22,129 research outputs found
Nanometer lithography on silicon and hydrogenated amorphous silicon with low-energy electrons
We report the local oxidation of hydrogen terminated silicon (Si) surfaces induced with the scanning-tunneling microscope (STM) operating in air and by a beam of free low-energy electrons. With STM, oxide lines were written in Si(100) and Si(110) and transferred into the substrate by wet etching. In case of Si(110) trenches with a width as small as 35 nm and a depth of 300 nm were made. The same process has also successfully been applied to the patterning of hydrogenated amorphous silicon (a-Si:H) thin films. We demonstrate the fabrication of metallic ‘nanowires’ using a-Si:H as resist layer. With regard to the process of oxidation, it is found that the oxide written with STM is apparently not proportional to the electron current, in contrast to results obtained with a beam of free electrons in an oxygen gas-environment. The dose needed to remove the hydrogen was determined as a function of electron energy. This dose is minimal for 100 eV electrons amounting to 4 mC/cm2
Gimbal bearing design considerations and friction control
The design considerations of bearing selection, bearing fits, bearing installation, and thermal control are discussed for a gimbal with a high stiffness, low friction torque requirement. Tradeoffs between a quad set of small diameter spread apart or a large diameter bearing pair resulted in a cleaner, lighter, stiffer unit with the latter selection. Bearing fits were designed to eliminate clearances with tolerances of .00127 mm 00005 in) on the bearing shafts and housings. The problems in metrology are discussed and a perferred technique for measurement of small cross-section bearings described. A technique for installation to assure proper seating of the bearing is offered. Where transient thermal conditions are involved, a method of controlling bearing friction by active control of bearing temperature gradients including the use of bearing unload test curves is described
A New Universality for Random Sequential Deposition of Needles
Percolation and jamming phenomena are investigated for random sequential
deposition of rectangular needles on square lattices. Associated
thresholds and are determined for various needle
sizes. Their ratios are found to be a constant for all sizes. In addition the ratio of jamming thresholds for
respectively square blocks and needles is also found to be a constant . These constants exhibit some universal connexion in the geometry of
jamming and percolation for both anisotropic shapes (needles versus square
lattices) and isotropic shapes (square blocks on square lattices). A universal
empirical law is proposed for all three thresholds as a function of .Comment: 9 pages, latex, 4 eps figures include
Geodetic Precession and the Binary Pulsar B1913+16
A change of the component separation in the profiles of the binary pulsar PSR
B1913+16 has been observed for the first time (Kramer 1998) as expected by
geodetic precession. In this work we extend the previous work by accounting for
recent data from the Effelsberg 100-m telescope and Arecibo Observatory and
testing model predictions. We demonstrate how the new information will provide
additional information on the solutions of the system geometry.Comment: 2 pages, 1 figure, IAU 177 Colloquium: Pulsar Astronomy - 2000 and
Beyon
Pulsar-black hole binaries: prospects for new gravity tests with future radio telescopes
The anticipated discovery of a pulsar in orbit with a black hole is expected
to provide a unique laboratory for black hole physics and gravity. In this
context, the next generation of radio telescopes, like the Five-hundred-metre
Aperture Spherical radio Telescope (FAST) and the Square Kilometre Array (SKA),
with their unprecedented sensitivity, will play a key role. In this paper, we
investigate the capability of future radio telescopes to probe the spacetime of
a black hole and test gravity theories, by timing a pulsar orbiting a
stellar-mass-black-hole (SBH). Based on mock data simulations, we show that a
few years of timing observations of a sufficiently compact pulsar-SBH (PSR-SBH)
system with future radio telescopes would allow precise measurements of the
black hole mass and spin. A measurement precision of one per cent can be
expected for the spin. Measuring the quadrupole moment of the black hole,
needed to test GR's no-hair theorem, requires extreme system configurations
with compact orbits and a large SBH mass. Additionally, we show that a PSR-SBH
system can lead to greatly improved constraints on alternative gravity theories
even if they predict black holes (practically) identical to GR's. This is
demonstrated for a specific class of scalar-tensor theories. Finally, we
investigate the requirements for searching for PSR-SBH systems. It is shown
that the high sensitivity of the next generation of radio telescopes is key for
discovering compact PSR-SBH systems, as it will allow for sufficiently short
survey integration times.Comment: 20 pages, 11 figures, 1 table, accepted for publication in MNRA
Diamagnetic Phase Transition and Phase Diagrams in Beryllium
The model of diamagnetic phase transition in beryllium which takes into
account the quasi 2-dimensional shape of the Fermi surface of beryllium is
proposed. It explains correctly the recent experimental data on observation of
non-homogeneous phase in beryllium at the conditions of strong dHvA effect when
the strong correlation of electron gas results in instability of homogeneous
phase and formation of Condon domain structure.Comment: 5 pages, 4 figure
Singularity free cosmological solutions of Einstein-Maxwell equations
We report on a new two-parameter class of cosmological solutions to the
Einstein-Maxwell equations. The solutions have everywhere regular curvature
invariants. We prove that the solutions are geodesically complete and globally
hyperbolic.Comment: 8 pages,latex; v2 some typos correcte
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