1,100 research outputs found
Theoretical and experimental investigations of collective microwave phenomena in solids Semiannual status report, 1 Oct. - 31 Dec. 1969
Theoretical and experimental investigations of collective microwave phenomena in solid
Theoretical and experimental investigations of collective microwave phenomena in solids
Methods of generating microwave shear waves efficiently were studied. A new technique of mode conversion from a longitudinal wave to a shear wave was investigated and virtually 100% conversion efficiency could be obtained. Several aspects of Gunn oscillators were studied. One involved the detailed study of domains in Gunn oscillators using long samples of bulk gallium arsenide. Several new techniques were devised for measuring the properties of the domains. A fundamental theory of the efficiency of a Gunn oscillator was worked out. A computer program was devised to study the transient effect of domains passing through Gunn diodes. Some of the first planar epitaxial Gunn oscillators were made using liquid epitaxial material grown by liquid phase techniques on semi-insulating substrate. A new theory which predicted the properties of RF signals propagating in a thin film layer of GaAs was developed. The theory of the operation of a traveling wave amplifier is described
Theoretical and experimental investigations of collective microwave phenomena in solids Semiannual status, 1 Apr. - 30 Sep. 1969
Microwave amplification in high resistivity gallium arsenide
Ultrasonic Measurements of Inhomogenous Stress Fields
An acoustic technique for measuring inhomogeneous stress fields in externally loaded solids is described. This method requires a measurement of transit time of a longitudinal acoustic wave through a stressed thin metal specimen using a small diameter water-coupled acoustic transducer. The transducer is mechanically scanned over the surface of the sample by a computer controlled system to take stress field contour plots. Samples investigated include an aluminum plate with a centrol hole, a double edge-notched panel and a single edge-notched panel. In addition to measuring stress fields, the nondestructive determination of stress intensity factors is also discussed
Optical Range Finder
Recently, a great deal of interest has been shown in making accurate range measurements with good transverse definition. This capability makes it possible, in machine vision systems, to extract geometrical shape information from the images. In robot position sensing, it is important to determine the absolute distance instead of distance change so that noncontinuous measurements can be made without the need for calibration at start-up. A third application of great importance is to measure the shape and size of machined parts with a noncontacting sensor
A quest for frustration driven distortion in Y2Mo2O7
We investigated the nature of the freezing in the geometrically frustrated
Heisenberg spin-glass Y2Mo2O7 by measuring the temperature dependence of the
static internal magnetic field distribution above the spin-glass temperature,
Tg, using the muSR technique. The evolution of the field distribution cannot be
explained by changes in the spin susceptibility alone and suggests a lattice
deformation. This possibility is addressed by numerical simulations of the
Heisenberg Hamiltonian with magneto-elastic coupling at T>0.Comment: 5 pages 4 figures. Accepted for publication in PR
Parametric Amplification in the Dynamic Radiation Force of Acoustic Waves in Fluids
We report on parametric amplification in dynamic radiation force produced by
a bichromatic acoustic beam in a fluid. To explain this effect we develop a
theory taking into account the nonlinearity of the fluid. The theory is
validated through an experiment to measure the dynamic radiation force on an
acrylic sphere. Results exhibit an amplification of 66 dB in water and 80 dB in
alcohol as the difference of the frequencies is increased from 10 Hz to 240
kHz
Miniature photonic-crystal hydrophone optimized for ocean acoustics
This work reports on an optical hydrophone that is insensitive to hydrostatic
pressure, yet capable of measuring acoustic pressures as low as the background
noise in the ocean in a frequency range of 1 Hz to 100 kHz. The miniature
hydrophone consists of a Fabry-Perot interferometer made of a photonic-crystal
reflector interrogated with a single-mode fiber, and is compatible with
existing fiber-optic technologies. Three sensors with different acoustic power
ranges placed within a sub-wavelength sized hydrophone head allow a high
dynamic range in the excess of 160 dB with a low harmonic distortion of better
than -30 dB. A method for suppressing cross coupling between sensors in the
same hydrophone head is also proposed. A prototype was fabricated, assembled,
and tested. The sensitivity was measured from 100 Hz to 100 kHz, demonstrating
a minimum detectable pressure down to 12 {\mu}Pa (1-Hz noise bandwidth), a
flatband wider than 10 kHz, and very low distortion
Energetics of Tev Blazars and Physical Constraints on their Emission Regions
Using multi-frequency spectra from TeV blazars in quiescent states, we obtain
the physical parameters of the emission region of blazars within the framework
of the one-zone synchrotron self-Compton (SSC) model. We numerically calculate
the steady-state energy spectra of electrons by self-consistently taking into
account the effects of radiative cooling with a proper account of the
Klein-Nishina effects. Here electrons are assumed to be injected with a
power-law spectrum and to escape on a finite time scale, which naturally leads
to the existence of a break energy scale. Although we do not use time
variabilities but utilize a model of electron escape to constrain the size of
the emission region, the resultant size turns out to be similar to that
obtained based on time variabilities. Through detailed comparison of the
predicted emission spectra with observations, we find that for Mrk 421, Mrk
501, and PKS 2155--304, the energy density of relativistic electrons is about
an order of magnitude larger than that of magnetic fields with an uncertainty
within a factor of a few.Comment: Accepted for publication in Ap
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