7,667 research outputs found
Helicity Amplitudes for Charmonium Production in Hadron-Hadron and Photon-Hadron Collisions
We present the gluon-gluon and photon-gluon helicity amplitudes for color
singlet and octet charmonium production in polarized and unpolarized
hadron-hadron and photon-hadron collisions.Comment: 11 pages amstex no figure
Analytical Expression for the RKKY Interaction in Doped Graphene
We obtain an analytical expression for the Ruderman-Kittel-Kasuya-Yosida
(RKKY) interaction in electron or hole doped graphene for linear Dirac
bands. The results agree very well with the numerical calculations for the full
tight-binding band structure in the regime where the linear band structure is
valid. The analytical result, expressed in terms of the Meijer G-function,
consists of a product of two oscillatory terms, one coming from the
interference between the two Dirac cones and the second coming from the finite
size of the Fermi surface. For large distances, the Meijer G-function behaves
as a sinusoidal term, leading to the result for moments located on the same sublattice. The
dependence, which is the same for the standard two-dimensional electron gas, is
universal irrespective of the sublattice location and the distance direction of
the two moments except when (undoped case), where it reverts to the
dependence. These results correct several inconsistencies found in the
literature.Comment: 5 pages, 5 figure
An AC electric trap for ground-state molecules
We here report on the realization of an electrodynamic trap, capable of
trapping neutral atoms and molecules in both low-field and high-field seeking
states. Confinement in three dimensions is achieved by switching between two
electric field configurations that have a saddle-point at the center of the
trap, i.e., by alternating a focusing and a defocusing force in each direction.
AC trapping of 15ND3 molecules is experimentally demonstrated, and the
stability of the trap is studied as a function of the switching frequency. A 1
mK sample of 15ND3 molecules in the high-field seeking component of the
|J,K>=|1,1> level, the ground-state of para-ammonia, is trapped in a volume of
about 1 mm^3
Local structure of liquid carbon controls diamond nucleation
Diamonds melt at temperatures above 4000 K. There are no measurements of the
steady-state rate of the reverse process: diamond nucleation from the melt,
because experiments are difficult at these extreme temperatures and pressures.
Using numerical simulations, we estimate the diamond nucleation rate and find
that it increases by many orders of magnitude when the pressure is increased at
constant supersaturation. The reason is that an increase in pressure changes
the local coordination of carbon atoms from three-fold to four-fold. It turns
out to be much easier to nucleate diamond in a four-fold coordinated liquid
than in a liquid with three-fold coordination, because in the latter case the
free-energy cost to create a diamond-liquid interface is higher. We speculate
that this mechanism for nucleation control is relevant for crystallization in
many network-forming liquids. On the basis of our calculations, we conclude
that homogeneous diamond nucleation is likely in carbon-rich stars and unlikely
in gaseous planets
Slowing heavy, ground-state molecules using an alternating gradient decelerator
Cold supersonic beams of molecules can be slowed down using a switched
sequence of electrostatic field gradients. The energy to be removed is
proportional to the mass of the molecules. Here we report deceleration of YbF,
which is 7 times heavier than any molecule previously decelerated. We use an
alternating gradient structure to decelerate and focus the molecules in their
ground state. We show that the decelerator exhibits the axial and transverse
stability required to bring these molecules to rest. Our work significantly
extends the range of molecules amenable to this powerful method of cooling and
trapping.Comment: 4 pages, 5 figure
Can Polymer Coils be modeled as "Soft Colloids"?
We map dilute or semi-dilute solutions of non-intersecting polymer chains
onto a fluid of ``soft'' particles interacting via a concentration dependent
effective pair potential, by inverting the pair distribution function of the
centers of mass of the initial polymer chains. A similar inversion is used to
derive an effective wall-polymer potential; these potentials are combined to
successfully reproduce the calculated exact depletion interaction induced by
non-intersecting polymers between two walls. The mapping opens up the
possibility of large-scale simulations of polymer solutions in complex
geometries.Comment: 4 pages, 3 figures ReVTeX[epsfig,multicol,amssymb] references update
Phase behavior and structure of model colloid-polymer mixtures confined between two parallel planar walls
Using Gibbs ensemble Monte Carlo simulations and density functional theory we
investigate the fluid-fluid demixing transition in inhomogeneous
colloid-polymer mixtures confined between two parallel plates with separation
distances between one and ten colloid diameters covering the complete range
from quasi two-dimensional to bulk-like behavior. We use the
Asakura-Oosawa-Vrij model in which colloid-colloid and colloid-polymer
interactions are hard-sphere like, whilst the pair potential between polymers
vanishes. Two different types of confinement induced by a pair of parallel
walls are considered, namely either through two hard walls or through two
semi-permeable walls that repel colloids but allow polymers to freely
penetrate. For hard (semi-permeable) walls we find that the capillary binodal
is shifted towards higher (lower) polymer fugacities and lower (higher) colloid
fugacities as compared to the bulk binodal; this implies capillary condensation
(evaporation) of the colloidal liquid phase in the slit. A macroscopic
treatment is provided by a novel symmetric Kelvin equation for general binary
mixtures, based on the proximity in chemical potentials of statepoints at
capillary coexistence and the reference bulk coexistence. Results for capillary
binodals compare well with those obtained from the classic version of the
Kelvin equation due to Evans and Marini Bettolo Marconi [J. Chem. Phys. 86,
7138 (1987)], and are quantitatively accurate away from the fluid-fluid
critical point, even at small wall separations. For hard walls the density
profiles of polymers and colloids inside the slit display oscillations due to
packing effects for all statepoints. For semi-permeable walls either similar
structuring or flat profiles are found, depending on the statepoint considered.Comment: 15 pages, 13 figure
The Gbt 67–93.6 Ghz Spectral Line Survey Of Orion-Kl
We present a 67--93.6 GHz spectral line survey of Orion-KL with the new 4 mm Receiver on the Green Bank Telescope (GBT). The survey reaches unprecedented depths and covers the low-frequency end of the 3 mm atmospheric window which has been relatively unexplored previously. The entire spectral-line survey is published electronically for general use by the astronomical community. The calibration and performance of 4 mm Receiver on the GBT is also summarized
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