3,425 research outputs found
Electron dynamics in graphene with gate-defined quantum dots
We use numerically exact Chebyshev expansion and kernel polynomial methods to
study transport through circular graphene quantum dots in the framework of a
tight-binding honeycomb lattice model. Our focus lies on the regime where
individual modes of the electrostatically defined dot dominate the charge
carrier dynamics. In particular, we discuss the scattering of an injected Dirac
electron wave packet for a single quantum dot, electron confinement in the dot,
the optical excitation of dot-bound modes, and the propagation of an electronic
excitation along a linear array of dots.Comment: revised version, 6 pages, 7 figure
Land vehicle antennas for satellite mobile communications
The RF performance, size, pointing system, and cost were investigated concepts are: for a mechanically steered 1 x 4 tilted microstrip array, a mechanically steered fixed-beam conformal array, and an electronically steered conformal phased array. Emphasis is on the RF performance of the tilted 1 x 4 antenna array and methods for pointing the various antennas studied to a geosynchronous satellite. An updated version of satellite isolations in a two-satellite system is presented. Cost estimates for the antennas in quantities of 10,000 and 100,000 unites are summarized
Tensor correlations in the Unitary Correlation Operator Method
We present a unitary correlation operator that explicitly induces into shell
model type many-body states short ranged two-body correlations caused by the
strong repulsive core and the pronounced tensor part of the nucleon-nucleon
interaction. Alternatively an effective Hamiltonian can be defined by applying
this unitary correlator to the realistic nucleon-nucleon interaction.
The momentum space representation shows that realistic interactions which
differ in their short range behaviour are mapped on the same correlated
Hamiltonian, indicating a successful provision for the correlations at high
momenta. Calculations for He4 using the one- and two-body part of the
correlated Hamiltonian compare favorably with exact many-body methods. For
heavier nuclei like O16 and Ca40 where exact many-body calculations are not
possible we compare our results with other approximations. The correlated
single-particle momentum distributions describe the occupation of states above
the Fermi momentum. The Unitary Correlation Operator Method (UCOM) can be used
in mean-field and shell model configuration spaces that are not able to
describe these repulsive and tensor correlations explicitly.Comment: 73 pages, 65 figure
Draft genome sequence of the naphthalene degrader Herbaspirillum sp. strain RV1423
Herbaspirillum sp. strain RV1423 was isolated from a site contaminated with alkanes and aromatic compounds and harbors the complete pathway for naphthalene degradation. The new features found in RV1423 increase considerably the versatility and the catabolic potential of a genus of bacteria previously considered mainly to be diazotrophic endophytes to plants
Proton-tetraneutron elastic scattering
We analyze the elastic scattering of protons on a 4n system. This was used as
part of the detection technique of a recent experiment [1] to search for the 4n
(tetraneutron) as a bound particle. We show that it is unlikely that this
process alone could yield the events reported in ref. [1], unless the 4n has an
anomalously large backward elastic scattering amplitude.Comment: 6 pages, 2 figures, accepted for publication in Phys. Rev.
Nuclear Structure based on Correlated Realistic Nucleon-Nucleon Potentials
We present a novel scheme for nuclear structure calculations based on
realistic nucleon-nucleon potentials. The essential ingredient is the explicit
treatment of the dominant interaction-induced correlations by means of the
Unitary Correlation Operator Method (UCOM). Short-range central and tensor
correlations are imprinted into simple, uncorrelated many-body states through a
state-independent unitary transformation. Applying the unitary transformation
to the realistic Hamiltonian leads to a correlated, low-momentum interaction,
well suited for all kinds of many-body models, e.g., Hartree-Fock or
shell-model. We employ the correlated interaction, supplemented by a
phenomenological correction to account for genuine three-body forces, in the
framework of variational calculations with antisymmetrised Gaussian trial
states (Fermionic Molecular Dynamics). Ground state properties of nuclei up to
mass numbers A<~60 are discussed. Binding energies, charge radii, and charge
distributions are in good agreement with experimental data. We perform angular
momentum projections of the intrinsically deformed variational states to
extract rotational spectra.Comment: 32 pages, 15 figure
Auswahl generativer prozessketten für die herstellung metallischer endprodukte
The application of generating processes for the creation of metal end-products can only be applied economically with small batch size and geometrically high-complex workpieces. This paper concerns the analytic evaluation of several additive methods and its process chains and compares these with traditional manufacturing technologies regarding the quality, processing time, expenses as well as the complexity of the product. The existing methods of process planning and the assurance of the quality are extended with a focus on the new manufacturing possibilities which can be realized by the application of additive processes.У даній статті розглядаються питання аналітичної оцінки процесу селективного спікання і його технологічних ланцюжків у порівнянні із традиційними технологіями виробництва. Критеріями порівняння є якість, час обробки, витрати, а також складність геометрії виробу. Існуючі методи планування й забезпечення якості повинні бути доповнені з урахуванням нових можливостей виготовлення виробів, які відкривають генеративні методи. Застосування генеративних технологій для виготовлення металевих кінцевих продуктів економічно обґрунтовано тільки для деталей зі складною геометрією й малим розміром
Recent Developments in the Nuclear Many-Body Problem
The study of quantum chromodynamics (QCD) over the past quarter century has
had relatively little impact on the traditional approach to the low-energy
nuclear many-body problem. Recent developments are changing this situation. New
experimental capabilities and theoretical approaches are opening windows into
the richness of many-body phenomena in QCD. A common theme is the use of
effective field theory (EFT) methods, which exploit the separation of scales in
physical systems. At low energies, effective field theory can explain how
existing phenomenology emerges from QCD and how to refine it systematically.
More generally, the application of EFT methods to many-body problems promises
insight into the analytic structure of observables, the identification of new
expansion parameters, and a consistent organization of many-body corrections,
with reliable error estimates.Comment: 15 pages, 10 figures, plenary talk at the 11th Conference on Recent
Progress in Many-Body Theories (MB 11), Manchester, England, 9-13 Jul 200
The Impact of Densification by Means of Informal Shacks in the Backyards of Low-Cost Houses on the Environment and Service Delivery in Cape Town, South Africa
This paper investigates the state-sponsored low cost housing provided to previously disadvantaged communities in the City of Cape Town. The strain imposed on municipal services by informal densification of unofficial backyard shacks was found to create unintended public health risks. Four subsidized low-cost housing communities were selected within the City of Cape Town in this cross-sectional survey. Data was obtained from 1080 persons with a response rate of 100%. Illegal electrical connections to backyard shacks that are made of flimsy materials posed increased fire risks. A high proportion of main house owners did not pay for water but sold water to backyard dwellers. The design of state-subsidised houses and the unplanned housing in the backyard added enormous pressure on the existing municipal infrastructure and the environment. Municipal water and sewerage systems and solid waste disposal cannot cope with the increased population density and poor sanitation behaviour of the inhabitants of these settlements. The low-cost housing program in South Africa requires improved management and prudent policies to cope with the densification of state-funded low-cost housing settlements
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