867 research outputs found
Use of AlInN layers in optical monitoring of growth of GaN-based structures on free-standing GaN substrates
When lattice matched to GaN, the AlInN ternary alloy has a refractive index ~7% lower than that of GaN. This characteristic can be exploited to perform in situ reflectometry during epitaxial growth of GaN-based multilayer structures on free-standing GaN substrates, by insertion of a suitable Al0.82In0.18N layer. The real-time information on growth rates and cumulative layer thicknesses thus obtainable is particularly valuable in the growth of optical resonant cavity structures. We illustrate this capability with reference to the growth of InGaN/GaN multiple quantum-well structures, including a doubly periodic structure with relatively thick GaN spacer layers between groups of wells. Al0.82In0.18N insertion layers can also assist in the fabrication of resonant cavity structures in postgrowth processing, for example, acting as sacrificial layers in a lift-off process exploiting etch selectivity between Al0.82In0.18N and GaN
Southern Hemisphere Observations of a eV Cosmic Ray Source Near the Direction of the Galactic Centre
We report on an analysis of data from the southern hemisphere SUGAR cosmic
ray detector. We confirm the existence of an excess of eV cosmic rays
from a direction close to the Galactic Centre, first reported by the AGASA
group. We find that the signal is consistent with that from a point source, and
we find no evidence for an excess of cosmic rays coming from the direction of
the Galactic Centre itself.Comment: 14 pages including 5 postscript figures, corrected the title
(replaced "Cosmic Ray Source" instead of "Cosmic Ray"
Optical spectroscopy of gan microcavities with thicknesses controlled using a plasma etch-back
The effect of an etch-back step to control the cavity length within GaN-based microcavities formed between two dielectric Bragg mirrors was investigated using photoluminescence and reflectivity. The structures are fabricated using a combination of a laser lift-off technique to separate epitaxial III-N layers from their sapphire substrates and electron-beam evaporation to deposit silica/zirconia multilayer mirrors. The photoluminescence measurements reveal cavity modes from both etched and nonetched microcavities. Similar cavity finesses are measured for 2.0 and 0.8 mm GaN cavities fabricated from the same wafer, indicating that the etchback has had little effect on the microcavity quality. For InGaN quantum well samples the etchback is shown to allow controllable reduction of the cavity length. Two etch steps of 100 nm are demonstrated with an accuracy of approximately 5%. The etchback, achieved using inductively coupled plasma and wet chemical etching, allows removal of the low-quality GaN nucleation layer, control of the cavity length, and modification of the surface resulting from lift-off
(In,Ga)N/GaN microcavities with double dielectric mirrors fabricated by selective removal of an (Al,In)N sacrificial layer
Comparable microcavities with 3/2 (~240 nm) active regions containing distributed (In,Ga)N quantum wells, grown on GaN substrates and bounded by two dielectric mirrors, have been fabricated by two different routes: one using laser lift-off to process structures grown on GaN-on-sapphire templates and the second using freestanding GaN substrates, which are initially processed by mechanical thinning. Both exploit the properties of an Al0.83In0.17N layer, lattice matched to the GaN substrate and spacer layers. In both cases cavity quality factors >400 are demonstrated by measurements of the cavity-filtered room-temperature excitonic emission near 410 nm
Growth and fabrication of gaN-based structures using aluminium indium nitride insertion layers
This speech was presented to the 2005 Annual Conference of the British Association for Crystal Growth, held in Sheffield on Sunday 4 - Tuesday 6 September 2005. The presentation focused on the design and growth of microcavities and the roles of AlInN layer in post-growth processing
Search for Global Dipole Enhancements in the HiRes-I Monocular Data above 10^{18.5} eV
Several proposed source models for Ultra-High Energy Cosmic Rays (UHECRs)
consist of dipole distributions oriented towards major astrophysical landmarks
such as the galactic center, M87, or Centaurus A. We use a comparison between
real data and simulated data to show that the HiRes-I monocular data for
energies above 10^{18.5} eV is, in fact, consistent with an isotropic source
model. We then explore methods to quantify our sensitivity to dipole source
models oriented towards the Galactic Center, M87, and Centaurus A.Comment: 17 pages, 31 figure
Observation of the Ankle and Evidence for a High-Energy Break in the Cosmic Ray Spectrum
We have measured the cosmic ray spectrum at energies above eV using
the two air fluorescence detectors of the High Resolution Fly's Eye experiment
operating in monocular mode. We describe the detector, PMT and atmospheric
calibrations, and the analysis techniques for the two detectors. We fit the
spectrum to models describing galactic and extragalactic sources. Our measured
spectrum gives an observation of a feature known as the ``ankle'' near eV, and strong evidence for a suppression near eV.Comment: 14 pages, 9 figures. To appear in Physics Letters B. Accepted versio
Numerical study of the noninertial systems: applicationto train coupler systems
Car coupler forces have a significant effect on the longitudinal train dynamics and stability. Because the coupler inertia is relatively small in comparison with the car inertia; the high stiffness associated with the coupler components can lead to high frequencies that adversely impact the computational efficiency of train models. The objective of this investigation is to study the effect of the coupler inertia on the train dynamics and on the computational efficiency as measured by the simulation time. To this end, two different models are developed for the car couplers; one model, called the inertial coupler model, includes the effect of the coupler inertia, while in the other model, called the noninertial model, the effect of the coupler inertia is neglected. Both inertial and noninertial coupler models used in this investigation are assumed to have the same coupler kinematic degrees of freedom that capture geometric nonlinearities and allow for the relative translation of the draft gears and end of car cushioning (EOC) devices as well as the relative rotation of the coupler shank. In both models, the coupler kinematic equations are expressed in terms of the car body and coupler coordinates. Both the inertial and noninertial models used in this study lead to a system of differential and algebraic equations that are solved simultaneously in order to determine the coordinates of the cars and couplers. In the case of the inertial model, the coupler kinematics is described using the absolute Cartesian coordinates, and the algebraic equations describe the kinematic constraints imposed on the motion of the system. In this case of the inertial model, the constraint equations are satisfied at the position, velocity, and acceleration levels. In the case of the noninertial model, the equations of motion are developed using the relative joint coordinates, thereby eliminating systematically the algebraic equations that represent the kinematic constraints. A quasistatic force analysis is used to determine a set of coupler nonlinear force algebraic equations for a given car configuration. These nonlinear force algebraic equations are solved iteratively to determine the coupler noninertial coordinates which enter into the formulation of the equations of motion of the train cars. The results obtained in this study showed that the neglect of the coupler inertia eliminates high frequency oscillations that can negatively impact the computational efficiency. The effect of these high frequencies that are attributed to the coupler inertia on the simulation time is examined using frequency and eigenvalue analyses. While the neglect of the coupler inertia leads, as demonstrated in this investigation, to a much more efficient model, the results obtained using the inertial and noninertial coupler models show good agreement, demonstrating that the coupler inertia can be neglected without having an adverse effect on the accuracy of the solutio
Asymptotic properties of Born-improved amplitudes with gauge bosons in the final state
For processes with gauge bosons in the final state we show how to
continuously connect with a single Born-improved amplitude the resonant region,
where resummation effects are important, with the asymptotic region far away
from the resonance, where the amplitude must reduce to its tree-level form.
While doing so all known field-theoretical constraints are respected, most
notably gauge-invariance, unitarity and the equivalence theorem. The
calculations presented are based on the process , mediated by a
possibly resonant Higgs boson; this process captures all the essential
features, and can serve as a prototype for a variety of similar calculations.
By virtue of massive cancellations the resulting closed expressions for the
differential and total cross-sections are particularly compact.Comment: 23 pages, Latex, 4 Figures, uses axodra
Aspects of and Production in Polarized Proton-Proton Collisions
Several topics of relevance to low transverse momentum and
production in polarized proton-proton collisions
are discussed. The leading contributions to the low
production cross-sections via , , and
initial states are calculated as well as the corresponding spin-spin
asymmetries. We find that production increases relative to direct
and production, providing up to of the observable
pairs arising from decays in collisions at . The spin-dependence of production, however, is much smaller
than for either direct or production and so will likely be far
less useful than either process in probing the polarized gluon structure
function of the proton. A subset of the radiative
corrections to production involving initial state quarks are also
performed and compared to leading order predictions.Comment: 21 pages + 5 postscript figures (included), OCIP/C-94-
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