36,428 research outputs found
Design of magnetic traps for neutral atoms with vortices in type-II superconducting micro-structures
We design magnetic traps for atoms based on the average magnetic field of
vortices induced in a type-II superconducting thin film. This magnetic field is
the critical ingredient of the demonstrated vortex-based atom traps, which
operate without transport current. We use Bean's critical-state method to model
the vortex field through mesoscopic supercurrents induced in the thin strip.
The resulting inhomogeneous magnetic fields are studied in detail and compared
to those generated by multiple normally-conducting wires with transport
currents. Various vortex patterns can be obtained by programming different
loading-field and transport current sequences. These variable magnetic fields
are employed to make versatile trapping potentials.Comment: 11 pages, 14 figure
Conductivity of disordered quantum lattice models at infinite temperature: Many-body localization
We reinvestigate the behavior of the conductivity of several disordered
quantum lattice models at infinite temperature using exact diagonalization.
Contrary to the conclusion drawn in a recent investigation of similar
quantities in identical systems, we find evidence of a localized regime for
strong random fields. We estimate the location of the critical field for the
many-body localization transition for the random-field XXZ spin chain, and
compare our findings with recent investigations in related systems.Comment: 5 pages, 4 figures. Accepted for publication in Phys. Rev.
Rotational States of Magnetic Molecules
We study a magnetic molecule that exhibits spin tunneling and is free to
rotate about its anisotropy axis. Exact low-energy eigenstates of the molecule
that are superpositions of spin and rotational states are obtained. We show
that parameter determines the ground state of
the molecule. Here is the spin, is the moment of inertia, and
is the tunnel splitting. The magnetic moment of the molecule is zero
at . At the spin of the molecule localizes in one of
the directions along the anisotropy axis.Comment: 4 pages, 3 figure
Ignition and combustion characteristics of metallized propellants
Research designed to develop detailed knowledge of the secondary atomization and ignition characteristics of aluminum slurry propellants was started. These processes are studied because they are the controlling factors limiting the combustion efficiency of aluminum slurry propellants in rocket applications. A burner and spray rig system allowing the study of individual slurry droplets having diameters from about 10 to 100 microns was designed and fabricated. The burner generates a near uniform high temperature environment from the merging of 72 small laminar diffusion flames above a honeycomb matrix. This design permits essentially adiabatic operation over a wide range of stoichiometries without danger of flashback. A single particle sizing system and velocimeter also were designed and assembled. Light scattered from a focused laser beam is related to the particle (droplet) size, while the particle velocity is determined by its transit time through the focal volume. Light from the combustion of aluminum is also sensed to determine if ignition was achieved. These size and velocity measurements will allow the determination of disruption and ignition times as functions of drop sizes and ambient conditions
Exploring molecular complexity with ALMA (EMoCA): Detection of three new hot cores in Sagittarius B2(N)
The SgrB2 molecular cloud contains several sites forming high-mass stars.
SgrB2(N) is one of its main centers of activity. It hosts several compact and
UCHII regions, as well as two known hot molecular cores (SgrB2(N1) and
SgrB2(N2)), where complex organic molecules are detected. Our goal is to use
the high sensitivity of ALMA to characterize the hot core population in
SgrB2(N) and shed a new light on the star formation process. We use a complete
3 mm spectral line survey conducted with ALMA to search for faint hot cores in
SgrB2(N). We report the discovery of three new hot cores that we call
SgrB2(N3), SgrB2(N4), and SgrB2(N5). The three sources are associated with
class II methanol masers, well known tracers of high-mass star formation, and
SgrB2(N5) also with a UCHII region. The chemical composition of the sources and
the column densities are derived by modelling the whole spectra under the
assumption of LTE. The H2 column densities are computed from ALMA and SMA
continuum emission maps. The H2 column densities of these new hot cores are
found to be 16 up to 36 times lower than the one of the main hot core Sgr
B2(N1). Their spectra have spectral line densities of 11 up to 31 emission
lines per GHz, assigned to 22-25 molecules. We derive rotational temperatures
around 140-180 K for the three new hot cores and mean source sizes of 0.4 for
SgrB2(N3) and 1.0 for SgrB2(N4) and SgrB2(N5). SgrB2(N3) and SgrB2(N5) show
high velocity wing emission in typical outflow tracers, with a bipolar
morphology in their integrated intensity maps suggesting the presence of an
outflow, like in SgrB2(N1). The associations of the hot cores with class II
methanol masers, outflows, and/or UCHII regions tentatively suggest the
following age sequence: SgrB2(N4), SgrB2(N3), SgrB2(N5), SgrB2(N1). The status
of SgrB2(N2) is unclear. It may contain two distinct sources, a UCHII region
and a very young hot core.Comment: Accepted for publication in A&A, 24 pages, 23 figure
Differential regulation of Ota and Otb, two primary glycine betaine transporters in the methanogenic archaeon Methanosarcina mazei go1
Methanogenic archaea accumulate glycine betaine in response to hypersalinity, but the regulation of proteins involved, their mechanism of activation and regulation of the corresponding genes are largely unknown. Methanosarcina mazei differs from most other methanoarchaea in having two gene clusters both encoding a potential glycine betaine transporter, Ota and Otb. Western blot as well as quantitative real-time PCR revealed that Otb is not regulated by osmolarity. On the other hand, cellular levels of Ota increased with increasing salt concentrations. A maximum was reached at 300-500 m M NaCl. Ota concentrations reached a maximum 4 h after an osmotic upshock. Hyperosmolarity also caused an increase in cellular Ota concentrations. In addition to osmolarity Ota expression was regulated by the growth phase. Expression of Ota as well as transport of betaine was downregulated in the presence of glycine betaine. Copyright (c) 2007 S. Karger AG, Basel
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