42,730 research outputs found
Testing and evaluation of solid lubricants for gas bearings
The testing and results of testing solid film lubricants for gas lubricated bearing applications are reported. The tests simulated operational hazards of tilting pad gas bearings. The presence of a low coefficient of friction and the endurance of the solid film lubricant were the criteria for judging superior performance. All solid lubricants tested were applied to a plasma sprayed chrome oxide surface. Molybdenum disulfide and graphite fluoride were the solid lubricants tested; other test parameters included the method of application of the solid lubricant and the surface finish of the plasma sprayed coating. In general, the application of a solid film lubricant was found to significantly improve the coefficient of friction of the rubbing surfaces
High resolution frequency selective photochemistry of phycobilisomes at cryogenic temperatures
Electronic structure of an electron on the gyroid surface, a helical labyrinth
Previously reported formulation for electrons on curved periodic surfaces is
used to analyze the band structure of an electron bound on the gyroid surface
(the only triply-periodic minimal surface that has screw axes). We find that an
effect of the helical structure appears as the bands multiply sticking together
on the Brillouin zone boundaries. We elaborate how the band sticking is lifted
when the helical and inversion symmetries of the structure are degraded. We
find from this that the symmetries give rise to prominent peaks in the density
of states.Comment: RevTeX, 4 pages, 6 figure
Low-Temperature Excitations of Dilute Lattice Spin Glasses
A new approach to exploring low-temperature excitations in finite-dimensional
lattice spin glasses is proposed. By focusing on bond-diluted lattices just
above the percolation threshold, large system sizes can be obtained which
lead to enhanced scaling regimes and more accurate exponents. Furthermore, this
method in principle remains practical for any dimension, yielding exponents
that so far have been elusive. This approach is demonstrated by determining the
stiffness exponent for dimensions , (the upper critical dimension),
and . Key is the application of an exact reduction algorithm, which
eliminates a large fraction of spins, so that the reduced lattices never exceed
variables for sizes as large as L=30 in , L=9 in , or L=8
in . Finite size scaling analysis gives for ,
significantly improving on previous work. The results for and ,
and , are entirely new and are compared with
mean-field predictions made for d>=6.Comment: 7 pages, LaTex, 7 ps-figures included, added result for stiffness in
d=7, as to appear in Europhysics Letters (see
http://www.physics.emory.edu/faculty/boettcher/ for related information
Small-Angle Excess Scattering: Glassy Freezing or Local Orientational Ordering?
We present Monte Carlo simulations of a dense polymer melt which shows
glass-transition-like slowing-down upon cooling, as well as a build up of
nematic order. At small wave vectors q this model system shows excess
scattering similar to that recently reported for light-scattering experiments
on some polymeric and molecular glass-forming liquids. For our model system we
can provide clear evidence that this excess scattering is due to the onset of
short-range nematic order and not directly related to the glass transition.Comment: 3 Pages of Latex + 4 Figure
Consequences Of Fully Dressing Quark-Gluon Vertex Function With Two-Point Gluon Lines
We extend recent studies of the effects of quark-gluon vertex dressing upon
the solutions of the Dyson-Schwinger equation for the quark propagator. A
momentum delta function is used to represent the dominant infrared strength of
the effective gluon propagator so that the resulting integral equations become
algebraic. The quark-gluon vertex is constructed from the complete set of
diagrams involving only 2-point gluon lines. The additional diagrams, including
those with crossed gluon lines, are shown to make an important contribution to
the DSE solutions for the quark propagator, because of their large color
factors and the rapid growth in their number
Numerical Studies of the Compressible Ising Spin Glass
We study a two-dimensional compressible Ising spin glass at constant volume.
The spin interactions are coupled to the distance between neighboring particles
in the Edwards-Anderson model with +/- J interactions. We find that the energy
of a given spin configuration is shifted from its incompressible value, E_0, by
an amount quadratic in E_0 and proportional to the coupling strength. We then
construct a simple model expressed only in terms of spin variables that
predicts the existence of a critical value of the coupling above which the
spin-glass transition disappears.Comment: REVTeX, 4 pages, 4 figures. Submitted to Phys. Rev. Let
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An iron shuttle for deepwater silica in Late Archean and early Paleoproterozoic iron formation
Iron formations are typically thinly bedded or laminated sedimentary rocks containing 15% or more of iron and a large proportion of silica (commonly > 40%). In the ca. 2590-2460 Ma Campbellrand-Kuruman Complex, Transvaal Supergroup, South Africa, iron formation occurs as a sediment-starved deepwater facies distal to carbonates and shales. Iron minerals, primarily siderite, define the lamination. The silica primarily occurs as thin beds and nodules of diagenetic chert (now microcrystalline quartz), filling pore space and replacing iron formation minerals and co-occurring deepwater lithologies. Mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production, anoxygenic photosynthesis, abiotic photochemistry, and chemoautotrophy using Fe(II) as an electron donor. The origin and mechanism of silica precipitation in these deposits have received less attention. Here we present a conceptual model of iron formation that offers insight into the deposition of silica. The model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides. Soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides, which precipitate. Fe(OH)_3 binds silica and sinks from the surface ocean along with organic matter, shuttling silica to basinal waters and sediments. Fe(III) respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters, driving precipitation of siderite from Fe2+ and respiration-influenced CO2. Silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases. This model explains many of the mineralogical, textural, and environmental features of Late Archean and earliest Paleo-proterozoic iron formation
Free-induction decay and envelope modulations in a narrowed nuclear spin bath
We evaluate free-induction decay for the transverse components of a localized
electron spin coupled to a bath of nuclear spins via the Fermi contact
hyperfine interaction. Our perturbative treatment is valid for special
(narrowed) bath initial conditions and when the Zeeman energy of the electron
exceeds the total hyperfine coupling constant : . Using one unified
and systematic method, we recover previous results reported at short and long
times using different techniques. We find a new and unexpected modulation of
the free-induction-decay envelope, which is present even for a purely isotropic
hyperfine interaction without spin echoes and for a single nuclear species. We
give sub-leading corrections to the decoherence rate, and show that, in
general, the decoherence rate has a non-monotonic dependence on electron Zeeman
splitting, leading to a pronounced maximum. These results illustrate the
limitations of methods that make use of leading-order effective Hamiltonians
and re-exponentiation of short-time expansions for a strongly-interacting
system with non-Markovian (history-dependent) dynamics.Comment: 13 pages, 9 figure
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