10,912 research outputs found
Low energy universality and scaling of Van der Waals forces
At long distances interactions between neutral ground state atoms can be
described by the Van der Waals potential V(r) =-C6/r^6-C8/r^8 - ... . In the
ultra-cold regime atom-atom scattering is dominated by s-waves phase shifts
given by an effective range expansion p cot d0 (p) = -1/a0 + r0 p^2/2 + ... in
terms of the scattering length a0 and the effective range r0. We show that
while for these potentials the scattering length cannot be predicted, the
effective range is given by the universal low energy theorem r0 = A + B/a0+
C/a0^2 where A,B and C depend on the dispersion coefficients Cn and the reduced
di-atom mass. We confront this formula to about a hundred determinations of r0
and a0 and show why the result is dominated by the leading dispersion
coefficient C6. Universality and scaling extends much beyond naive dimensional
analysis estimates.Comment: 4 pages, 3 figure
Enhancement and evaluation of Skylab photography for potential land use inventories, part 1
The author has identified the following significant results. Three sites were evaluated for land use inventory: Finger Lakes - Tompkins County, Lower Hudson Valley - Newburgh, and Suffolk County - Long Island. Special photo enhancement processes were developed to standardize the density range and contrast among S190A negatives. Enhanced black and white enlargements were converted to color by contact printing onto diazo film. A color prediction model related the density values on each spectral band for each category of land use to the spectral properties of the various diazo dyes. The S190A multispectral system proved to be almost as effective as the S190B high resolution camera for inventorying land use. Aggregate error for Level 1 averaged about 12% while Level 2 aggregate error averaged about 25%. The S190A system proved to be much superior to LANDSAT in inventorying land use, primarily because of increased resolution
Fine-Structure FeII* Emission and Resonant MgII Emission in z = 1 Star-Forming Galaxies
We present a study of the prevalence, strength, and kinematics of ultraviolet
FeII and MgII emission lines in 212 star-forming galaxies at z = 1 selected
from the DEEP2 survey. We find FeII* emission in composite spectra assembled on
the basis of different galaxy properties, indicating that FeII* emission is
prevalent at z = 1. In these composites, FeII* emission is observed at roughly
the systemic velocity. At z = 1, we find that the strength of FeII* emission is
most strongly modulated by dust attenuation, and is additionally correlated
with redshift, star-formation rate, and [OII] equivalent width, such that
systems at higher redshifts with lower dust levels, lower star-formation rates,
and larger [OII] equivalent widths show stronger FeII* emission. We detect MgII
emission in at least 15% of the individual spectra and we find that objects
showing stronger MgII emission have higher specific star-formation rates,
smaller [OII] linewidths, larger [OII] equivalent widths, lower dust
attenuations, and lower stellar masses than the sample as a whole. MgII
emission strength exhibits the strongest correlation with specific
star-formation rate, although we find evidence that dust attenuation and
stellar mass also play roles in the regulation of MgII emission. Future
integral field unit observations of the spatial extent of FeII* and MgII
emission in galaxies with high specific star-formation rates, low dust
attenuations, and low stellar masses will be important for probing the
morphology of circumgalactic gas.Comment: 29 pages, 22 figures, 2 tables; accepted to Ap
Sympathetic cooling and collisional properties of a Rb-Cs mixture
We report on measurements of the collisional properties of a mixture of
Cs and Rb atoms in a magnetic trap at
temperatures. By selectively evaporating the Rb atoms using a radio-frequency
field, we achieved sympathetic cooling of Cs down to a few . The
inter-species collisional cross-section was determined through rethermalization
measurements, leading to an estimate of for the s-wave scattering
length for Rb in the and Cs in the magnetic
states. We briefly speculate on the prospects for reaching Bose-Einstein
condensation of Cs inside a magnetic trap through sympathetic cooling
Geometric Measure of Indistinguishability for Groups of Identical Particles
The concept of p-orthogonality (1=< p =< n) between n-particle states is
introduced. It generalizes common orthogonality, which is equivalent to
n-orthogonality, and strong orthogonality between fermionic states, which is
equivalent to 1-orthogonality. Within the class of non p-orthogonal states a
finer measure of non p-orthogonality is provided by Araki's angles between
p-internal spaces. The p-orthogonality concept is a geometric measure of
indistinguishability that is independent of the representation chosen for the
quantum states. It induces a new hierarchy of approximations for group function
methods. The simplifications that occur in the calculation of matrix elements
between p-orthogonal group functions are presented
Transform-limited pulses are not optimal for resonant multiphoton transitions
Maximizing nonlinear light-matter interactions is a primary motive for
compressing laser pulses to achieve ultrashort transform limited pulses. Here
we show how, by appropriately shaping the pulses, resonant multiphoton
transitions can be enhanced significantly beyond the level achieved by
maximizing the pulse's peak intensity. We demonstrate the counterintuitive
nature of this effect with an experiment in a resonant two-photon absorption,
in which, by selectively removing certain spectral bands, the peak intensity of
the pulse is reduced by a factor of 40, yet the absorption rate is doubled.
Furthermore, by suitably designing the spectral phase of the pulse, we increase
the absorption rate by a factor of 7.Comment: 4 pages, 3 figure
Hot Disks And Delayed Bar Formation
We present observational evidence for the inhibition of bar formation in
dispersion-dominated (dynamically hot) galaxies by studying the relationship
between galactic structure and host galaxy kinematics in a sample of 257
galaxies between 0.1 z 0.84 from the All-Wavelength Extended Groth
Strip International Survey (AEGIS) and the Deep Extragalactic Evolutionary
Probe 2 (DEEP2) survey. We find that bars are preferentially found in galaxies
that are massive and dynamically cold (rotation-dominated) and on the stellar
Tully-Fisher relationship, as is the case for barred spirals in the local
Universe. The data provide at least one explanation for the steep (3)
decline in the overall bar fraction from z=0 to z=0.84 in L and brighter
disks seen in previous studies. The decline in the bar fraction at high
redshift is almost exclusively in the lower mass (10 log M(\Msun)
11), later-type and bluer galaxies. A proposed explanation for this
"downsizing" of the bar formation / stellar structure formation is that the
lower mass galaxies may not form bars because they could be dynamically hotter
than more massive systems from the increased turbulence of accreting gas,
elevated star formation, and/or increased interaction/merger rate at higher
redshifts. The evidence presented here provides observational support for this
hypothesis. However, the data also show that not every disk galaxy that is
massive and cold has a stellar bar, suggesting that mass and dynamic coldness
of a disk are necessary but not sufficient conditions for bar formation -- a
secondary process, perhaps the interaction history between the dark matter halo
and the baryonic matter, may play an important role in bar formation.Comment: In press, ApJ, 13 pages, 5 figures (3 color
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