23,238 research outputs found
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
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
Matter-wave solitons with a periodic, piecewise-constant nonlinearity
Motivated by recent proposals of ``collisionally inhomogeneous''
Bose-Einstein condensates (BECs), which have a spatially modulated scattering
length, we study the existence and stability properties of bright and dark
matter-wave solitons of a BEC characterized by a periodic, piecewise-constant
scattering length. We use a ``stitching'' approach to analytically approximate
the pertinent solutions of the underlying nonlinear Schr\"odinger equation by
matching the wavefunction and its derivatives at the interfaces of the
nonlinearity coefficient. To accurately quantify the stability of bright and
dark solitons, we adapt general tools from the theory of perturbed Hamiltonian
systems. We show that solitons can only exist at the centers of the constant
regions of the piecewise-constant nonlinearity. We find both stable and
unstable configurations for bright solitons and show that all dark solitons are
unstable, with different instability mechanisms that depend on the soliton
location. We corroborate our analytical results with numerical computations.Comment: 16 pages, 7 figures (some with multiple parts), to appear in Physical
Review
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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
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