19,651 research outputs found
Selection of the Argentine indicator region
Determined from available Argentine crop statistics, selection of the Indicator Region was based on the highest wheat, corn, and soybean producing provinces, which were: Buenos Aires, Cordoba, Entre Rios, and Santa Fe. Each province in Argentina was examined for the availability of LANDSAT data; area, yield and production statistics; crop calendars; and other ancillary data. The Argentine Indicator Region is described
Supergroup BF action for supergravity
General relativity can be formulated as a SU(2) BF-theory with constraints,
as has been shown, by Pleba\'nski. The cosmological constant term can be
obtained from the constraint term, following from the consistency of the
equations of motion, as recently shown by Krasnov. We consider an
invariant, supergravity extension of this theory, for which the consistency of
the equations of motion and the constraints contribute as well to the
cosmological constant terms of Townsend's supergravity. The Kalb-Ramond
invariance is shortly discussed.Comment: 14 page
Lithium abundances in nearby FGK dwarf and subgiant stars: internal destruction, Galactic chemical evolution, and exoplanets
We derive atmospheric parameters and lithium abundances for 671 stars and
include our measurements in a literature compilation of 1381 dwarf and subgiant
stars. First, a "lithium desert" in the effective temperature (Teff) versus
lithium abundance (A_Li) plane is observed such that no stars with Teff~6075 K
and A_Li~1.8 are found. We speculate that most of the stars on the low A_Li
side of the desert have experienced a short-lived period of severe surface
lithium destruction as main-sequence or subgiant stars. Next, we search for
differences in the lithium content of thin-disk and thick-disk stars, but we
find that internal processes have erased from the stellar photospheres their
possibly different histories of lithium enrichment. Nevertheless, we note that
the maximum lithium abundance of thick-disk stars is nearly constant from
[Fe/H]=-1.0 to -0.1, at a value that is similar to that measured in very
metal-poor halo stars (A_Li~2.2). Finally, differences in the lithium abundance
distribution of known planet-host stars relative to otherwise ordinary stars
appear when restricting the samples to narrow ranges of Teff or mass, but they
are fully explained by age and metallicity biases. We confirm the lack of a
connection between low lithium abundance and planets. However, we find that no
low A_Li planet-hosts are found in the desert Teff window. Provided that subtle
sample biases are not responsible for this observation, this suggests that the
presence of gas giant planets inhibit the mechanism responsible for the lithium
desert.Comment: ApJ, in press. Complete Tables 1 and 3 are available upon reques
Realizing Colloidal Artificial Ice on Arrays of Optical Traps
We demonstrate how a colloidal version of artificial ice can be realized on
optical trap lattices. Using numerical simulations, we show that this system
obeys the ice rules and that for strong colloid-colloid interactions, an
ordered ground state appears. We show that the ice rule ordering can occur for
systems with as few as twenty-four traps and that the ordering transition can
be observed at constant temperature by varying the barrier strength of the
traps.Comment: 4 pages, 3 postscript figures; version to appear in Phys. Rev. Let
Gamma-Ray Burst and Relativistic Shells: The Surface Filling Factor
The variability observed in many complex gamma-ray bursts (GRBs) is
inconsistent with causally connected variations in a single, symmetric,
relativistic shell interacting with the ambient material ("external shocks").
Rather, the symmetry of the shell must be broken on an angular scale much
smaller than Gamma^{-1} where Gamma is the bulk Lorentz factor for the shell.
The observed variability in the external shock models arises from the number of
causally connected regions that (randomly) become active. We define the
"surface filling factor" to be the ratio of the area of causally connected
regions that become active to the observable area of the shell. From the
observed variability in 52 BATSE bursts, we estimate the surface filling factor
to be typically 0.005 although some values are near unity. We find that the
surface filling factor is about 0.1 Delta T/T in both the constant Gamma phase
(which probably produces the GRB) and the deaccelerating phase (which probably
produces the x-ray afterglows). Here, \Delta T is a typical time scale of
variability and T is the time since the initial signal. We analyze the 2 hr
flare seen by ASCA 36 hr after the GRB and conclude that the surface filling
factor must be small (0.001) in the x-ray afterglow phase as well. Explanations
for low surface filling factor can either require more or less energy (by a
factor of about 1000) compared to that expected for a symmetric shell.Comment: 26 pages, 5 embedded figures, Latex, revised version as in press,
ApJ, added figure to show the possible expanding shell geometries that can
give low filling facto
Anti-ferromagnetic ordering in arrays of superconducting pi-rings
We report experiments in which one dimensional (1D) and two dimensional (2D)
arrays of YBa2Cu3O7-x-Nb pi-rings are cooled through the superconducting
transition temperature of the Nb in various magnetic fields. These pi-rings
have degenerate ground states with either clockwise or counter-clockwise
spontaneous circulating supercurrents. The final flux state of each ring in the
arrays was determined using scanning SQUID microscopy. In the 1D arrays,
fabricated as a single junction with facets alternating between alignment
parallel to a [100] axis of the YBCO and rotated 90 degrees to that axis,
half-fluxon Josephson vortices order strongly into an arrangement with
alternating signs of their magnetic flux. We demonstrate that this ordering is
driven by phase coupling and model the cooling process with a numerical
solution of the Sine-Gordon equation. The 2D ring arrays couple to each other
through the magnetic flux generated by the spontaneous supercurrents. Using
pi-rings for the 2D flux coupling experiments eliminates one source of disorder
seen in similar experiments using conventional superconducting rings, since
pi-rings have doubly degenerate ground states in the absence of an applied
field. Although anti-ferromagnetic ordering occurs, with larger negative bond
orders than previously reported for arrays of conventional rings, long-range
order is never observed, even in geometries without geometric frustration. This
may be due to dynamical effects. Monte-Carlo simulations of the 2D array
cooling process are presented and compared with experiment.Comment: 10 pages, 15 figure
Slow spin relaxation in a highly polarized cooperative paramagnet
We report measurements of the ac susceptibility of the cooperative paramagnet
Tb2Ti2O7 in a strong magnetic field. Our data show the expected saturation
maximum in chi(T) and also an unexpected low frequency dependence (< 1 Hz) of
this peak, suggesting very slow spin relaxations are occurring. Measurements on
samples diluted with nonmagnetic Y3+ or Lu3+ and complementary measurements on
pure and diluted Dy2Ti2O7 strongly suggest that the relaxation is associated
with dipolar spin correlations, representing unusual cooperative behavior in a
paramagnetic system.Comment: Accepted for publication in Physical Review Letter
- …