20,678 research outputs found
Magnetoresistance and spin polarization in the insulating regime of a Si two-dimensional electron system
We have studied the magnetoresistance in a high-mobility Si inversion layer
down to low electron concentrations at which the longitudinal resistivity
has an activated temperature dependence. The angle of the magnetic
field was controlled so as to study the orbital effect proportional to the
perpendicular component for various total strengths . A
dip in , which corresponds to the Landau level filling factor of
, survives even for high resistivity of at
. The linear -dependence of the value of
at the dip for low indicates that a ferromagnetic
instability does not occur even in the far insulating regime.Comment: 6 pages, 7 figures, submitted to PR
Aperture excited dielectric antennas
The results of a comprehensive experimental and theoretical study of the effect of placing dielectric objects over the aperture of waveguide antennas are presented. Experimental measurements of the radiation patterns, gain, impedance, near-field amplitude, and pattern and impedance coupling between pairs of antennas are given for various Plexiglas shapes, including the sphere and the cube, excited by rectangular, circular, and square waveguide feed apertures. The waveguide excitation of a dielectric sphere is modeled using the Huygens' source, and expressions for the resulting electric fields, directivity, and efficiency are derived. Calculations using this model show good overall agreement with experimental patterns and directivity measurements. The waveguide under an infinite dielectric slab is used as an impedance model. Calculations using this model agree qualitatively with the measured impedance data. It is concluded that dielectric loaded antennas such as the waveguide excited sphere, cube, or sphere-cylinder can produce directivities in excess of that obtained by a uniformly illuminated aperture of the same cross section, particularly for dielectric objects with dimensions of 2 wavelengths or less. It is also shown that for certain configurations coupling between two antennas of this type is less than that for the same antennas without dielectric loading
A Comparison of the High-Frequency Magnetic Fluctuations in Insulating and Superconducting La2-xSrxCuO4
Inelastic neutron scattering performed at a spallation source is used to make
absolute measurements of the dynamic susceptibility of insulating La2CuO4 and
superconducting La2-xSrxCuO4 over the energy range 15<EN<350 meV. The effect of
Sr doping on the magnetic excitations is to cause a large broadening in
wavevector and a substantial change in the spectrum of the local spin
fluctuations. Comparison of the two compositions reveals a new energy scale of
22 meV in La1.86Sr0.14CuO4.Comment: RevTex, 7 Pages, 4 postscript figure
Modelling and experimental investigation of carangiform locomotion for control
We propose a model for planar carangiform swimming based on conservative equations for the interaction of a rigid body and an incompressible fluid. We account for the generation of thrust due to vortex shedding through controlled coupling terms. We investigate the correct form of this coupling experimentally with a robotic propulsor, comparing its observed behavior to that predicted by unsteady hydrodynamics. Our analysis of thrust generation by an oscillating hydrofoil allows us to characterize and evaluate certain families of gaits. Our final swimming model takes the form of a control-affine nonlinear system
Correlation of the Quasi-Periodic Oscillation Frequencies of White Dwarf, Neutron Star, and Black Hole Binaries
Using data obtained in 1994 June/July with the Extreme Ultraviolet Explorer
deep survey photometer and in 2001 January with the Chandra X-ray Observatory
Low Energy Transmission Grating Spectrograph, we investigate the
extreme-ultraviolet (EUV) and soft X-ray oscillations of the dwarf nova SS Cyg
in outburst. We find quasi-periodic oscillations (QPOs) at nu_0 ~ 0.012 Hz and
nu_1 ~ 0.13 Hz in the EUV flux and at nu_0 ~ 0.0090 Hz, nu_1 ~ 0.11 Hz, and
possibly nu_2 ~ nu_0 + nu_1 ~ 0.12 Hz in the soft X-ray flux. These data,
combined with the optical data of Woudt & Warner for VW Hyi, extend the
Psaltis, Belloni, & van der Klis nu_high-nu_low correlation for neutron star
and black hole low-mass X-ray binaries (LMXBs) nearly two orders of magnitude
in frequency, with nu_low ~ 0.08 nu_high. This correlation identifies the
high-frequency quasi-coherent oscillations (so-called ``dwarf nova
oscillations'') of cataclysmic variables (CVs) with the kilohertz QPOs of
LMXBs, and the low-frequency QPOs of CVs with the horizontal branch
oscillations (or the broad noise component identified as such) of LMXBs.
Assuming that the same mechanisms produce the QPOs in white dwarf, neutron
star, and black hole binaries, we find that the data exclude the relativistic
precession model and the magnetospheric and sonic-point beat-frequency models
(as well as any model requiring the presence or absence of a stellar surface or
magnetic field); more promising are models that interpret QPOs as
manifestations of disk accretion onto any low-magnetic field compact object.Comment: 15 pages including 4 encapsulated postscript figures; LaTeX format,
uses aastex.cls; accepted on 2002 July 23 for publication in The
Astrophysical Journa
HST followup observations of two bright z ~ 8 candidate galaxies from the BoRG pure-parallel survey
We present followup imaging of two bright (L > L*) galaxy candidates at z > 8
from the Brightest of Reionizing Galaxies (BoRG) survey with the F098M filter
on HST/WFC3. The F098M filter provides an additional constraint on the flux
blueward of the spectral break, and the observations are designed to
discriminate between low- and high-z photometric redshift solutions for these
galaxies. Our results confirm one galaxy, BoRG 0116+1425 747, as a highly
probable z ~ 8 source, but reveal that BoRG 0116+1425 630 - previously the
brightest known z > 8 candidate (mAB = 24.5) - is likely to be a z ~ 2
interloper. As this source was substantially brighter than any other z > 8
candidate, removing it from the sample has a significant impact on the derived
UV luminosity function in this epoch. We show that while previous BoRG results
favored a shallow power-law decline in the bright end of the luminosity
function prior to reionization, there is now no evidence for departure from a
Schechter function form and therefore no evidence for a difference in galaxy
formation processes before and after reionization.Comment: Accepted by ApJL, 7 pages, 4 figure
Time-resolved, multi-color photometry and spectroscopy of Virgo 4 (OU Vir): a high orbital inclination, short orbital period dwarf nova
We present multi-color photometry and time resolved spectroscopy of OU Vir.
The analysis of the quiescent light curve shows that OU Vir is characterized by
i) strong cycle-to-cycle brightness variations, and ii) hot spot modulated
light curve with grazing eclipse of the impact region. Colors are derived both
in- and out- of eclipse. The time-resolved spectroscopy allows us to produce
the radial velocity curve from the H accretion disk emission line which
possibly reveals only weak evidence for hot spot line emission. The hot spot is
believed to be a turbulent optically thick region, producing mostly continuum
emission.Comment: 8 pages (including figures), 7 figures. To Be published in A&
A poor man's positive energy theorem: II. Null geodesics
We show that positivity of energy for stationary, or strongly uniformly
Schwarzschildian, asymptotically flat, non-singular domains of outer
communications can be proved using Galloway's null rigidity theorem.Comment: Latex2e, 24 A4 pages, minor change
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