8,971 research outputs found
Facility for interferometric testing of 1.25-m mirrors at liquid helium temperatures
A concept is presented for a national cryogenic optics test facility capable of optical characterization of 1.25 m diameter optics having focal lengths up to 6.2 m at temperatures from 300 K to near 4 K. The facility will be comprised of a large Dewar with a phase shift interferometer, a two stage vacuum system employing a turbomolecular pump, and an integral vibration isolation system. The entire facility will be housed in a concrete site with a massive floor to assist in reducing vibration during optical tests. By providing interchangeable sections, the overall height of the Dewar can be adjusted to provide for testing of shorter focal length optics. The background for the facility is discussed along with the facility location, and the requirements and the performance considerations which drive the Dewar design with respect to the vibration isolation system, vacuum system, and optical interferometry
Mixed symmetry localized modes and breathers in binary mixtures of Bose-Einstein condensates in optical lattices
We study localized modes in binary mixtures of Bose-Einstein condensates
embedded in one-dimensional optical lattices. We report a diversity of
asymmetric modes and investigate their dynamics. We concentrate on the cases
where one of the components is dominant, i.e. has much larger number of atoms
than the other one, and where both components have the numbers of atoms of the
same order but different symmetries. In the first case we propose a method of
systematic obtaining the modes, considering the "small" component as
bifurcating from the continuum spectrum. A generalization of this approach
combined with the use of the symmetry of the coupled Gross-Pitaevskii equations
allows obtaining breather modes, which are also presented.Comment: 11 pages, 16 figure
Performance of all-metal demountable cryogenic seals at superfluid helium temperatures
Two all-metal demountable cryogenic seals with an outside diameter of 36.6 mm, inside diameter of 27.2 mm, and thickness of 0.51 mm were leak-tested at room temperature (300 K), liquid nitrogen temperature (21 cycles at 77 K), liquid helium temperature (9 cycles at 4.2 K), and susperfluid helium temperature (4 cycles at 1.6 K). Each seal was mounted and demounted for 13 cycles. Thickness measurements at 90 deg intervals along the circumference showed a maximum seal compression of 0.038 mm. Leak-rate measurements at all temperatures showed no detectable leak above the helium background level, typically 0.1 x 10(-9) std-cc/sec, during testing
Thermal conductance measurements of pressed OFHC copper contacts at liquid helium temperatures
The thermal conductance of oxygen-free high conductivity (OFHC) copper sample pairs with surface finishes ranging from 0.1 to 1.6-micrometers rms roughness was investigated over the range of 1.6 to 6.0-K under applied contact forces up to 670 N. The thermal conductance increases with increasing contact force; however, no correlation can be drawn with respect to surface finish
Thermal conductance of pressed contacts at liquid helium temperatures
The thermal contact conductance of a 0.4 micrometer surface finish OFHC copper sample pair has been investigated from 1.6 to 3.8 K for a range of applied contact forces up to 670 N. Experimental data have been fitted to the relation Q = the integral alpha T to the nth power dt by assuming that the thermal contact conductance is a simple power function of the sample temperature. It has been found that the conductance is proportional to T squared and that conductance increases with an increase in applied contact force. These results confirm earlier work
Entangling power of permutation invariant quantum states
We investigate the von Neumann entanglement entropy as function of the size
of a subsystem for permutation invariant ground states in models with finite
number of states per site, e.g., in quantum spin models. We demonstrate that
the entanglement entropy of sites in a system of length generically
grows as , where is the on-site spin
and is a function depending only on magnetization.Comment: 6 pages, 2 figure
Coating thickness and coverage effects on the forces between silica nanoparticles in water
The structure and interactions of coated silica nanoparticles have been
studied in water using molecular dynamics simulations. For 5 nm diameter
amorphous silica nanoparticles we studied the effects of varying the chain
length and grafting density of polyethylene oxide (PEO) on the nanoparticle
coating's shape and on nanoparticle-nanoparticle effective forces. For short
ligands of length and repeat units, the coatings are radially
symmetric while for longer chains () the coatings are highly
anisotropic. This anisotropy appears to be governed primarily by chain length,
with coverage playing a secondary role. For the largest chain lengths
considered, the strongly anisotropic shape makes fitting to a simple radial
force model impossible. For shorter ligands, where the coatings are isotropic,
we found that the force between pairs of nanoparticles is purely repulsive and
can be fit to the form where is the separation
between the center of the nanoparticles, is the radius of the
silica core, and is measured to be between 2.3 and 4.1.Comment: 20 pages, 6 figure
Thermal conductance of pressed aluminum and stainless steel contacts at liquid helium temperatures
The thermal conductance of aluminum and stainless steel 304 sample pairs with surface finishes ranging from 0.1 to 1.6 microns rms roughness was investigated over a temperature range from 1.6 to 6.0 k. The thermal conductance follows a simple power law function of temperature, with the exponent ranging from 0.5 to 2.25, increases asymptotically with increasing applied force, and exhibits an anomaly for surface finishes in the 0.4 micron region
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