1,536 research outputs found
A Freedom Forum Presentation: What Is Communism?
https://digitalcommons.acu.edu/crs_books/1490/thumbnail.jp
Thermal Re-emission Model
Starting from a continuum description, we study the non-equilibrium
roughening of a thermal re-emission model for etching in one and two spatial
dimensions. Using standard analytical techniques, we map our problem to a
generalized version of an earlier non-local KPZ (Kardar-Parisi-Zhang) model. In
2+1 dimensions, the values of the roughness and the dynamic exponents
calculated from our theory go like and in 1+1
dimensions, the exponents resemble the KPZ values for low vapor pressure,
supporting experimental results. Interestingly, Galilean invariance is
maintained althrough.Comment: 4 pages, minor textual corrections and typos, accepted in Physical
Review B (rapid
Current-Induced Step Bending Instability on Vicinal Surfaces
We model an apparent instability seen in recent experiments on current
induced step bunching on Si(111) surfaces using a generalized 2D BCF model,
where adatoms have a diffusion bias parallel to the step edges and there is an
attachment barrier at the step edge. We find a new linear instability with
novel step patterns. Monte Carlo simulations on a solid-on-solid model are used
to study the instability beyond the linear regime.Comment: 4 pages, 4 figure
Multiscale Kinetic Monte-Carlo for Simulating Epitaxial Growth
We present a fast Monte-Carlo algorithm for simulating epitaxial surface
growth, based on the continuous-time Monte-Carlo algorithm of Bortz, Kalos and
Lebowitz. When simulating realistic growth regimes, much computational time is
consumed by the relatively fast dynamics of the adatoms. Continuum and
continuum-discrete hybrid methods have been developed to approach this issue;
however in many situations, the density of adatoms is too low to efficiently
and accurately simulate as a continuum. To solve the problem of fast adatom
dynamics, we allow adatoms to take larger steps, effectively reducing the
number of transitions required. We achieve nearly a factor of ten speed up, for
growth at moderate temperatures and large D/F.Comment: 7 pages, 6 figures; revised text, accepted by PR
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An overview of ISCAT 2000
The Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) took place over the timer period of 15 November to 31 December in the year 2000. The study location was the Amundsen Scott Station in Antarctica. ISCAT 2000 defines the second phase of a program designed to explore tropospheric chemistry in Antarctica. As in 1998, the 2000 ISCAT study revealed a strong oxidizing environment at South Pole (SP). During the 2000 investigation, however, the suite of measurements was greatly expanded. These new measurements established the recycling of reactive nitrogen as a critical component of this unique environment. This paper first presents the historical background leading up to the ISCAT 2000 observations; then it focuses on providing a summary of the year 2000 results and contrasts these with those recorded during 1998. Important developments made during the 2000 study included the recording of SP data for several species being emitted from the snowpack. These included NO, H 2O2 and CH2O. In this context, eddy-diffusion flux measurements provided the first quantitative estimates of the SP NO and NOx snow-to-atmosphere fluxes. This study also revealed that HNO 3 and HO2NO2 were major sink species for HOx and NOx radicals. And, it identified the critical factors responsible for SP NO levels exceeding those at other polar sites by nearly an order of magnitude. Finally, it reports on the levels of gas phase sulfur species and provides evidence indicating that the absence of DMS at SP is most likely due to its greatly shorten chemical lifetime in the near vicinity of the plateau. It is proposed that this is due to the influence of NO on the distribution of OH in the lower free troposphere over a region that extends well beyond the plateau itself. Details related to each of the above findings plus others can be found in the 11 accompanying Special Issue papers. © 2004 Elsevier Ltd. All rights reserved
Precision Measurement of the Radiative Decay of the Free Neutron
The standard model predicts that, in addition to a proton, an electron, and
an antineutrino, a continuous spectrum of photons is emitted in the
decay of the free neutron. We report on the RDK II experiment which measured
the photon spectrum using two different detector arrays. An annular array of
bismuth germanium oxide scintillators detected photons from 14 to 782~keV. The
spectral shape was consistent with theory, and we determined a branching ratio
of 0.00335 0.00005 [stat] 0.00015 [syst]. A second detector array
of large area avalanche photodiodes directly detected photons from 0.4 to
14~keV. For this array, the spectral shape was consistent with theory, and the
branching ratio was determined to be 0.00582 0.00023 [stat] 0.00062
[syst]. We report the first precision test of the shape of the photon energy
spectrum from neutron radiative decay and a substantially improved
determination of the branching ratio over a broad range of photon energies
Level Set Approach to Reversible Epitaxial Growth
We generalize the level set approach to model epitaxial growth to include
thermal detachment of atoms from island edges. This means that islands do not
always grow and island dissociation can occur. We make no assumptions about a
critical nucleus. Excellent quantitative agreement is obtained with kinetic
Monte Carlo simulations for island densities and island size distributions in
the submonolayer regime.Comment: 7 pages, 9 figure
Fabrication and evaluation of superplastically formed/weld-brazed corrugated compression panels with beaded webs
A study was made to investigate the feasibility of superplastically forming corrugated panels with beaded webs and to demonstrate the structural integrity of these panels by testing. The test panels in the study consist of superplastically formed titanium alloy Ti-6Al-4V half-hat elements that are joined by weld-brazing to titanium alloy Ti-6Al-4V caps to form either single-corrugation compression panels or multiple-corrugation compression panels. Stretching and subsequent thinning of the titanium sheet during superplastic forming is reduced by approximately 35 percent with a shallow half-hat die concept instead of a deep die concept and results in a more uniform thickness across the beaded webs. The complete panels are tested in end compression at room temperature and the results compared with analysis. The heavily loaded panels failed at loads approaching the yield strength of the titanium material. At maximum load, the caps wrinkled locally accompanied with separation of the weld-braze joint in the wrinkle. None of the panels tested, however, failed catastrophically in the weld-braze joint. Experimental test results are in good agreement with structural analysis of the panels
Stress distribution and the fragility of supercooled melts
We formulate a minimal ansatz for local stress distribution in a solid that
includes the possibility of strongly anharmonic short-length motions. We
discover a broken-symmetry metastable phase that exhibits an aperiodic,
frozen-in stress distribution. This aperiodic metastable phase is characterized
by many distinct, nearly degenerate configurations. The activated transitions
between the configurations are mapped onto the dynamics of a long range
classical Heisenberg model with 6-component spins and anisotropic couplings. We
argue the metastable phase corresponds to a deeply supercooled non-polymeric,
non-metallic liquid, and further establish an order parameter for the
glass-to-crystal transition. The spin model itself exhibits a continuous range
of behaviors between two limits corresponding to frozen-in shear and uniform
compression/dilation respectively. The two regimes are separated by a
continuous transition controlled by the anisotropy in the spin-spin
interaction, which is directly related to the Poisson ratio of the
material. The latter ratio and the ultra-violet cutoff of the theory determine
the liquid configurational entropy. Our results suggest that liquid's fragility
depends on the Poisson ratio in a non-monotonic way. The present ansatz
provides a microscopic framework for computing the configurational entropy and
relaxational spectrum of specific substances.Comment: 11 pages, 5 figures, Final version published in J Phys Chem
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