80,801 research outputs found
Preliminary testing of a prototype portable X-ray fluorescence spectrometer
A portable X-ray fluorescence spectrometer for use as an analyzer in mineral resource investigative work was built and tested. The prototype battery powered spectrometer, measuring 11 by 12 by 5 inches and weighing only about 15 pounds, was designed specifically for field use. The spectrometer has two gas proportional counters and two radioactive sources, Cd (10a) and Fe (55). Preliminary field and laboratory tests on rock specimens and rock pulps have demonstrated the capability of the spectrometer to detect 33 elements to date. Characteristics of the system present some limitations, however, and further improvements are recommended
Mobile radio alternative systems study, executive summary
Present day mobile communication technologies, systems and equipment are described from background in evaluating the concepts generated in the study. Average propagation ranges are calculated for terrestrial installations in each of seven physiographic areas of the contiguous states to determine the number of installations that would be required for nationwide coverage. Four system concepts are defined and analyzed to determine how well terrestrial systems can fulfill the requirements at acceptable costs
Coarse-graining a restricted solid-on-solid model
A procedure suggested by Vvedensky for obtaining continuum equations as the
coarse-grained limit of discrete models is applied to the restricted
solid-on-solid model with both adsorption and desorption. Using an expansion of
the master equation, discrete Langevin equations are derived; these agree
quantitatively with direct simulation of the model. From these, a continuum
differential equation is derived, and the model is found to exhibit either
Edwards-Wilkinson or Kardar-Parisi-Zhang exponents, as expected from symmetry
arguments. The coefficients of the resulting continuum equation remain
well-defined in the coarse-grained limit.Comment: Accepted for pubication in PR
Supercurrent induced by tunneling Bogoliubov excitations in a Bose-Einstein condensate
We study the tunneling of Bogoliubov excitations through a barrier in a
Bose-Einstein condensate. We extend our previous work [Phys. Rev. A
\textbf{78}, 013628 (2008)] to the case when condensate densities are different
between the left and right of the barrier potential. In the framework of the
Bogoliubov mean-field theory, we calculate the transmission probability and
phase shift, as well as the energy flux and quasiparticle current carried by
Bogoliubov excitations. We find that Bogoliubov phonons twist the condensate
phase due to a back-reaction effect, which induces the Josephson supercurrent.
While the total current given by the sum of quasiparticle current and induced
supercurrent is conserved, the quasiparticle current flowing through the
barrier potential is shown to be remarkably enhanced in the low energy region.
When the condensate densities are different between the left and right of the
barrier, the excess quasiparticle current, as well as the induced supercurrent,
remains finite far away from the barrier. We also consider the tunneling of
excitations and atoms through the boundary between the normal and superfluid
regions. We show that supercurrent can be generated inside the condensate by
injecting free atoms from outside. On the other hand, atoms are emitted when
the
Bogoliubov phonons propagate toward the phase boundary from the superfluid
region.Comment: 36 pages, 12 figures, revised version as accepted by Phys. Rev.
Lorentz-covariant reduced spin density matrix and EPR-Bohm correlations
We show that it is possible to define a Lorentz-covariant reduced spin
density matrix for massive particles. Such a matrix allows one to calculate the
mean values of observables connected with spin measurements (average
polarizations). Moreover, it contains not only information about polarization
of the particle but also information about its average kinematical state. We
also use our formalism to calculate the correlation function in the
Einstein--Podolsky--Rosen--Bohm type experiment with massive relativistic
particles.Comment: 7 pages, 1 figur
Fractionalization and confinement in the U(1) and gauge theories of strongly correlated systems
Recently, we have elucidated the physics of electron fractionalization in
strongly interacting electron systems using a gauge theory formulation.
Here we discuss the connection with the earlier U(1) gauge theory approaches
based on the slave boson mean field theory. In particular, we identify the
relationship between the holons and Spinons of the slave-boson theory and the
true physical excitations of the fractionalized phases that are readily
described in the approach.Comment: 4 page
The Link between General Relativity and Shape Dynamics
We show that one can construct two equivalent gauge theories from a linking
theory and give a general construction principle for linking theories which we
use to construct a linking theory that proves the equivalence of General
Relativity and Shape Dynamics, a theory with fixed foliation but spatial
conformal invariance. This streamlines the rather complicated construction of
this equivalence performed previously. We use this streamlined argument to
extend the result to General Relativity with asymptotically flat boundary
conditions. The improved understanding of linking theories naturally leads to
the Lagrangian formulation of Shape Dynamics, which allows us to partially
relate the degrees of freedom.Comment: 19 pages, LaTeX, no figure
New interpretation of variational principles for gauge theories. I. Cyclic coordinate alternative to ADM split
I show how there is an ambiguity in how one treats auxiliary variables in
gauge theories including general relativity cast as 3 + 1 geometrodynamics.
Auxiliary variables may be treated pre-variationally as multiplier coordinates
or as the velocities corresponding to cyclic coordinates. The latter treatment
works through the physical meaninglessness of auxiliary variables' values
applying also to the end points (or end spatial hypersurfaces) of the
variation, so that these are free rather than fixed. [This is also known as
variation with natural boundary conditions.] Further principles of dynamics
workings such as Routhian reduction and the Dirac procedure are shown to have
parallel counterparts for this new formalism. One advantage of the new scheme
is that the corresponding actions are more manifestly relational. While the
electric potential is usually regarded as a multiplier coordinate and Arnowitt,
Deser and Misner have regarded the lapse and shift likewise, this paper's
scheme considers new {\it flux}, {\it instant} and {\it grid} variables whose
corresponding velocities are, respectively, the abovementioned previously used
variables. This paper's way of thinking about gauge theory furthermore admits
interesting generalizations, which shall be provided in a second paper.Comment: 11 page
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Coking-Resistant Sub-Nano Dehydrogenation Catalysts: PtnSnx/SiO2 (n=4, 7)
We present a combined experimental/theoretical study of Pt/SiO and
PtSn/SiO (n = 4, 7) model catalysts for the endothermic
dehydrogenation of hydrocarbons, using the ethylene intermediate as a model
reactant. Supported pure Ptn clusters are found to be highly active toward
dehydrogenation of C2D4, quickly deactivating due to a combination of carbon
deposition and sintering, resulting in loss of accessible Pt sites. Addition of
Sn to Ptn clusters results in the complete suppression of C2D4 dehydrogenation
and carbon deposition, and also stabilizes the clusters against thermal
sintering. Theory shows that both systems have thermal access to a multitude of
cluster structures and adsorbate configurations that form a statistical
ensemble. While Pt4/SiO2 clusters bind ethylene in both di-sigma and pi-bonded
configurations, PtSn/SiO binds C2H4 only in the pi-mode, with
di-sigma bonding suppressed by a combination of electronic and geometric
features of the PtSn clusters. Dehydrogenation reaction profiles on the
accessible cluster isomers were calculated using the climbing image nudged
elastic band (CI-NEB) method
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