15,692 research outputs found
Automation, regulation, and collaboration: Threats and opportunities for clinical medical physics careers in diagnostic imaging and nuclear medicine.
An approach for assessing software prototypes
A procedure for evaluating a software prototype is presented. The need to assess the prototype itself arises from the use of prototyping to demonstrate the feasibility of a design or development stategy. The assessment procedure can also be of use in deciding whether to evolve a prototype into a complete system. The procedure consists of identifying evaluations criteria, defining alterative design approaches, and ranking the alternatives according to the criteria
Localized gap soliton trains of Bose-Einstein condensates in an optical lattice
We develop a systematic analytical approach to study the linear and nonlinear
solitary excitations of quasi-one-dimensional Bose-Einstein condensates trapped
in an optical lattice. For the linear case, the Bloch wave in the energy
band is a linear superposition of Mathieu's functions and ;
and the Bloch wave in the band gap is a linear superposition of
and . For the nonlinear case, only solitons inside the band gaps are
likely to be generated and there are two types of solitons -- fundamental
solitons (which is a localized and stable state) and sub-fundamental solitons
(which is a lacalized but unstable state). In addition, we find that the
pinning position and the amplitude of the fundamental soliton in the lattice
can be controlled by adjusting both the lattice depth and spacing. Our
numerical results on fundamental solitons are in quantitative agreement with
those of the experimental observation [Phys. Rev. Lett. {\bf92}, 230401
(2004)]. Furthermore, we predict that a localized gap soliton train consisting
of several fundamental solitons can be realized by increasing the length of the
condensate in currently experimental conditions.Comment: 9 pages, 6 figures, accepted for publicaiton in PR
(Never) Mind your p's and q's: Von Neumann versus Jordan on the Foundations of Quantum Theory
In two papers entitled "On a new foundation [Neue Begr\"undung] of quantum
mechanics," Pascual Jordan (1927b,g) presented his version of what came to be
known as the Dirac-Jordan statistical transformation theory. As an alternative
that avoids the mathematical difficulties facing the approach of Jordan and
Paul A. M. Dirac (1927), John von Neumann (1927a) developed the modern Hilbert
space formalism of quantum mechanics. In this paper, we focus on Jordan and von
Neumann. Central to the formalisms of both are expressions for conditional
probabilities of finding some value for one quantity given the value of
another. Beyond that Jordan and von Neumann had very different views about the
appropriate formulation of problems in quantum mechanics. For Jordan, unable to
let go of the analogy to classical mechanics, the solution of such problems
required the identication of sets of canonically conjugate variables, i.e., p's
and q's. For von Neumann, not constrained by the analogy to classical
mechanics, it required only the identication of a maximal set of commuting
operators with simultaneous eigenstates. He had no need for p's and q's. Jordan
and von Neumann also stated the characteristic new rules for probabilities in
quantum mechanics somewhat differently. Jordan (1927b) was the first to state
those rules in full generality. Von Neumann (1927a) rephrased them and, in a
subsequent paper (von Neumann, 1927b), sought to derive them from more basic
considerations. In this paper we reconstruct the central arguments of these
1927 papers by Jordan and von Neumann and of a paper on Jordan's approach by
Hilbert, von Neumann, and Nordheim (1928). We highlight those elements in these
papers that bring out the gradual loosening of the ties between the new quantum
formalism and classical mechanics.Comment: New version. The main difference with the old version is that the
introduction has been rewritten. Sec. 1 (pp. 2-12) in the old version has
been replaced by Secs. 1.1-1.4 (pp. 2-31) in the new version. The paper has
been accepted for publication in European Physical Journal
Phase-resolved far-ultraviolet HST spectroscopy of the peculiar magnetic white dwarf RE J0317-853
We present phase resolved FUV HST FOS spectra of the rapidly rotating, highly
magnetic white dwarf RE J0317-853. Using these data, we construct a new model
for the magnetic field morphology across the stellar surface. From an expansion
into spherical harmonics, we find the range of magnetic field strengths present
is 180-800MG. For the first time we could identify an absorption feature
present at certain phases at 1160A as a ``forbidden'' 1s_0 -> 2s_0 component,
due to the combined presence of an electric and magnetic field.Comment: 15 pages including 4 figures. Accepted for publication in ApJ Letter
Symmetric three-particle motion in Stokes flow: equilibrium for heavy spheres in contrast to "end-of-world" for point forces
A stationary stable solution of the Stokes equations for three identical
heavy solid spheres falling in a vertical plane is found. It has no analog in
the point-particle approximation. Three spheres aligned horizontally at equal
distances evolve towards the equilibrium relative configuration while the point
particles collapse onto a single point in a finite time.Comment: 4 pages, 7 figure
Chiral Spin Textures of Strongly Interacting Particles in Quantum Dots
We probe for statistical and Coulomb induced spin textures among the
low-lying states of repulsively-interacting particles confined to potentials
that are both rotationally and time-reversal invariant. In particular, we focus
on two-dimensional quantum dots and employ configuration-interaction techniques
to directly compute the correlated many-body eigenstates of the system. We
produce spatial maps of the single-particle charge and spin density and verify
the annular structure of the charge density and the rotational invariance of
the spin field. We further compute two-point spin correlations to determine the
correlated structure of a single component of the spin vector field. In
addition, we compute three-point spin correlation functions to uncover chiral
structures. We present evidence for both chiral and quasi-topological spin
textures within energetically degenerate subspaces in the three- and
four-particle system.Comment: 13 pages, 17 figures, 1 tabl
Electrochemical Energy Storage Subsystems Study, Volume 2
The effects on life cycle costs (LCC) of major design and performance technology parameters for multi kW LEO and GEO energy storage subsystems using NiCd and NiH2 batteries and fuel cell/electrolysis cell devices were examined. Design, performance and LCC dynamic models are developed based on mission and system/subsystem requirements and existing or derived physical and cost data relationships. The models are exercised to define baseline designs and costs. Then the major design and performance parameters are each varied to determine their influence on LCC around the baseline values
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