31,321 research outputs found
Poisoning of Hydrogen Dissociation at Pd (100) by Adsorbed Sulfur Studied by ab initio Quantum Dynamics and ab initio Molecular Dynamics
We report calculations of the dissociative adsorption of H_2 at Pd (100)
covered with 1/4 monolayer of sulfur using quantum dynamics as well as
molecular dynamics and taking all six degrees of freedom of the two H atoms
fully into account. The ab initio potential-energy surface (PES) is found to be
very strongly corrugated. In particular we discuss the influence of tunneling,
zero-point vibrations, localization of the nuclei's wave function when narrow
valleys of the PES are passed, steering of the approaching H_2 molecules
towards low energy barrier configurations, and the time scales of the center of
mass motion and the other degrees of freedom. Several ``established'' concepts,
which were derived from low-dimensional dynamical studies, are shown to be not
valid.Comment: 4 pages, 3 figures, submitted to Surf. Sci. Lett. Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Gauge Fields and Space-Time
In this article I attempt to collect some ideas,opinions and formulae which
may be useful in solving the problem of gauge/ string / space-time
correspondence This includes the validity of D-brane representation, counting
of gauge-invariant words, relations between the null states and the Yang-Mills
equations and the discussion of the strong coupling limit of the string sigma
model. The article is based on the talk given at the "Odyssey 2001" conference.Comment: 20 page
Gauging the three-nucleon spectator equation
We derive relativistic three-dimensional integral equations describing the
interaction of the three-nucleon system with an external electromagnetic field.
Our equations are unitary, gauge invariant, and they conserve charge. This has
been achieved by applying the recently introduced gauging of equations method
to the three-nucleon spectator equations where spectator nucleons are always on
mass shell. As a result, the external photon is attached to all possible places
in the strong interaction model, so that current and charge conservation are
implemented in the theoretically correct fashion. Explicit expressions are
given for the three-nucleon bound state electromagnetic current, as well as the
transition currents for the scattering processes
\gamma He3 -> NNN, Nd -> \gamma Nd, and \gamma He3 -> Nd. As a result, a
unified covariant three-dimensional description of the NNN-\gamma NNN system is
achieved.Comment: 23 pages, REVTeX, epsf, 4 Postscript figure
Emergent bipartiteness in a society of knights and knaves
We propose a simple model of a social network based on so-called
knights-and-knaves puzzles. The model describes the formation of networks
between two classes of agents where links are formed by agents introducing
their neighbours to others of their own class. We show that if the proportion
of knights and knaves is within a certain range, the network self-organizes to
a perfectly bipartite state. However, if the excess of one of the two classes
is greater than a threshold value, bipartiteness is not observed. We offer a
detailed theoretical analysis for the behaviour of the model, investigate its
behaviou r in the thermodynamic limit, and argue that it provides a simple
example of a topology-driven model whose behaviour is strongly reminiscent of a
first-order phase transitions far from equilibrium.Comment: 12 pages, 5 figure
Formation of nanosized strontium substituted hydroxyapatites
Incorporation of specific elements into calcium phosphates offers the combination of a bioactive material and a therapeutic effect. This is important for improving the integration of implants as well as treating medical conditions. Strontium is a suitable candidate and displays the ability to stimulate bone growth and reducing bone resorption. This study investigated the formation of strontium carbonated hydroxyapatite nanoparticles from an amorphous phase. Crystallization of carbonated hydroxyapatite occurred at 585 oC, but samples with an intended 25% and 75% replacement of calcium with strontium crystallized at 624 oC. Heat treatment at the crystallization temperature revealed that strontium free apatite does not crystallize in 5 minutes, but an increasing strontium concentration leads to a higher rate of crystallization. X-ray diffraction patterns suggest that it may be difficult to include strontium, but higher strontium concentrations are possibly included with ease in the lattice. This work has produced a nanosized apatite accompanied by an amorphous phase after a short heat-treatment time. This offers a range of features that collectively show great promise for significantly enhancing the release of strontium for improved bone therapeutic effects
The stability of the spectator, Dirac, and Salpeter equations for mesons
Mesons are made of quark-antiquark pairs held together by the strong force.
The one channel spectator, Dirac, and Salpeter equations can each be used to
model this pairing. We look at cases where the relativistic kernel of these
equations corresponds to a time-like vector exchange, a scalar exchange, or a
linear combination of the two. Since the model used in this paper describes
mesons which cannot decay physically, the equations must describe stable
states. We find that this requirement is not always satisfied, and give a
complete discussion of the conditions under which the various equations give
unphysical, unstable solutions
Calculating the Rest Tension for a Polymer of String Bits
We explore the application of approximation schemes from many body physics,
including the Hartree-Fock method and random phase approximation (RPA), to the
problem of analyzing the low energy excitations of a polymer chain made up of
bosonic string bits. We accordingly obtain an expression for the rest tension
of the bosonic relativistic string in terms of the parameters
characterizing the microscopic string bit dynamics. We first derive an exact
connection between the string tension and a certain correlation function of the
many-body string bit system. This connection is made for an arbitrary
interaction potential between string bits and relies on an exact dipole sum
rule. We then review an earlier calculation by Goldstone of the low energy
excitations of a polymer chain using RPA. We assess the accuracy of the RPA by
calculating the first order corrections. For this purpose we specialize to the
unique scale invariant potential, namely an attractive delta function potential
in two (transverse) dimensions. We find that the corrections are large, and
discuss a method for summing the large terms. The corrections to this improved
RPA are roughly 15\%.Comment: 44 pages, phyzzx, psfig required, Univ. of Florida preprint,
UFIFT-HEP-94
Detection of the tagged or untagged photons in acousto-optic imaging of thick highly scattering media by photorefractive adaptive holography
We propose an original adaptive wavefront holographic setup based on the
photorefractive effect (PR), to make real-time measurements of acousto-optic
signals in thick scattering media, with a high flux collection at high rates
for breast tumor detection. We describe here our present state of art and
understanding on the problem of breast imaging with PR detection of the
acousto-optic signal
Electronic, dynamical and superconducting properties of CaBeSi
We report first-principles calculations on the normal and superconducting
state of CaBe(x)Si(2-x) (x=1), in the framework of density functional theory
for superconductors (SCDFT). CaBeSi is isostructural and isoelectronic to MgB2
and this makes possible a direct comparison of the electronic and vibrational
properties and the electron-phonon interaction of the two materials. Despite
the many similarities with MgB2 (e.g. sigma bands at the Fermi level and a
larger Fermi surface nesting), according to our calculations CaBeSi has a very
low critical temperature (Tc ~ 0.4 K, consistent with the experiment). CaBeSi
exhibits a complex gap structure, with three gaps at Fermi level: besides the
two sigma and pi gaps, present also in MgB2, the appearance of a third gap is
related to the anisotropy of the Coulomb repulsion, acting in different way on
the bonding and antibonding electronic pi states.Comment: 6 pages, 5 figure
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