7,655 research outputs found
Ionization of Rydberg atoms embedded in an ultracold plasma
We have studied the behavior of cold Rydberg atoms embedded in an ultracold
plasma. We demonstrate that even deeply bound Rydberg atoms are completely
ionized in such an environment, due to electron collisions. Using a fast pulse
extraction of the electrons from the plasma we found that the number of excess
positive charges, which is directly related to the electron temperature Te, is
not strongly affected by the ionization of the Rydberg atoms. Assuming a
Michie-King equilibrium distribution, in analogy with globular star cluster
dynamics, we estimate Te. Without concluding on heating or cooling of the
plasma by the Rydberg atoms, we discuss the range for changing the plasma
temperature by adding Rydberg atoms.Comment: To be published in P.R.
Elementary derivation of Spitzer's asymptotic law for Brownian windings and some of its physical applications
A simple derivation of Spitzer'z asymptotic law for Brownian windings
[Trans.Am.Math.Soc.87,187 (1958)]is presented along with its generalizations
>.These include the cases of planar Brownian walks interacting with a single
puncture and Brownian walks on a single truncated cone with variable conical
angle interacting with the truncated conical tip.Such situations are typical in
the theories of quantum Hall effect and 2+1 quantum gravity, respectively .They
also have some applications in polymer physic
Transport of interface states in the Heisenberg chain
We demonstrate the transport of interface states in the one-dimensional
ferromagnetic Heisenberg model by a time dependent magnetic field. Our analysis
is based on the standard Adiabatic Theorem. This is supplemented by a numerical
analysis via the recently developed time dependent DMRG method, where we
calculate the adiabatic constant as a function of the strength of the magnetic
field and the anisotropy of the interaction.Comment: minor revision, final version; 13 pages, 4 figure
Excimer laser annealing for fabrication of low-cost solar cells
Pulsed excimer laser annealing was successfully performed using a 50 w laser. Both polished and texturized cells were tried, however, there are serious problems with nonuniformity on texturized cells. A number of cells were produced and compared to diffusion furnace annealed cells. There was no clear economic advantage in using an excimer laser and there was a small penalty on average efficiency. The conclusion was that the excimer laser anneal process must be able to produce superior cells to be considered as a viable process option
Partially asymmetric exclusion models with quenched disorder
We consider the one-dimensional partially asymmetric exclusion process with
random hopping rates, in which a fraction of particles (or sites) have a
preferential jumping direction against the global drift. In this case the
accumulated distance traveled by the particles, x, scales with the time, t, as
x ~ t^{1/z}, with a dynamical exponent z > 0. Using extreme value statistics
and an asymptotically exact strong disorder renormalization group method we
analytically calculate, z_{pt}, for particlewise (pt) disorder, which is argued
to be related to the dynamical exponent for sitewise (st) disorder as
z_{st}=z_{pt}/2. In the symmetric situation with zero mean drift the particle
diffusion is ultra-slow, logarithmic in time.Comment: 4 pages, 3 figure
Mechanisms of the Vertical Secular Heating of a Stellar Disk
We investigate the nonlinear growth stages of bending instability in stellar
disks with exponential radial density profiles.We found that the unstable modes
are global (the wavelengths are larger than the disk scale lengths) and that
the instability saturation level is much higher than that following from a
linear criterion. The instability saturation time scales are of the order of
one billion years or more. For this reason, the bending instability can play an
important role in the secular heating of a stellar disk in the direction.
In an extensive series of numerical -body simulations with a high spatial
resolution, we were able to scan in detail the space of key parameters (the
initial disk thickness , the Toomre parameter , and the ratio of dark
halo mass to disk mass ). We revealed three distinct
mechanisms of disk heating in the direction: bending instability of the
entire disk, bending instability of the bar, and heating on vertical
inhomogeneities in the distribution of stellar matter.Comment: 22 pages including 8 figures. To be published in Astronomy Letters
(v.29, 2003
Heating Hot Atmospheres with Active Galactic Nuclei
High resolution X-ray spectroscopy of the hot gas in galaxy clusters has
shown that the gas is not cooling to low temperatures at the predicted rates of
hundreds to thousands of solar masses per year. X-ray images have revealed
giant cavities and shock fronts in the hot gas that provide a direct and
relatively reliable means of measuring the energy injected into hot atmospheres
by active galactic nuclei (AGN). Average radio jet powers are near those
required to offset radiative losses and to suppress cooling in isolated giant
elliptical galaxies, and in larger systems up to the richest galaxy clusters.
This coincidence suggests that heating and cooling are coupled by feedback,
which suppresses star formation and the growth of luminous galaxies. How jet
energy is converted to heat and the degree to which other heating mechanisms
are contributing, eg. thermal conduction, are not well understood. Outburst
energies require substantial late growth of supermassive black holes. Unless
all of the approximately 10E62 erg required to suppress star formation is
deposited in the cooling regions of clusters, AGN outbursts must alter
large-scale properties of the intracluster medium.Comment: 60 pages, 12 figures, to appear in 1997 Annual Reviews of Astronomy
and Astrophysics. This version supersedes the April 2007 version in Reviews
in Advance (references and minor corrections were added), and is similar to
the one scheduled to appear in Volume 45 of ARA
Induced two-photon decay of the 2s level and the rate of cosmological hydrogen recombination
Induced emission due to the presence of soft CMB photons slightly increases
the two-photon decay rate of the 2s level of hydrogen defining the rate of
cosmological recombination. This correspondingly changes the degree of
ionization, the visibility function and the resulting primordial temperature
anisotropies and polarization of the CMB on the percent level. These changes
exceed the precision of the widely used CMBFAST and CAMB codes by more than one
order of magnitude and can be easily taken into account.Comment: 5 pages, 5 figure, accepted by Astronomy and Astrophysic
Determination of complex dielectric functions of ion implanted and implantedāannealed amorphous silicon by spectroscopic ellipsometry
Measuring with a spectroscopic ellipsometer (SE) in the 1.8ā4.5 eV photon energy region we determined the complex dielectric function (Ļµ = Ļµ1 + iĻµ2) of different kinds of amorphous silicon prepared by selfāimplantation and thermal relaxation (500āĀ°C, 3 h). These measurements show that the complex dielectric function (and thus the complex refractive index) of implanted aāSi (iāaāSi) differs from that of relaxed (annealed) aāSi (rāaāSi). Moreover, its Ļµ differs from the Ļµ of evaporated aāSi (eāaāSi) found in the handbooks as Ļµ for aāSi. If we use this Ļµ to evaluate SE measurements of ion implanted silicon then the fit is very poor. We deduced the optical band gap of these materials using the DavisāMott plot based on the relation: (Ļµ2E2)1/3 ā¼ (Eā Eg). The results are: 0.85 eV (iāaāSi), 1.12 eV (eāaāSi), 1.30 eV (rāaāSi). We attribute the optical change to annihilation of point defects
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