80 research outputs found
Universal Quantum Viscosity in a Unitary Fermi Gas
A Fermi gas of atoms with resonant interactions is predicted to obey
universal hydrodynamics, where the shear viscosity and other transport
coefficients are universal functions of the density and temperature. At low
temperatures, the viscosity has a universal quantum scale where
is the density, while at high temperatures the natural scale is
where is the thermal momentum. We employ breathing mode damping to
measure the shear viscosity at low temperature. At high temperature , we
employ anisotropic expansion of the cloud to find the viscosity, which exhibits
precise scaling. In both experiments, universal hydrodynamic
equations including friction and heating are used to extract the viscosity. We
estimate the ratio of the shear viscosity to the entropy density and compare to
that of a perfect fluid.Comment: 13 pages, 3 figure
High-flux beam source for cold, slow atoms or molecules
We demonstrate and characterize a high-flux beam source for cold, slow atoms
or molecules. The desired species is vaporized using laser ablation, then
cooled by thermalization in a cryogenic cell of buffer gas. The beam is formed
by particles exiting a hole in the buffer gas cell. We characterize the
properties of the beam (flux, forward velocity, temperature) for both an atom
(Na) and a molecule (PbO) under varying buffer gas density, and discuss
conditions for optimizing these beam parameters. Our source compares favorably
to existing techniques of beam formation, for a variety of applications.Comment: 5 Pages, 4 figure
Microwave traps for cold polar molecules
We discuss the possibility of trapping polar molecules in the standing-wave
electromagnetic field of a microwave resonant cavity. Such a trap has several
novel features that make it very attractive for the development of ultracold
molecule sources. Using commonly available technologies, microwave traps can be
built with large depth (up to several Kelvin) and acceptance volume (up to
several cm^3), suitable for efficient loading with currently available sources
of cold polar molecules. Unlike most previous traps for molecules, this
technology can be used to confine the strong-field seeking absolute ground
state of the molecule, in a free-space maximum of the microwave electric field.
Such ground state molecules should be immune to inelastic collisional losses.
We calculate elastic collision cross-sections for the trapped molecules, due to
the electrical polarization of the molecules at the trap center, and find that
they are extraordinarily large. Thus, molecules in a microwave trap should be
very amenable to sympathetic and/or evaporative cooling. The combination of
these properties seems to open a clear path to producing large samples of polar
molecules at temperatures much lower than has been possible previously.Comment: 10 pages, 3 figure
Quantitation of Cellular Dynamics in Growing Arabidopsis Roots with Light Sheet Microscopy
To understand dynamic developmental processes, living tissues must be imaged
frequently and for extended periods of time. Root development is extensively
studied at cellular resolution to understand basic mechanisms underlying
pattern formation and maintenance in plants. Unfortunately, ensuring continuous
specimen access, while preserving physiological conditions and preventing
photo-damage, poses major barriers to measurements of cellular dynamics in
indeterminately growing organs such as plant roots. We present a system that
integrates optical sectioning through light sheet fluorescence microscopy with
hydroponic culture that enables us to image at cellular resolution a vertically
growing Arabidopsis root every few minutes and for several consecutive days. We
describe novel automated routines to track the root tip as it grows, track
cellular nuclei and identify cell divisions. We demonstrate the system's
capabilities by collecting data on divisions and nuclear dynamics.Comment: * The first two authors contributed equally to this wor
NOF1 Encodes an Arabidopsis Protein Involved in the Control of rRNA Expression
The control of ribosomal RNA biogenesis is essential for the regulation of protein synthesis in eukaryotic cells. Here, we report the characterization of NOF1 that encodes a putative nucleolar protein involved in the control of rRNA expression in Arabidopsis. The gene has been isolated by T-DNA tagging and its function verified by the characterization of a second allele and genetic complementation of the mutants. The nof1 mutants are affected in female gametogenesis and embryo development. This result is consistent with the detection of NOF1 mRNA in all tissues throughout plant life's cycle, and preferentially in differentiating cells. Interestingly, the closely related proteins from zebra fish and yeast are also necessary for cell division and differentiation. We showed that the nof1-1 mutant displays higher rRNA expression and hypomethylation of rRNA promoter. Taken together, the results presented here demonstrated that NOF1 is an Arabidopsis gene involved in the control of rRNA expression, and suggested that it encodes a putative nucleolar protein, the function of which may be conserved in eukaryotes
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