5,665 research outputs found
Formation of van der Waals molecules in buffer gas cooled magnetic traps
We show that a large class of helium-containing cold polar molecules form
readily in a cryogenic buffer gas, achieving densities as high as 10^12 cm^-3.
We explore the spin relaxation of these molecules in buffer gas loaded magnetic
traps, and identify a loss mechanism based on Landau-Zener transitions arising
from the anisotropic hyperfine interaction. Our results show that the recently
observed strong T^6 thermal dependence of spin change in buffer gas trapped
silver (Ag) is accounted for by the formation and spin change of AgHe, thus
providing evidence for molecular formation in a buffer gas trap.Comment: 4 pages, 4 figure
Diffusion-limited deposition of dipolar particles
Deposits of dipolar particles are investigated by means of extensive Monte
Carlo simulations. We found that the effect of the interactions is described by
an initial, non-universal, scaling regime characterized by orientationally
ordered deposits. In the dipolar regime, the order and geometry of the clusters
depend on the strength of the interactions and the magnetic properties are
tunable by controlling the growth conditions. At later stages, the growth is
dominated by thermal effects and the diffusion-limited universal regime
obtains, at finite temperatures. At low temperatures the crossover size
increases exponentially as T decreases and at T=0 only the dipolar regime is
observed.Comment: 5 pages, 4 figure
Universality and Crossover of Directed Polymers and Growing Surfaces
We study KPZ surfaces on Euclidean lattices and directed polymers on
hierarchical lattices subject to different distributions of disorder, showing
that universality holds, at odds with recent results on Euclidean lattices.
Moreover, we find the presence of a slow (power-law) crossover toward the
universal values of the exponents and verify that the exponent governing such
crossover is universal too. In the limit of a 1+epsilon dimensional system we
obtain both numerically and analytically that the crossover exponent is 1/2.Comment: LateX file + 5 .eps figures; to appear on Phys. Rev. Let
High power impulse magnetron sputtering discharges: Instabilities and plasma self-organization
We report on instabilities in high power impulse magnetron sputtering plasmas which are likely to be of the generalized drift wave type. They are characterized by well defined regions of high and low plasma emissivity along the racetrack of the magnetron and cause periodic shifts in floating potential. The azimuthal mode number m depends on plasma current, plasma density, and gas pressure. The structures rotate in × direction at velocities of ∼10 km s−1 and frequencies up to 200 kHz. Collisions with residual gas atoms slow down the rotating wave, whereas increasing ionization degree of the gas and plasma conductivity speeds it up
A Beam Driven Plasma-Wakefield Linear Collider: From Higgs Factory to Multi-TeV
Plasma wakefield acceleration (PWFA) holds much promise for advancing the
energy frontier because it can potentially provide a 1000-fold or more increase
in acceleration gradient with excellent power efficiency in respect with
standard technologies. Most of the advances in beam-driven plasma wakefield
acceleration were obtained by a UCLA/USC/SLAC collaboration working at the SLAC
FFTB[ ]. These experiments have shown that plasmas can accelerate and focus
both electron and positron high energy beams, and an accelerating gradient in
excess of 50 GeV/m can be sustained in an 85 cm-long plasma. The FFTB
experiments were essentially proof-of-principle experiments that showed the
great potential of plasma accelerators.
The FACET[ ] test facility at SLAC will in the period 2012-2016 further study
several issues that are directly related to the applicability of PWFA to a
high-energy collider, in particular two-beam acceleration where the witness
beam experiences high beam loading (required for high efficiency), small energy
spread and small emittance dilution (required to achieve luminosity).
The PWFA-LC concept presented in this document is an attempt to find the best
design that takes advantage of the PWFA, identify the critical parameters to be
achieved and eventually the necessary R&D to address their feasibility. It best
benefits from the extensive R&D that has been performed for conventional rf
linear colliders during the last twenty years, especially ILC[ ] and CLIC[ ],
with a potential for a comparably lower power consumption and cost.Comment: Submitted to the proceedings of the Snowmass Process CSS2013. Work
supported in part by the U.S. Department of Energy under contract number
DE-AC02-76SF0051
Levy-Nearest-Neighbors Bak-Sneppen Model
We study a random neighbor version of the Bak-Sneppen model, where "nearest
neighbors" are chosen according to a probability distribution decaying as a
power-law of the distance from the active site, P(x) \sim |x-x_{ac
}|^{-\omega}. All the exponents characterizing the self-organized critical
state of this model depend on the exponent \omega. As \omega tends to 1 we
recover the usual random nearest neighbor version of the model. The pattern of
results obtained for a range of values of \omega is also compatible with the
results of simulations of the original BS model in high dimensions. Moreover,
our results suggest a critical dimension d_c=6 for the Bak-Sneppen model, in
contrast with previous claims.Comment: To appear on Phys. Rev. E, Rapid Communication
Finite size effects in nonequilibrium wetting
Models with a nonequilibrium wetting transition display a transition also in
finite systems. This is different from nonequilibrium phase transitions into an
absorbing state, where the stationary state is the absorbing one for any value
of the control parameter in a finite system. In this paper, we study what kind
of transition takes place in finite systems of nonequilibrium wetting models.
By solving exactly a microscopic model with three and four sites and performing
numerical simulations we show that the phase transition taking place in a
finite system is characterized by the average interface height performing a
random walk at criticality and does not discriminate between the bounded-KPZ
classes and the bounded-EW class. We also study the finite size scaling of the
bKPZ universality classes, showing that it presents peculiar features in
comparison with other universality classes of nonequilibrium phase transitions.Comment: 14 pages, 6figures, major change
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