1,522 research outputs found
An atom fiber for guiding cold neutral atoms
We present an omnidirectional matter wave guide on an atom chip. The
rotational symmetry of the guide is maintained by a combination of two current
carrying wires and a bias field pointing perpendicular to the chip surface. We
demonstrate guiding of thermal atoms around more than two complete turns along
a spiral shaped 25mm long curved path (curve radii down to 200m) at
various atom--surface distances (35-450m). An extension of the scheme for
the guiding of Bose-Einstein condensates is outlined
Trapping and manipulating neutral atoms with electrostatic fields
We report on experiments with cold thermal Li atoms confined in combined
magnetic and electric potentials. A novel type of three-dimensional trap was
formed by modulating a magnetic guide using electrostatic fields. We observed
atoms trapped in a string of up to six individual such traps, a controlled
transport of an atomic cloud over a distance of 400m, and a dynamic
splitting of a single trap into a double well potential. Applications for
quantum information processing are discussed.Comment: 4 pages, 4 figure
Origin of molecular oxygen in Comet 67P/Churyumov-Gerasimenko
Molecular oxygen has been detected in the coma of comet
67P/Churyumov-Gerasimenko with abundances in the 1-10% range by the ROSINA-DFMS
instrument on board the Rosetta spacecraft. Here we find that the radiolysis of
icy grains in low-density environments such as the presolar cloud may induce
the production of large amounts of molecular oxygen. We also show that
molecular oxygen can be efficiently trapped in clathrates formed in the
protosolar nebula, and that its incorporation as crystalline ice is highly
implausible because this would imply much larger abundances of Ar and N2 than
those observed in the coma. Assuming that radiolysis has been the only O2
production mechanism at work, we conclude that the formation of comet
67P/Churyumov-Gerasimenko is possible in a dense and early protosolar nebula in
the framework of two extreme scenarios: (1) agglomeration from pristine
amorphous icy grains/particles formed in ISM and (2) agglomeration from
clathrates that formed during the disk's cooling. The former scenario is found
consistent with the strong correlation between O2 and H2O observed in 67P/C-G's
coma while the latter scenario requires that clathrates formed from ISM icy
grains that crystallized when entering the protosolar nebula.Comment: The Astrophysical Journal Letters, in pres
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Characterization of the Subsurface of 67P/Churyumov-Gerasimenko's Abydos Site
We investigate the structure of the subsurface of the Abydos site using a cometary nucleus model with parameters adapted to comet 67P/Churyumov-Gerasimenko and the Abydos landing site. We aim to compare the production rates derived from our model with those of the main molecules measured by Ptolemy. This will allow us to retrieve the depths at which the different molecules still exist in solid form
Health impact assessment for promoting sustainable development: the HIA4SD project
Health is central to sustainable development, and thus a cross-cutting issue of the SustainableDevelopment Goal (SDG) 2030 agenda. Natural resource extraction projects in Africa haveconsiderable potential to impact on health-related targets of the SDGs. This paper introducesthe rationale and organization of the HIA4SD Project; a 6-year research for development (r4d)project that aims to inform and facilitate a policy dialogue at the national and internationallevel on whether current regulatory approaches to impact assessment in Africa promotesustainable development, placing emphasis on SDG3Good Health and Well-being. TheHIA4SD Project has a focus on large-scale natural resource extraction projects and is imple-mented in four African countries, namely Burkina Faso, Ghana, Mozambique and Tanzania
Interaction of a Magnetic Impurity with Strongly Correlated Conduction Electrons
We consider a magnetic impurity which interacts by hybridization with a
system of strongly correlated conduction electrons. The latter are described by
a Hubbard Hamiltonian. By means of a canconical transformation the charge
degrees of freedom of the magnetic impurity are eliminated. The resulting
effective Hamiltonian is investigated and various limiting cases
are considered. If the Hubbard interaction between the conduction electrons
is neglected reduces to a form obtained by the Schrieffer-Wolff
transformation, which is essentially the Kondo Hamiltonian. If is large and
the correlations are strong is changed. One modification concerns
the coefficient of the dominant exchange coupling of the magnetic impurity with
the nearest lattice site. When the system is hole doped, there is also an
antiferromagnetic coupling to the nearest neighbors of that site involving
additionally a hole. Furthermore, it is found that the magnetic impurity
attracts a hole. In the case of electron doping, double occupancies are
repelled by the impurity. In contrast to the hole-doped case, we find no
magnetic coupling which additionally involves a doubly occupied site.Comment: 16 pages, Revtex 3.
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Low atmospheric CO2 levels before the rise of forested ecosystems
The emergence of forests on Earth (~385 million years ago, Ma)1 has been linked to an order-of-magnitude decline in atmospheric CO2 levels and global climatic cooling by altering continental weathering processes, but observational constraints on atmospheric CO2 before the rise of forests carry large, often unbound, uncertainties. Here, we calibrate a mechanistic model for gas exchange in modern lycophytes and constrain atmospheric CO2 levels 410–380 Ma from related fossilized plants with bound uncertainties of approximately ±100 ppm (1 sd). We find that the atmosphere contained ~525–715 ppm CO2 before continents were afforested, and that Earth was partially glaciated according to a palaeoclimate model. A process-driven biogeochemical model (COPSE) shows the appearance of trees with deep roots did not dramatically enhance atmospheric CO2 removal. Rather, shallow-rooted vascular ecosystems could have simultaneously caused abrupt atmospheric oxygenation and climatic cooling long before the rise of forests, although earlier CO2 levels are still unknown
Lattice vibrations of BeO
Parts of the dispersion relations for BeO have been measured. These relations are for waves moving in the [100] direction with their polarization vectors in the (0001) plane. The initial slopes of the measured acoustical branches agree with those obtained from ultrasonics. Branch intercepts at the zone centers of 84 meV (1.27 x 1013 rad/sec) and possibly 5 meV (8 x 1012 rad/sec) were found. The dispersion relations are correlated with infrared. Reststrahlen and "frequency spectra" measurements and proposed models.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33368/1/0000766.pd
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