25,894 research outputs found
Generalization of Dirac Non-Linear Electrodynamics, and Spinning Charged Particles
In this note we generalized the Dirac non-linear electrodynamics, by
introducing two potentials (namely, the vector potential A and the
pseudo-vector potential gamma^5 B of the electromagnetic theory with charges
and magnetic monopoles) and by imposing the pseudoscalar part of the product
omega.omega* to be zero, with omega = A + gamma^5 B. We show that the field
equations of such a theory possess a soliton-like solution which can represent
a priori a "charged particle", since it is endowed with a Coulomb field plus
the field of a magnetic dipole. The rest energy of the soliton is finite, and
the angular momentum stored in its electromagnetic field can be identified
--for suitable choices of the parameters-- with the spin of the charged
particle. Thus this approach seems to yield a classical model for the charged
(spinning) particle, which does not meet the problems met by earlier attempts
in the same direction.Comment: standard LaTeX file; 16 pages; it is a corrected version of a paper
appeared in Found. Phys. (issue in honour of A.O.Barut) 23 (1993) 46
A Direct Multigrid Poisson Solver for Oct-Tree Adaptive Meshes
We describe a finite-volume method for solving the Poisson equation on
oct-tree adaptive meshes using direct solvers for individual mesh blocks. The
method is a modified version of the method presented by Huang and Greengard
(2000), which works with finite-difference meshes and does not allow for shared
boundaries between refined patches. Our algorithm is implemented within the
FLASH code framework and makes use of the PARAMESH library, permitting
efficient use of parallel computers. We describe the algorithm and present test
results that demonstrate its accuracy.Comment: 10 pages, 6 figures, accepted by the Astrophysical Journal; minor
revisions in response to referee's comments; added char
Towards a knowledge-based system to assist the Brazilian data-collecting system operation
A study is reported which was carried out to show how a knowledge-based approach would lead to a flexible tool to assist the operation task in a satellite-based environmental data collection system. Some characteristics of a hypothesized system comprised of a satellite and a network of Interrogable Data Collecting Platforms (IDCPs) are pointed out. The Knowledge-Based Planning Assistant System (KBPAS) and some aspects about how knowledge is organized in the IDCP's domain are briefly described
Superconducting charge qubits from a microscopic many-body perspective
The quantised Josephson junction equation that underpins the behaviour of
charge qubits and other tunnel devices is usually derived through cannonical
quantisation of the classical macroscopic Josephson relations. However, this
approach may neglect effects due to the fact that the charge qubit consists of
a superconducting island of finite size connected to a large superconductor.
We show that the well known quantised Josephson equation can be derived
directly and simply from a microscopic many-body Hamiltonian. By choosing the
appropriate strong coupling limit we produce a highly simplified Hamiltonian
that nevertheless allows us to go beyond the mean field limit and predict
further finite-size terms in addition to the basic equation.Comment: Accepted for J Phys Condensed Matte
The ecomics of ecosystems and biodiversity: scoping the scale
The G8 decided in March 2007 to initiate a “Review on the economics of biodiversity loss”, in the so called Potsdam Initiative: 'In a global study we will initiate the process of analysing the global economic benefit of biological diversity, the costs of the loss of biodiversity and the failure to take protective measures versus the costs of effective conservation. The study is being supported by the European Commission (together with the European Environmental Agency and in cooperation with the German Government. “The objective of the current study is to provide a coherent overview of existing scientific knowledge upon which to base the economics of the Review, and to propose a coherent global programme of scientific work, both for Phase 2 (consolidation) and to enable more robust future iterations of the Review beyond 2010.
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Solid Freeform Fabrication of Functional Silicon Nitride Ceramics by Laminated Object Manufacturing 1
The processing of silicon nitride (Si3N4) structural ceramics by Laminated Object
Manufacturing (LOM) using ceramic tape preforms was investigated. The key processing stages
involved green shape formation (which used the LOM process), followed by the burnout of all
organics, and final densification by pressureless sintering. Two material systems were
considered. These were a) monolithic Si3N4 and b) a preceramic polymer infiltrated Si3N4. The
raw materials for the process were tape preforms of Si3N4, which were fabricated by standard
tape casting techniques.
Mechanical property data obtained for the LOM processed Si3N4 showed high strength and
fracture toughness values. The room temperature and high temperature (1260 o
C) flexural
strengths were in the range of 700-900 MPa and 360-400 MPa, respectively. The fracture
toughness averaged from 5.5-7.5 MPa.m1/2. These strength and fracture toughness values are
comparable to those reported for conventionally prepared Si3N4 ceramics. Thus, this research
demonstrated that the LOM technique is a viable method for preparing functional Si3N4 ceramics
with good physical and mechanical properties.Mechanical Engineerin
Rigorous Formulation of Duality in Gravitational Theories
In this paper we evince a rigorous formulation of duality in gravitational
theories where an Einstein like equation is valid, by providing the conditions
under which the Hodge duals (with respect to the metric tensor g) of T^a and
R_b^a may be considered as the torsion and curvature 2-forms associated with a
connection D', part of a Riemann-Cartan structure (M,g',D'), in the cases g =
g' and g does not equal g', once T^a and R_b^a are the torsion and curvature
2-forms associated with a connection D part of a Riemann-Cartan structure
(M,g,D). A new form for the Einstein equation involving the dual of the Riemann
tensor of D is also provided, and the result is compared with others appearing
in the literature.Comment: 15 page
Noise properties of two single electron transistors coupled by a nanomechanical resonator
We analyze the noise properties of two single electron transistors (SETs)
coupled via a shared voltage gate consisting of a nanomechanical resonator.
Working in the regime where the resonator can be treated as a classical system,
we find that the SETs act on the resonator like two independent heat baths. The
coupling to the resonator generates positive correlations in the currents
flowing through each of the SETs as well as between the two currents. In the
regime where the dynamics of the resonator is dominated by the back-action of
the SETs, these positive correlations can lead to parametrically large
enhancements of the low frequency current noise. These noise properties can be
understood in terms of the effects on the SET currents of fluctuations in the
state of a resonator in thermal equilibrium which persist for times of order
the resonator damping time.Comment: Accepted for publication in Phys. Rev.
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