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Search Results: 1 - 10 of 454406 matches for " R. J. Warburton "
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Analytic Approximations of Projectile Motion with Quadratic Air Resistance  [PDF]
R. D. H. Warburton, J. Wang, J. Burgd?rfer
Journal of Service Science and Management (JSSM) , 2010, DOI: 10.4236/jssm.2010.31012
Abstract: We study projectile motion with air resistance quadratic in speed. We consider three regimes of approximation: low-angle trajectory where the horizontal velocity, u, is assumed to be much larger than the vertical velocity w; high-angle trajectory where ; and split-angle trajectory where . Closed form solutions for the range in the first regime are obtained in terms of the Lambert W function. The approximation is simple and accurate for low angle ballistics problems when compared to measured data. In addition, we find a surprising behavior that the range in this approximation is symmetric about , although the trajectories are asymmetric. We also give simple and practical formulas for accurate evaluations of the Lambert W function.
GPU accelerated spectral finite elements on all-hex meshes
J. -F. Remacle,R. Gandham,T. Warburton
Computer Science , 2015,
Abstract: This paper presents a spectral element finite element scheme that efficiently solves elliptic problems on unstructured hexahedral meshes. The discrete equations are solved using a matrix-free preconditioned conjugate gradient algorithm. An additive Schwartz two-scale preconditioner is employed that allows h-independence convergence. An extensible multi-threading programming API is used as a common kernel language that allows runtime selection of different computing devices (GPU and CPU) and different threading interfaces (CUDA, OpenCL and OpenMP). Performance tests demonstrate that problems with over 50 million degrees of freedom can be solved in a few seconds on an off-the-shelf GPU.
Laser micro-fabrication of concave, low-roughness features in silica
D. Hunger,C. Deutsch,R. J. Barbour,R. J. Warburton
AIP Advances , 2012, DOI: 10.1063/1.3679721
Abstract: We describe a micro-fabrication method to create concave features with ultra-low roughness in silica, either on optical fibers or on flat substrates. The machining uses a single CO2 laser pulse train. Parameters are chosen such that evaporation removes material while a low-viscosity melt layer produces excellent surface quality. A surface roughness σ ~ 0.2 nm is regularly obtained. The concave depressions are near-spherical close to the center with radii of curvature between 20 and 2000 μm. The method allows fabrication of low-scatter micro-optical devices such as mirror substrates for high-finesse cavities or negative lenses on fiber tips, extending the range of micro-optical components.
Kondo excitons in self-assembled quantum dots
A. O. Govorov,K. Karrai,R. J. Warburton
Physics , 2002, DOI: 10.1103/PhysRevB.67.241307
Abstract: We describe novel excitons in quantum dots by allowing for an interaction with a Fermi sea of electrons. We argue that these excitons can be realized very simply with self-assembled quantum dots, using the wetting layer as host for the Fermi sea. We show that a tunnel hybridization of a charged exciton with the Fermi sea leads to two striking effects in the optical spectra. First, the photoluminescence lines become strongly dependent on the vertical bias. Secondly, if the exciton spin is nonzero, the Kondo effect leads to peculiar photoluminescence line shapes with a line width determined by the Kondo temperature.
Entanglement on demand through time reordering
J. E. Avron,G. Bisker,D. Gershoni,N. H. Lindner,E. A. Meirom,R. J. Warburton
Physics , 2007, DOI: 10.1103/PhysRevLett.100.120501
Abstract: Entangled photons can be generated "on demand" in a novel scheme involving unitary time reordering of the photons emitted in a radiative decay cascade. The scheme yields polarization entangled photon pairs, even though prior to reordering the emitted photons carry significant "which path information" and their polarizations are unentangled. This shows that quantum chronology can be manipulated in a way that is lossless and deterministic (unitary). The theory can, in principle, be tested and applied to the biexciton cascade in semiconductor quantum dots.
Exciton-photon coupling in a ZnSe based microcavity fabricated using epitaxial liftoff
A. Curran,J. K. Morrod,K. A. Prior,A. K. Kar,R. J. Warburton
Physics , 2007, DOI: 10.1088/0268-1242/22/11/001
Abstract: We report the observation of strong exciton-photon coupling in a ZnSe based microcavity fabricated using epitaxial liftoff. Molecular beam epitaxial grown ZnSe/Zn$_{0.9}$Cd$_{0.1}$Se quantum wells with a one wavelength optical length at the exciton emission were transferred to a SiO$_2$/Ta$_2$O$_5$ mirror with a reflectance of 96% to form finesse matched microcavities. Analysis of our angle resolved transmission spectra reveals key features of the strong coupling regime: anticrossing with a normal mode splitting of $23.6 meV$ at $20 K$; composite evolution of the lower and upper polaritons; and narrowing of the lower polariton linewidth near resonance. The heavy hole exciton oscillator strength per quantum well is also deduced to be $1.78 \times 10^{13} cm^{-2}$.
The Health Benefits and Challenges of Exercise Training in Persons Living with Schizophrenia: A Pilot Study
Shannon S. D. Bredin,Darren E. R. Warburton,Donna J. Lang
Brain Sciences , 2013, DOI: 10.3390/brainsci3020821
Abstract: Background: In addition to the hallmark cognitive and functional impairments mounting evidence indicates that schizophrenia is also associated with an increased risk for the development of secondary complications, in particular cardio-metabolic disease. This is thought to be the result of various factors including physical inactivity and the metabolic side effects of psychotropic medications. Therefore, non-pharmacological approaches to improving brain health, physical health, and overall well-being have been promoted increasingly. Methods: We report on the health-related physical fitness (body composition, blood pressure, heart rate, and aerobic fitness) and lipid profile of persons living with schizophrenia and effective means to address the challenges of exercise training in this population. Results: There was a markedly increased risk for cardio-metabolic disease in 13 persons living with schizophrenia (Age = 31 ± 7 years) including low aerobic fitness (76% ± 34% of predicted), reduced HDL (60% of cohort), elevated resting heart rate (80% of cohort), hypertension (40% of cohort), overweight and obesity (69% of cohort), and abdominal obesity (54% of cohort). Individualized exercise prescription (3 times/week) was well tolerated, with no incidence of adverse exercise-related events. The exercise adherence rate was 81% ± 21% (Range 48%–100%), and 69% of the participants were able to complete the entire exercise training program. Exercise training resulted in clinically important changes in physical activity, aerobic fitness, exercise tolerance, blood pressure, and body composition. Conclusion: Persons living with schizophrenia appear to be at an increased risk for cardio-metabolic disease. An individualized exercise program has shown early promise for the treatment of schizophrenia and the various cognitive, functional, and physiological impairments that ultimately affect health and well-being.
Kondo-excitons and Auger processes in self-assembled quantum dots
A. O. Govorov,K. Karrai,R. J. Warburton,A. V. Kalameitsev
Physics , 2003,
Abstract: We describe theoretically novel excitons in self-assembled quantum dots interacting with a two-dimensional electron gas in the wetting layer. In the presence of the Fermi sea, the optical lines become strongly voltage-dependent. If the electron spin is nonzero, the width of optical lines is given by k_BT_K, where T_K is Kondo temperature. If the spin is zero, the exciton couples with the continuum due to Auger-like processes. This leads to anticrossings in a magnetic field. Some of the described phenomena are observed in recent experiments.
Polarized excitons in nanorings and the `optical' Aharonov-Bohm effect
A. O. Govorov,S. E. Ulloa,K. Karrai,R. J. Warburton
Physics , 2002, DOI: 10.1103/PhysRevB.66.081309
Abstract: The quantum nature of matter lies in the wave function phases that accumulate while particles move along their trajectories. A prominent example is the Aharonov-Bohm phase, which has been studied in connection with the conductance of nanostructures. However, optical response in solids is determined by neutral excitations, for which no sensitivity to magnetic flux would be expected. We propose a new mechanism for the topological phase of a neutral particle, a polarized exciton confined to a semiconductor quantum ring. We predict that this magnetic-field induced phase may strongly affect excitons in a system with cylindrical symmetry, resulting in switching between `bright' exciton ground states and novel `dark' states with nearly infinite lifetimes. Since excitons determine the optical response of semiconductors, the predicted phase can be used to tailor photon emission from quantum nanostructures.
Zero-field spin splitting in InAs-AlSb quantum wells revisited
S. Brosig,K. Ensslin,R. J. Warburton,C. Nguyen,B. Brar,M. Thomas,H. Kroemer
Physics , 1999, DOI: 10.1103/PhysRevB.60.R13989
Abstract: We present magnetotransport experiments on high-quality InAs-AlSb quantum wells that show a perfectly clean single-period Shubnikov-de Haas oscillation down to very low magnetic fields. In contrast to theoretical expectations based on an asymmetry induced zero-field spin splitting, no beating effect is observed. The carrier density has been changed by the persistent photo conductivity effect as well as via the application of hydrostatic pressure in order to influence the electric field at the interface of the electron gas. Still no indication of spin splitting at zero magnetic field was observed in spite of highly resolved Shubnikov- de Haas oscillations up to filling factors of 200. This surprising and unexpected result is discussed in view of other recently published data.
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