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A variable timestep generalized Runge-Kutta method for the numerical integration of the space-time diffusion equations
A generalized Runge-Kutta method has been employed in the numerical integration of the stiff space-time diffusion equations. The method is fourth-order accurate, using an embedded third-order solution to arrive at an estimate of the truncation error for automatic timestep control. The efficiency of the Runge-Kutta method is enhanced by a block-factorization technique that exploits the sparse structure of the matrix system resulting from the space and energy discretized form of the time-dependent neutron diffusion equations. Preliminary numerical evaluation using a one-dimensional finite difference code shows the sparse matrix implementation of the generalized Runge-Kutta method to be highly accurate and efficient when compared to an optimized iterative theta method. 12 refs., 5 figs., 4 tabs
Magnetoconductivity of Hubbard bands induced in Silicon MOSFETs
Sodium impurities are diffused electrically to the oxide-semiconductor
interface of a silicon MOSFET to create an impurity band. At low temperature
and at low electron density, the band is split into an upper and a lower
sections under the influence of Coulomb interactions. We used
magnetoconductivity measurements to provide evidence for the existence of
Hubbard bands and determine the nature of the states in each band.Comment: In press in Physica
Surfactant effect in heteroepitaxial growth. The Pb - Co/Cu(111) case
A MonteCarlo simulations study has been performed in order to study the
effect of Pb as surfactant on the initial growth stage of Co/Cu(111). The main
characteristics of Co growing over Cu(111) face, i.e. the decorated double
layer steps, the multiple layer islands and the pools of vacancies, disappear
with the pre-evaporation of a Pb monolayer. Through MC simulations, a full
picture of these complex processes is obtained. Co quickly diffuses through the
Pb monolayer exchanging place with Cu atoms at the substrate. The exchange
process diffusion inhibits the formation of pure Co islands, reducing the
surface stress and then the formation of multilayer islands and the pools of
vacancies. On the other hand, the random exchange also suppress the nucleation
preferential sites generated by Co atoms at Cu steps, responsible of the step
decoration.Comment: 4 pages, latex, 2 figures embedded in the tex
Meditation-induced bliss viewed as release from conditioned neural (thought) patterns that block reward signals in the brain pleasure center
The nucleus accumbens orchestrates processes related to reward and pleasure,
including the addictive consequences of repeated reward (e.g., drug addiction and
compulsive gambling) and the accompanying feelings of craving and anhedonia.
The neurotransmitters dopamine and endogenous opiates play interactive roles in
these processes. They are released by natural rewards (i.e., food, water, sex, money,
play, etc.) and are released or mimicked by drugs of abuse. Repeated drug use
induces conditioned down-regulation of these neurotransmitters, thus causing
painful suppression of everyday pleasure. As with many spiritual traditions,
Buddhism provides strong advice against the pursuit of worldly pleasures to
attain the ‘‘good life.’’ In contrast, many forms of meditation give rise to an
immense and abiding joy. Most of these practices involve ‘‘stilling the mind,’’
whereby all content-laden thought (e.g., fantasies, daydreams, plans) ceases, and
the mind enters a state of openness, formlessness, clarity, and bliss. This can be
explained by the Buddhist suggestion that almost all of our everyday thoughts are
a form of addiction. It follows that if we turn off this internal ‘‘gossip of ego,’’ we
will find relief from the biochemical dopamine/opiate down-regulation, which is,
perhaps, the perpetual concomitant of our daily rumination
Segregation to and structure of [001] twist grain boundaries in Cu---Ni alloys
The segregation, thermodynamic, and structural properties of [001] twist boundaries in Cu---Ni alloys have been examined within a wide range of misorientations and temperatures. Cu always segregates to the boundary. The concentration of the first layer adjacent to the boundary increases monotonically with misorientation and no obvious cusps are observed. All other thermodynamic properties vary smoothly with the misorientation, with the exception of the vibrational entropy of the boundaries without segregation. The unsegregated vibrational entropy shows a large peak at the misorientation corresponding to the [Sigma]17 boundary and two minima around the [Sigma]13 and [Sigma]5 boundary orientations. The concentration distribution within the plane of the grain boundaries can be described by the same structural unit model established for [001] twist boundaries in pure materials. Regions of large tensile stress show greater segregation than do regions of compressive stress. Regions of large shear stress tend to show reduced segragation compared with regions of small shear stress.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30624/1/0000265.pd
An Effective-Medium Tight-Binding Model for Silicon
A new method for calculating the total energy of Si systems is presented. The
method is based on the effective-medium theory concept of a reference system.
Instead of calculating the energy of an atom in the system of interest a
reference system is introduced where the local surroundings are similar. The
energy of the reference system can be calculated selfconsistently once and for
all while the energy difference to the reference system can be obtained
approximately. We propose to calculate it using the tight-binding LMTO scheme
with the Atomic-Sphere Approximation(ASA) for the potential, and by using the
ASA with charge-conserving spheres we are able to treat open system without
introducing empty spheres. All steps in the calculational method is {\em ab
initio} in the sense that all quantities entering are calculated from first
principles without any fitting to experiment. A complete and detailed
description of the method is given together with test calculations of the
energies of phonons, elastic constants, different structures, surfaces and
surface reconstructions. We compare the results to calculations using an
empirical tight-binding scheme.Comment: 26 pages (11 uuencoded Postscript figures appended), LaTeX,
CAMP-090594-
Standard‐space atlas of the viscoelastic properties of the human brain
Standard anatomical atlases are common in neuroimaging because they facilitate data analyses and comparisons across subjects and studies. The purpose of this study was to develop a standardized human brain atlas based on the physical mechanical properties (i.e., tissue viscoelasticity) of brain tissue using magnetic resonance elastography (MRE). MRE is a phase contrast-based MRI method that quantifies tissue viscoelasticity noninvasively and in vivo thus providing a macroscopic representation of the microstructural constituents of soft biological tissue. The development of standardized brain MRE atlases are therefore beneficial for comparing neural tissue integrity across populations. Data from a large number of healthy, young adults from multiple studies collected using common MRE acquisition and analysis protocols were assembled (N = 134; 78F/ 56 M; 18–35 years). Nonlinear image registration methods were applied to normalize viscoelastic property maps (shear stiffness, μ, and damping ratio, ξ) to the MNI152 standard structural template within the spatial coordinates of the ICBM-152. We find that average MRE brain templates contain emerging and symmetrized anatomical detail. Leveraging the substantial amount of data assembled, we illustrate that subcortical gray matter structures, white matter tracts, and regions of the cerebral cortex exhibit differing mechanical characteristics. Moreover, we report sex differences in viscoelasticity for specific neuroanatomical structures, which has implications for understanding patterns of individual differences in health and disease. These atlases provide reference values for clinical investigations as well as novel biophysical signatures of neuroanatomy. The templates are made openly available (github.com/mechneurolab/mre134) to foster collaboration across research institutions and to support robust cross-center comparisons
New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
(abridged) The heating mechanism at high densities during M dwarf flares is
poorly understood. Spectra of M dwarf flares in the optical and
near-ultraviolet wavelength regimes have revealed three continuum components
during the impulsive phase: 1) an energetically dominant blackbody component
with a color temperature of T 10,000 K in the blue-optical, 2) a smaller
amount of Balmer continuum emission in the near-ultraviolet at lambda 3646
Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer
lines. These properties are not reproduced by models that employ a typical
"solar-type" flare heating level in nonthermal electrons, and therefore our
understanding of these spectra is limited to a phenomenological interpretation.
We present a new 1D radiative-hydrodynamic model of an M dwarf flare from
precipitating nonthermal electrons with a large energy flux of erg
cm s. The simulation produces bright continuum emission from a
dense, hot chromospheric condensation. For the first time, the observed color
temperature and Balmer jump ratio are produced self-consistently in a
radiative-hydrodynamic flare model. We find that a T 10,000 K
blackbody-like continuum component and a small Balmer jump ratio result from
optically thick Balmer and Paschen recombination radiation, and thus the
properties of the flux spectrum are caused by blue light escaping over a larger
physical depth range compared to red and near-ultraviolet light. To model the
near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer
lines, we include the extra Balmer continuum opacity from Landau-Zener
transitions that result from merged, high order energy levels of hydrogen in a
dense, partially ionized atmosphere. This reveals a new diagnostic of ambient
charge density in the densest regions of the atmosphere that are heated during
dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar
Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015):
updated to include comments by Guest Editor. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-
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