418,516 research outputs found
Thermal gradient driven domain wall dynamics
The issue of whether a thermal gradient acts like a magnetic field or an
electric current in the domain wall (DW) dynamics is investigated. Broadly
speaking, magnetization control knobs can be classified as energy-driving or
angular-momentum driving forces. DW propagation driven by a static magnetic
field is the best-known example of the former in which the DW speed is
proportional to the energy dissipation rate, and the current-driven DW motion
is an example of the latter. Here we show that DW propagation speed driven by a
thermal gradient can be fully explained as the angular momentum transfer
between thermally generated spin current and DW. We found DW-plane rotation
speed increases as DW width decreases. Both DW propagation speed along the wire
and DW-plane rotation speed around the wire decrease with the Gilbert damping.
These facts are consistent with the angular momentum transfer mechanism, but
are distinct from the energy dissipation mechanism. We further show that
magnonic spin-transfer torque (STT) generated by a thermal gradient has both
damping-like and field-like components. By analyzing DW propagation speed and
DW-plane rotation speed, the coefficient ( \b{eta}) of the field-like STT
arising from the non-adiabatic process, is obtained. It is found that \b{eta}
does not depend on the thermal gradient; increases with uniaxial anisotropy
K_(||) (thinner DW); and decreases with the damping, in agreement with the
physical picture that a larger damping or a thicker DW leads to a better
alignment between the spin-current polarization and the local magnetization, or
a better adiabaticity
Detector measures power in 50 to 30,000 GHz radiation band
Broadband power detector assembly measures electromagnetic radiation in the 50 to 30,000 GHz band. The assembly includes a matched pair of detectors which incorporate thin-film radiation absorbers. The detector is effective with either coherent or incoherent radiation
Supernova pencil beam survey
Type Ia supernovae (SNe Ia) can be calibrated to be good standard candles at
cosmological distances. We propose a supernova pencil beam survey that could
yield between dozens to hundreds of SNe Ia in redshift bins of 0.1 up to
, which would compliment space based SN searches, and enable the proper
consideration of the systematic uncertainties of SNe Ia as standard candles, in
particular, luminosity evolution and gravitational lensing. We simulate SNe Ia
luminosities by adding weak lensing noise (using empirical fitting formulae)
and scatter in SN Ia absolute magnitudes to standard candles placed at random
redshifts. We show that flux-averaging is powerful in reducing the combined
noise due to gravitational lensing and scatter in SN Ia absolute magnitudes.
The SN number count is not sensitive to matter distribution in the universe; it
can be used to test models of cosmology or to measure the SN rate. The SN
pencil beam survey can yield a wealth of data which should enable accurate
determination of the cosmological parameters and the SN rate, and provide
valuable information on the formation and evolution of galaxies.
The SN pencil beam survey can be accomplished on a dedicated 4 meter
telescope with a square degree field of view. This telescope can be used to
conduct other important observational projects compatible with the SN pencil
beam survey, such as QSOs, Kuiper belt objects, and in particular, weak lensing
measurements of field galaxies, and the search for gamma-ray burst afterglows.Comment: Final version, to appear in ApJ, 531, #2 (March 10, 2000). 22 pages
including 5 figures. Improved presentatio
Spin Exchange Rates in Electron-Hydrogen Collisions
The spin temperature of neutral hydrogen, which determines the 21 cm optical
depth and brightness temperature, is set by the competition between radiative
and collisional processes. In the high-redshift intergalactic medium, the
dominant collisions are typically those between hydrogen atoms. However,
collisions with electrons couple much more efficiently to the spin state of
hydrogen than do collisions with other hydrogen atoms and thus become important
once the ionized fraction exceeds ~1%. Here we compute the rate at which
electron-hydrogen collisions change the hydrogen spin. Previous calculations
included only S-wave scattering and ignored resonances near the n=2 threshold.
We provide accurate results, including all partial wave terms through the
F-wave, for the de-excitation rate at temperatures T_K < 15,000 K; beyond that
point, excitation to n>=2 hydrogen levels becomes significant. Accurate
electron-hydrogen collision rates at higher temperatures are not necessary,
because collisional excitation in this regime inevitably produces Lyman-alpha
photons, which in turn dominate spin exchange when T_K > 6200 K even in the
absence of radiative sources. Our rates differ from previous calculations by
several percent over the temperature range of interest. We also consider some
simple astrophysical examples where our spin de-excitation rates are useful.Comment: submitted to MNRAS, 9 pages, 5 figure
Thermodynamics of de Sitter Black Holes: Thermal Cosmological Constant
We study the thermodynamic properties associated with the black hole event
horizon and the cosmological horizon for black hole solutions in asymptotically
de Sitter spacetimes. We examine thermodynamics of these horizons on the basis
of the conserved charges according to Teitelboim's method. In particular, we
have succeeded in deriving the generalized Smarr formula among thermodynamical
quantities in a simple and natural way. We then show that cosmological constant
must decrease when one takes into account the quantum effect. These
observations have been obtained if and only if cosmological constant plays the
role of a thermodynamical state variable. We also touch upon the relation
between inflation of our universe and a phase transition of black holes.Comment: Revtex4, 11page
Detecting and diagnosing faults in dynamic stochastic distributions using a rational b-splines approximation to output PDFs
Describes the process of detecting and diagnosing faults in dynamic stochastic distributions using a rational b-splines approximation to output PDFs
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