275 research outputs found
Development of a Kohn-Sham like potential in the Self-Consistent Atomic Deformation Model
This is a brief description of how to derive the local ``atomic'' potentials
from the Self-Consistent Atomic Deformation (SCAD) model density function.
Particular attention is paid to the spherically averaged case.Comment: 5 Pages, LaTeX, no figure
Qualitative treatment-subgroup interactions in a randomized clinical trial of treatments for adolescents with ADHD:Exploring what cognitive-behavioral treatment works for whom
On a Testing Methodology for the Mechanical Property Assessment of a New Low-Cost Titanium Alloy Derived from Synthetic Rutile
Mechanical property data of a low-cost titanium alloy derived directly from synthetic rutile is reported. A small-scale testing approach comprising consolidation via field-assisted sintering technology, followed by axisymmetric compression testing, has been designed to yield mechanical property data from small quantities of titanium alloy powder. To validate this approach and provide a benchmark, Ti-6Al-4V powder has been processed using the same methodology and compared with material property data generated from thermo-physical simulation software. Compressive yield strength and strain to failure of the synthetic rutile-derived titanium alloy were revealed to be similar to that of Ti-6Al-4V
Properties and Performance of Two Wide Field of View Cherenkov/Fluorescence Telescope Array Prototypes
A wide field of view Cherenkov/fluorescence telescope array is one of the
main components of the Large High Altitude Air Shower Observatory project. To
serve as Cherenkov and fluorescence detectors, a flexible and mobile design is
adopted for easy reconfiguring of the telescope array. Two prototype telescopes
have been constructed and successfully run at the site of the ARGO-YBJ
experiment in Tibet. The features and performance of the telescopes are
presented
Casimir interaction between two concentric cylinders: exact versus semiclassical results
The Casimir interaction between two perfectly conducting, infinite,
concentric cylinders is computed using a semiclassical approximation that takes
into account families of classical periodic orbits that reflect off both
cylinders. It is then compared with the exact result obtained by the
mode-by-mode summation technique. We analyze the validity of the semiclassical
approximation and show that it improves the results obtained through the
proximity theorem.Comment: 28 pages, 5 figures include
Phase transitions in BaTiO from first principles
We develop a first-principles scheme to study ferroelectric phase transitions
for perovskite compounds. We obtain an effective Hamiltonian which is fully
specified by first-principles ultra-soft pseudopotential calculations. This
approach is applied to BaTiO, and the resulting Hamiltonian is studied
using Monte Carlo simulations. The calculated phase sequence, transition
temperatures, latent heats, and spontaneous polarizations are all in good
agreement with experiment. The order-disorder vs.\ displacive character of the
transitions and the roles played by different interactions are discussed.Comment: 13 page
Molecular dynamics simulation of the order-disorder phase transition in solid NaNO
We present molecular dynamics simulations of solid NaNO using pair
potentials with the rigid-ion model. The crystal potential surface is
calculated by using an \emph{a priori} method which integrates the \emph{ab
initio} calculations with the Gordon-Kim electron gas theory. This approach is
carefully examined by using different population analysis methods and comparing
the intermolecular interactions resulting from this approach with those from
the \emph{ab initio} Hartree-Fock calculations. Our numerics shows that the
ferroelectric-paraelectric phase transition in solid NaNO is triggered by
rotation of the nitrite ions around the crystallographical c axis, in agreement
with recent X-ray experiments [Gohda \textit{et al.}, Phys. Rev. B \textbf{63},
14101 (2000)]. The crystal-field effects on the nitrite ion are also addressed.
Remarkable internal charge-transfer effect is found.Comment: RevTeX 4.0, 11 figure
Calculating Casimir Energies in Renormalizable Quantum Field Theory
Quantum vacuum energy has been known to have observable consequences since
1948 when Casimir calculated the force of attraction between parallel uncharged
plates, a phenomenon confirmed experimentally with ever increasing precision.
Casimir himself suggested that a similar attractive self-stress existed for a
conducting spherical shell, but Boyer obtained a repulsive stress. Other
geometries and higher dimensions have been considered over the years. Local
effects, and divergences associated with surfaces and edges have been studied
by several authors. Quite recently, Graham et al. have re-examined such
calculations, using conventional techniques of perturbative quantum field
theory to remove divergences, and have suggested that previous self-stress
results may be suspect. Here we show that the examples considered in their work
are misleading; in particular, it is well-known that in two dimensions a
circular boundary has a divergence in the Casimir energy for massless fields,
while for general dimension not equal to an even integer the corresponding
Casimir energy arising from massless fields interior and exterior to a
hyperspherical shell is finite. It has also long been recognized that the
Casimir energy for massive fields is divergent for . These conclusions
are reinforced by a calculation of the relevant leading Feynman diagram in
and three dimensions. There is therefore no doubt of the validity of the
conventional finite Casimir calculations.Comment: 25 pages, REVTeX4, 1 ps figure. Revision includes new subsection 4B
and Appendix, and other minor correction
Local and Global Casimir Energies: Divergences, Renormalization, and the Coupling to Gravity
From the beginning of the subject, calculations of quantum vacuum energies or
Casimir energies have been plagued with two types of divergences: The total
energy, which may be thought of as some sort of regularization of the
zero-point energy, , seems manifestly divergent. And
local energy densities, obtained from the vacuum expectation value of the
energy-momentum tensor, , typically diverge near
boundaries. The energy of interaction between distinct rigid bodies of whatever
type is finite, corresponding to observable forces and torques between the
bodies, which can be unambiguously calculated. The self-energy of a body is
less well-defined, and suffers divergences which may or may not be removable.
Some examples where a unique total self-stress may be evaluated include the
perfectly conducting spherical shell first considered by Boyer, a perfectly
conducting cylindrical shell, and dilute dielectric balls and cylinders. In
these cases the finite part is unique, yet there are divergent contributions
which may be subsumed in some sort of renormalization of physical parameters.
The divergences that occur in the local energy-momentum tensor near surfaces
are distinct from the divergences in the total energy, which are often
associated with energy located exactly on the surfaces. However, the local
energy-momentum tensor couples to gravity, so what is the significance of
infinite quantities here? For the classic situation of parallel plates there
are indications that the divergences in the local energy density are consistent
with divergences in Einstein's equations; correspondingly, it has been shown
that divergences in the total Casimir energy serve to precisely renormalize the
masses of the plates, in accordance with the equivalence principle.Comment: 53 pages, 1 figure, invited review paper to Lecture Notes in Physics
volume in Casimir physics edited by Diego Dalvit, Peter Milonni, David
Roberts, and Felipe da Ros
Molecular Dynamics Simulation of Nanoindentation-induced Mechanical Deformation and Phase Transformation in Monocrystalline Silicon
This work presents the molecular dynamics approach toward mechanical deformation and phase transformation mechanisms of monocrystalline Si(100) subjected to nanoindentation. We demonstrate phase distributions during loading and unloading stages of both spherical and Berkovich nanoindentations. By searching the presence of the fifth neighboring atom within a non-bonding length, Si-III and Si-XII have been successfully distinguished from Si-I. Crystallinity of this mixed-phase was further identified by radial distribution functions
- …