina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Instituto de BotĂĄnica Darwinion. Academia Nacional de Ciencias Exactas, FĂsicas y Naturales. Instituto de BotĂĄnica Darwinion; ArgentinaFil: Reinheimer, Renata. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Santa Fe. Instituto de AgrobiotecnologĂa del Litoral. Universidad Nacional del Litoral. Instituto de AgrobiotecnologĂa del Litoral; ArgentinaFil: Zuloaga, Fernando Omar. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Instituto de BotĂĄnica Darwinion. Academia Nacional de Ciencias Exactas, FĂsicas y Naturales. Instituto de BotĂĄnica Darwinion; Argentin
Spatial heterogeneity of habitat suitability for Rift Valley fever occurrence in Tanzania: an ecological niche modelling approach
Despite the long history of Rift Valley fever (RVF) in Tanzania, extent of its suitable habitat in the country remains unclear. In this study we investigated potential effects of temperature, precipitation, elevation, soil type, livestock density, rainfall pattern, proximity to wild animals, protected areas and forest on the habitat suitability for RVF occurrence in Tanzania. Presence-only records of 193 RVF outbreak locations from 1930 to 2007 together with potential predictor variables were used to model and map the suitable habitats for RVF occurrence using ecological niche modelling. Ground-truthing of the model outputs was conducted by comparing the levels of RVF virus specific antibodies in cattle, sheep and goats sampled from locations in Tanzania that presented different predicted habitat suitability values. Habitat suitability values for RVF occurrence were higher in the northern and central-eastern regions of Tanzania than the rest of the regions in the country. Soil type and precipitation of the wettest quarter contributed equally to habitat suitability (32.4% each), followed by livestock density (25.9%) and rainfall pattern (9.3%). Ground-truthing of model outputs revealed that the odds of an animal being seropositive for RVFV when sampled from areas predicted to be most suitable for RVF occurrence were twice the odds of an animal sampled from areas least suitable for RVF occurrence (95% CI: 1.43, 2.76, p < 0.001). The regions in the northern and central-eastern Tanzania were more suitable for RVF occurrence than the rest of the regions in the country. The modelled suitable habitat is characterised by impermeable soils, moderate precipitation in the wettest quarter, high livestock density and a bimodal rainfall pattern. The findings of this study should provide guidance for the design of appropriate RVF surveillance, prevention and control strategies which target areas with these characteristics
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