25,316 research outputs found
Comparison of ground and satellite based measurements of the fraction of photosynthetically active radiation intercepted by tall-grass prairie
The fraction, of photosynthetically active radiation absorbed by vegetation, F sub ipar, is an important requirement for estimating vegetation biomass productivity and related quantities. This was an integral part of a large international effort; the First ISLSCP Field Experiment (FIFE). The main objective of FIFE was to study the effects of vegetation on the land atmosphere interactions and to determine if these interactions can be assessed from satellite spectral measurements. The specific purpose of this experiment was to find out how well measurements of F sub ipar relate to ground, helicopter, and satellite based spectral reflectance measurements. Concurrent measurements of F sub ipar and ground, helicopter, and satellite based measurements were taken at 13 tall grass prairie sites in Kansas. The sites were subjected to various combinations of burning and grazing managements
Magnitude of Magnetic Field Dependence of a Possible Selective Spin Filter in ZnSe/Zn_{1-x}Mn_{x}Se Multilayer Heterostructure
Spin-polarized transport through a band-gap-matched ZnSe/Zn_{1-x}Mn_{x}
Se/ZnSe/Zn_{1-x}Mn_{x}Se/ZnSe multilayer structure is investigated. The
resonant transport is shown to occur at different energies for different spins
owing to the split of spin subbands in the paramagnetic layers. It is found
that the polarization of current density can be reversed in a certain range of
magnetic field, with the peak of polarization moving towards a stronger
magnetic field for increasing the width of central ZnSe layer while shifting
towards an opposite direction for increasing the width of paramagnetic layer.
The reversal is limited in a small-size system. A strong suppression of the
spin up component of the current density is present at high magnetic field. It
is expected that such a reversal of the polarization could act as a possible
mechanism for a selective spin filter device
The Fano resonance for Anderson impurity systems
We present a general theory for the Fano resonance in Anderson impurity
systems. It is shown that the broadening of the impurity level leads to an
additional and important contribution to the Fano resonance around the Fermi
surface, especially in the mixed valence regime. This contribution results from
the interference between the Kondo resonance and the broadened impurity level.
Being applied to the scanning tunnelling microscopic experiments, we find that
our theory gives a consistent and quantitative account for the Fano resonance
lineshapes for both Co and Ti impurities on Au or Ag surfaces. The Ti systems
are found to be in the mixed valence regime.Comment: 4 pages, 5 figures, published versio
Debris Disks around Solar-Type Stars: Observations of the Pleiades with Spitzer Space Telescope
We present Spitzer MIPS observations at 24 um of 37 solar-type stars in the
Pleiades and combine them with previous observations to obtain a sample of 71
stars. We report that 23 stars, or 32 +/- 6.8%, have excesses at 24 um at least
10% above their photospheric emission. We compare our results with studies of
debris disks in other open clusters and with a study of A stars to show that
debris disks around solar-type stars at 115 Myr occur at nearly the same rate
as around A-type stars. We analyze the effects of binarity and X-ray activity
on the excess flux. Stars with warm excesses tend not to be in equal-mass
binary systems, possibly due to clearing of planetesimals by binary companions
in similar orbits. We find that the apparent anti-correlations in the incidence
of excess and both the rate of stellar rotation and also the level of activity
as judged by X-ray emission are statistically weak.Comment: 34 pages; accepted for publication in ApJ; new version included
corrections of typos, etc to match published versio
Neutron scattering study of commensurate magnetic ordering in single crystal CeSb
Temperature and field-dependent magnetization measurements and
neutron scattering study of a single crystal CeSb are presented. Several
anomalies in the magnetization curves have been confirmed at low magnetic
field, i.e., 15.6 K, 12 K, and 9.8 K. These three transitions are all
metamagnetic transitions (MMT), which shift to lower temperatures as the
magnetic field increases. The anomaly at 15.6 K has been suggested as
paramagnetic (PM) to ferromagnetic (FM) phase transition. The anomaly located
at around 12 K is antiferromagnetic-like transition, and this turning point
will clearly split into two when the magnetic field T. Neutron
scattering study reveals that the low temperature ground state of CeSb
orders antiferromagnetically with commensurate propagation wave vectors
and , with N\'eel
temperature K. This transition is of first-order, as shown in the
hysteresis loop observed by the field cooled cooling (FCC) and field cooled
warming (FCW) processes.Comment: 7 pages,9 figure
Analytical stripe phase solution for the Hubbard model
The self-consistent solution for the spin-charge solitonic superstructure in
quasi-one-dimensional electron system is obtained in the framework of the
Hubbard model as a function of a hole doping. Effects of interchain
interactions on the ground state are discussed. Results are used for the
interpretation of the observed stripe phases in doped antiferromagnets.Comment: 9 pages, LaTex file, no figure
Mott-Peierls Transition in the extended Peierls-Hubbard model
The one-dimensional extended Peierls-Hubbard model is studied at several band
fillings using the density matrix renormalization group method. Results show
that the ground state evolves from a Mott-Peierls insulator with a correlation
gap at half-filling to a soliton lattice with a small band gap away from
half-filling. It is also confirmed that the ground state of the Peierls-Hubbard
model undergoes a transition to a metallic state at finite doping. These
results show that electronic correlations effects should be taken into account
in theoretical studies of doped polyacetylene. They also show that a
Mott-Peierls theory could explain the insulator-metal transition observed in
this material.Comment: 4 pages with 3 embedded eps figure
Precise and ultrafast molecular sieving through graphene oxide membranes
There has been intense interest in filtration and separation properties of
graphene-based materials that can have well-defined nanometer pores and exhibit
low frictional water flow inside them. Here we investigate molecular permeation
through graphene oxide laminates. They are vacuum-tight in the dry state but,
if immersed in water, act as molecular sieves blocking all solutes with
hydrated radii larger than 4.5A. Smaller ions permeate through the membranes
with little impedance, many orders of magnitude faster than the diffusion
mechanism can account for. We explain this behavior by a network of
nanocapillaries that open up in the hydrated state and accept only species that
fit in. The ultrafast separation of small salts is attributed to an 'ion
sponge' effect that results in highly concentrated salt solutions inside
graphene capillaries
Metal-insulator transition in the one-dimensional Holstein model at half filling
We study the one-dimensional Holstein model with spin-1/2 electrons at
half-filling. Ground state properties are calculated for long chains with great
accuracy using the density matrix renormalization group method and extrapolated
to the thermodynamic limit. We show that for small electron-phonon coupling or
large phonon frequency, the insulating Peierls ground state predicted by
mean-field theory is destroyed by quantum lattice fluctuations and that the
system remains in a metallic phase with a non-degenerate ground state and
power-law electronic and phononic correlations. When the electron-phonon
coupling becomes large or the phonon frequency small, the system undergoes a
transition to an insulating Peierls phase with a two-fold degenerate ground
state, long-range charge-density-wave order, a dimerized lattice structure, and
a gap in the electronic excitation spectrum.Comment: 6 pages (LaTex), 10 eps figure
The disk-outflow system in the S255IR area of high mass star formation
We report the results of our observations of the S255IR area with the SMA at
1.3 mm in the very extended configuration and at 0.8 mm in the compact
configuration as well as with the IRAM-30m at 0.8 mm. The best achieved angular
resolution is about 0.4 arcsec. The dust continuum emission and several tens of
molecular spectral lines are observed. The majority of the lines is detected
only towards the S255IR-SMA1 clump, which represents a rotating structure
(probably disk) around the young massive star. The achieved angular resolution
is still insufficient for conclusions about Keplerian or non-Keplerian
character of the rotation. The temperature of the molecular gas reaches 130-180
K. The size of the clump is about 500 AU. The clump is strongly fragmented as
follows from the low beam filling factor. The mass of the hot gas is
significantly lower than the mass of the central star. A strong DCN emission
near the center of the hot core most probably indicates a presence of a
relatively cold ( K) and rather massive clump there. High velocity
emission is observed in the CO line as well as in lines of high density tracers
HCN, HCO+, CS and other molecules. The outflow morphology obtained from
combination of the SMA and IRAM-30m data is significantly different from that
derived from the SMA data alone. The CO emission detected with the SMA traces
only one boundary of the outflow. The outflow is most probably driven by jet
bow shocks created by episodic ejections from the center. We detected a dense
high velocity clump associated apparently with one of the bow shocks. The
outflow strongly affects the chemical composition of the surrounding medium.Comment: 19 pages, 23 figures, accepted by The Astrophysical Journa
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