4,395 research outputs found
Coarsening Dynamics of Crystalline Thin Films
The formation of pyramid-like structures in thin-film growth on substrates
with a quadratic symmetry, e.g., {001} surfaces, is shown to exhibit
anisotropic scaling as there exist two length scales with different time
dependences. Analytical and numerical results indicate that for most
realizations coarsening of mounds is described by an exponent n=0.2357.
However, depending on material parameters, n may lie between 0 (logarithmic
coarsening) and 1/3. In contrast, growth on substrates with triangular
symmetries ({111} surfaces) is dominated by a single length scale and an
exponent n=1/3.Comment: RevTeX, 4 pages, 3 figure
A mechanical, thermal and electrical packaging design for a prototype power management and control system for the 30 cm mercury ion thruster
A prototype electric power management and thruster control system for a 30 cm ion thruster is described. The system meets all of the requirements necessary to operate a thruster in a fully automatic mode. Power input to the system can vary over a full two to one dynamic range (200 to 400 V) for the solar array or other power source. The power management and control system is designed to protect the thruster, the flight system and itself from arcs and is fully compatible with standard spacecraft electronics. The system is easily integrated into flight systems which can operate over a thermal environment ranging from 0.3 to 5 AU. The complete power management and control system measures 45.7 cm (18 in.) x 15.2 cm (6 in.) x 114.8 cm (45.2 in.) and weighs 36.2 kg (79.7 lb). At full power the overall efficiency of the system is estimated to be 87.4 percent. Three systems are currently being built and a full schedule of environmental and electrical testing is planned
Local gauge invariance implies Siegert's hypothesis
The nonrelativistic Ward-Takahashi identity, a consequence of local gauge
invariance in quantum mechanics, shows the necessity of exchange current
contributions in case of nonlocal and/or isospin-dependent potentials. It also
implies Siegert's hypothesis: in the nonrelativistic limit, two-body charge
densities identically vanish. Neither current conservation, which follows from
global gauge invariance, nor the constraints of (lowest order) relativity are
sufficient to arrive at this result. Furthermore, a low-energy theorem for
exchange contributions is established.Comment: 5 pages, REVTE
Envelope tracking of a radio frequency amplifier for Long Term Evolution using a three-level class-G modulator
In this contribution, efficiency enhancement of a radio frequency (RF) power
amplifier (PA) for Long Term Evolution (LTE) base stations using envelope
tracking is investigated. The dynamically adapted drain bias voltage is
provided by a three-level class-G modulator. The potential of this bias
modulator is demonstrated with a class AB PA built in gallium nitride (GaN)
technology with an operation frequency of 2.75 GHz, a maximum
output power of 41 dBm and a peak power-added efficiency (PAE) of
60%. When driven by an LTE signal with a bandwidth of
20 MHz, overall system efficiency can be increased from 17%
with a constant supply voltage up to 29% with envelope tracking. This is
accompanied by a reduction of linearity. Adjacent channel leakage ratio
(ACLR) decreases by 10 dB in the first and by 18 dB
in the second adjacent channel, respectively. Error vector magnitude (EVM)
increases from 9% up to 29%
Nuclear effects in positive pion electroproduction on the deuteron near threshold
Positive pion electroproduction from the deuteron near threshold has been
considered within an approach based on the unitary transformation method. The
gauge independence of the treatment is provided by using an explicitly gauge
independent expression for the reaction amplitude. The results of calculations
for kinematics of the experiments on forward-angle meson
electroproduction accomplished at Saclay and Jefferson Laboratory are discussed
and compared with those given by the impulse approximation. It is shown that
the observed behaviour of the cross sections is in accordance with the
calculations based on the pion-nucleon dynamics. In particular, the pion
production rate suppression in the reaction compared to that
for the one can be due to such ``nuclear medium'' effects as
nucleon motion and binding along with Pauli blocking in the final state.Comment: 15 pages, 8 figure
Fast coarsening in unstable epitaxy with desorption
Homoepitaxial growth is unstable towards the formation of pyramidal mounds
when interlayer transport is reduced due to activation barriers to hopping at
step edges. Simulations of a lattice model and a continuum equation show that a
small amount of desorption dramatically speeds up the coarsening of the mound
array, leading to coarsening exponents between 1/3 and 1/2. The underlying
mechanism is the faster growth of larger mounds due to their lower evaporation
rate.Comment: 4 pages, 4 PostScript figure
Pacific Islands families study: risk and protective factors associated with delinquent behaviour in Pacific 11-year-olds
This paper examines risk and protective factors associated with delinquent behaviour among Pacific youth living in New Zealand (NZ). As part of the longitudinal Pacific Islands Families study, 11-year-olds Pacific youth participated in multidisciplinary interviews which included questions about involvement in delinquent behaviours. Peer pressure was the strongest risk factor for delinquency, and protective factors were higher self-perception, teacher evaluation scores, and perceived support from friends. Pacific boys reported significantly more delinquent behaviours than Pacific girls. Maternal acculturation was significantly associated with the delinquent behaviour of youth. Youth of mothers categorized as integrators (high Pacific/high NZ) having lower odds for delinquency than youth of mothers categorized as assimilators (low Pacific/high NZ). Youth from the largest Pacific Island groups (Samoa, Tonga and Cook Islands) were also significantly more likely to engage in delinquent behaviour than those from smaller island groups. Implications of these findings for prevention and further research are discussed
Critical Exponents of the Chiral Potts Model from Conformal Field Theory
The -invariant chiral Potts model is considered as a perturbation of a
conformal field theory. In the self-dual case the renormalization group
equations become simple, and yield critical exponents and anisotropic scaling
which agree with exact results for the super-integrable lattice models.
Although the continuum theory is not Lorentz invariant, it respects a novel
type of space-time symmetry which allows for the observed spontaneous breaking
of translational symmetry in the ground state. The continuum theory is shown to
possess an infinite number of conserved charges on the self-dual line, which
remain conserved when the theory is perturbed by the energy operator.Comment: 15 page
Impurity-induced diffusion bias in epitaxial growth
We introduce two models for the action of impurities in epitaxial growth. In
the first, the interaction between the diffusing adatoms and the impurities is
``barrier''-like and, in the second, it is ``trap''-like. For the barrier
model, we find a symmetry breaking effect that leads to an overall down-hill
current. As expected, such a current produces Edwards-Wilkinson scaling. For
the trap model, no symmetry breaking occurs and the scaling behavior appears to
be of the conserved-KPZ type.Comment: 5 pages(with the 5 figures), latex, revtex3.0, epsf, rotate, multico
Glueballs and string breaking from full QCD
We present results on the static potential, and torelon and glueball masses
from simulations of QCD with two flavours of dynamical Wilson fermions on
and lattices at .Comment: Talk presented by Gunnar Bali at International Symposium on Lattice
Field Theories (Lattice 97), Edinburgh, July 1997, 3 pages LaTeX
(epscrc2.sty) with 4 eps figure
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