5,166 research outputs found
Quantum interference effects in particle transport through square lattices
We study the transport of a quantum particle through square lattices of
various sizes by employing the tight-binding Hamiltonian from quantum
percolation. Input and output semi-infinite chains are attached to the lattice
either by diagonal point to point contacts or by a busbar connection. We find
resonant transmission and reflection occuring whenever the incident particle's
energy is near an eigenvalue of the lattice alone (i.e., the lattice without
the chains attached). We also find the transmission to be strongly dependent on
the way the chains are attached to the lattice.Comment: 4 pages, 6 figures, submitted to Phys. Rev.
Experimental Evaluation of Steady-State Control Properties of an Electron-Bombardment Ion Thrustor
Examined in this report are the experimental steady-state gains, or changes, in beam-current output with changes in thrustor inputs for a 10-centimeter-diameter electron-bombardment ion thrustor. A general leveling off of beam current as a function of an input variable was found at the upper range of each variable, At the design operating point, the filament-heating current exhibited the greatest sensitivity, or percentage beam-current output change for a percentage change in the input (2 to 4). Accelerator voltage showed the least effect (0.04) while discharge voltage, magnetic field, and neutral propellant flow exhibited about the same sensitivity (0.4 to 0.5). Some effects of nonlinearity are shown that may require consideration in the design of a closed-loop control system
There are no abnormal solutions of the BetheSalpeter equation in the static model
The four-point Green's function of static QED, where a fermion and an
antifermion are located at fixed space positions, is calculated in covariant
gauges. The bound state spectrum does not display any abnormal state
corresponding to excitations of the relative time. The equation that was
established by Mugibayashi in this model and which has abnormal solutions does
not coincide with the BetheSalpeter equation. Gauge transformation from the
Coulomb gauge also confirms the absence of abnormal solutions in the
BetheSalpeter equation.Comment: 11 pages, late
Status of a five-centimeter-diameter ion thruster technology program
Prototype auxiliary propulsion subsystem with isolated single tank propellant feed system and 5-cm-diameter ion thruste
Markov chain analysis of random walks on disordered medium
We study the dynamical exponents and for a particle diffusing
in a disordered medium (modeled by a percolation cluster), from the regime of
extreme disorder (i.e., when the percolation cluster is a fractal at )
to the Lorentz gas regime when the cluster has weak disorder at and
the leading behavior is standard diffusion. A new technique of relating the
velocity autocorrelation function and the return to the starting point
probability to the asymptotic spectral properties of the hopping transition
probability matrix of the diffusing particle is used, and the latter is
numerically analyzed using the Arnoldi-Saad algorithm. We also present evidence
for a new scaling relation for the second largest eigenvalue in terms of the
size of the cluster, , which provides a
very efficient and accurate method of extracting the spectral dimension
where .Comment: 34 pages, REVTEX 3.
Bethe-Salpeter equation for doubly heavy baryons in the covariant instantaneous approximation
In the heavy quark limit, a doubly heavy baryon is regarded as composed of a
heavy diquark and a light quark. We establish the Bethe-Salpeter (BS) equations
for the heavy diquarks and the doubly heavy baryons, respectively, to leading
order in a expansion. The BS equations are solved numerically under
the covariant instantaneous approximation with the kernels containing scalar
confinement and one-gluon-exchange terms. The masses for the heavy diquarks and
the doubly heavy baryons are obtained and the non-leptonic decay widths for the
doubly heavy baryons emitting a pseudo-scalar meson are calculated within the
model.Comment: Corrections to the text, two references added, version accepted for
publication in Physical Review
A New Symmetry for QED
We demonstrate that QED exhibits a previously unobserved symmetry. Some
consequences are discussed.Comment: 9 pages, MZ-TH/93-02, DIAS-STP-93-0
An Interaction of a Magellanic Leading Arm High Velocity Cloud with the Milky Way Disk
The Leading Arm of the Magellanic System is a tidally formed HI feature
extending \sim 60\arcdeg from the Magellanic Clouds ahead of their direction
of motion. Using atomic hydrogen (HI) data from the Galactic All Sky-Survey
(GASS), supplemented with data from the Australia Telescope Compact Array, we
have found evidence for an interaction between a cloud in the Leading Arm and
the Galactic disk where the Leading Arm crosses the Galactic plane. The
interaction occurs at velocities permitted by Galactic rotation, which allows
us to derive a kinematic distance to the cloud of 21 kpc, suggesting that the
Leading Arm crosses the Galactic Plane at a Galactic radius of
kpc.Comment: 14 pages, 5 figures, accepted to Astrophysical Journal Letters. Full
resolution version available at
ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/LeadingArm_apjl.pd
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