2,095 research outputs found
Three-electron anisotropic quantum dots in variable magnetic fields: exact results for excitation spectra, spin structures, and entanglement
Exact-diagonalization calculations for N=3 electrons in anisotropic quantum
dots, covering a broad range of confinement anisotropies and strength of
inter-electron repulsion, are presented for zero and low magnetic fields. The
excitation spectra are analyzed as a function of the strength of the magnetic
field and for increasing quantum-dot anisotropy. Analysis of the intrinsic
structure of the many-body wave functions through spin-resolved two-point
correlations reveals that the electrons tend to localize forming Wigner
molecules. For certain ranges of dot parameters (mainly at strong anisotropy),
the Wigner molecules acquire a linear geometry, and the associated wave
functions with a spin projection S_z=1/2 are similar to the representative
class of strongly entangled states referred to as W-states. For other ranges of
parameters (mainly at intermediate anisotropy), the Wigner molecules exhibit a
more complex structure consisting of two mirror isosceles triangles. This
latter structure can be viewed as an embryonic unit of a zig-zag Wigner crystal
in quantum wires. The degree of entanglement in three-electron quantum dots can
be quantified through the use of the von Neumann entropy.Comment: To appear in Physical Review B. REVTEX4. 13 pages with 16 color
figures. To download a copy with higher-quality figures, go to publication
#78 in http://www.prism.gatech.edu/~ph274cy
Edge and bulk components of lowest-Landau-level orbitals, correlated fractional quantum Hall effect incompressible states, and insulating behavior in finite graphene samples
Many-body calculations of the total energy of interacting Dirac electrons in
finite graphene samples exhibit joint occurrence of cusps at angular momenta
corresponding to fractional fillings characteristic of formation of
incompressible (gapped) correlated states (nu=1/3 in particular) and opening of
an insulating energy gap (that increases with the magnetic field) at the Dirac
point, in correspondence with experiments. Single-particle basis functions
obeying the zigzag boundary condition at the sample edge are employed in exact
diagonalization of the interelectron Coulomb interaction, showing, at all
sizes, mixed equal-weight bulk and edge components. The consequent depletion of
the bulk electron density attenuates the fractional-quantum-Hall-effect
excitation energies and the edge charge accumulation results in a gap in the
many-body spectrum.Comment: 8 pages with 7 figures. REVTEX4. For related publications, see
http://www.prism.gatech.edu/~ph274c
Customized Software in Distributed Embedded Systems: ISOBUS and the Coming Revolution in Agriculture
The electrification of agricultural equipment has been evolving for many years and in some ways
is lagging behind other industries. However this strategy of following the lead of other industries
now offers Ag the opportunity to move forward at a revolutionary pace. Network standards
defined by the Society of Automotive Engineers (SAE) and the International Organization for
Standardization (ISO) committees are the basis for defining a rulebook for this industrystandardizing
worldwide electronics interoperability. ISOBUS (ISO 11783) which defines a
physical standard between tractors and implements will be an important enabler for most new
product definitions. The foundation of this coming revolution will be provided through software.
This paper outlines the electronics hardware and software architecture for off-road vehicles that
allows for implementation of customized machine control features. There are several key areas
discussed. The first enabler for this revolution is a software development and delivery system
that defines a design methodology for creating and delivering software modules for a distributed
set of controllers. This design methodology presents two advantages that today’s modern
electronic technologies can deliver: 1) Customization with commodity hardware and 2) Service
without replacing hardware parts anywhere in the world. The second enabler for this machine
revolution is an ‘agile’ process to develop the software. Many product ideas are being valuated
through a trial and error and continuous improvement process. Software will play an important
enabler for these product definitions. A comparison between the worldwide trend for software
processes, the Capability Maturity Model (CMM), and what type of process would fit the offroad
industry is based around the maturity of the new product ideas. The strong supply chain link
between dealers and customers for off-road machines, coupled with the emerging awareness of
electronic functions and controls, sets a basis for a specialized software development process. An
important enabler for this ‘agile’ process is the re-use of code and incremental testing with
reviews.
The history of the off-road machine business has been based on proven designs and long times
between model updates. However, the worldwide adoption of the ISOBUS standard is poised to
change this history. ISOBUS is not only establishing an open system for interoperability, it is
establishing a sequence of features for diagnostics, sequenced operations, and information
management. As customers discover these capabilities, they will expect them to be further
advanced and customized for their specific needs. This requires adding agility into the proven
durable processes so that manufacturers can respond faster to these growing needs. Electronics,
and especially well-planned software systems, offer an agile technology for meeting this coming need. This paper presents the benchmarking of various embedded software development projects
relating project content, project rigor, and quality. From this, insights into maintaining quality are
gained in order to include agility into a durable development project. Also, risk and rewards of
leveraging low cost country software development skills are addressed to stretch resources or
even develop common resources for software systems
Oxygen clamps in gold nanowires
We investigate how the insertion of an oxygen atom in an atomically thin gold
nanowire can affect its rupture. We find, using ab initio total energy density
functional theory calculations, that O atoms when inserted in gold nanowires
form not only stable but also very strong bonds, in such a way that they can
extract atoms from a stable tip, serving in this way as a clamp that could be
used to pull a string of gold atoms.Comment: 4 pages; 4 figure
Structural properties of electrons in quantum dots in high magnetic fields: Crystalline character of cusp states and excitation spectra
The crystalline or liquid character of the downward cusp states in N-electron
parabolic quantum dots (QD's) at high magnetic fields is investigated using
conditional probability distributions obtained from exact diagonalization.
These states are of crystalline character for fractional fillings covering both
low and high values, unlike the liquid Jastrow-Laughlin wave functions, but in
remarkable agreement with the rotating-Wigner-molecule ones [Phys. Rev. B 66,
115315 (2002)]. The crystalline arrangement consists of concentric polygonal
rings that rotate independently of each other, with the electrons on each ring
rotating coherently. We show that the rotation stabilizes the Wigner molecule
relative to the static one defined by the broken-symmetry
unrestricted-Hartree-Fock solution. We discuss the non-rigid behavior of the
rotating Wigner molecule and pertinent features of the excitation spectrum,
including the occurrence of a gap between the ground and first excited states
that underlies the incompressibility of the system. This leads us to conjecture
that the rotating crystal (and not the static one) remains the relevant ground
state for low fractional fillings even at the thermodynamic limit.Comment: Published version. Typos corrected. REVTEX4. 10 pages with 8
postscript figures (5 in color). For related papers, see
http://www.prism.gatech.edu/~ph274cy
Application of Robustness Analysis for Developing a Procedure for Better Urban Transportation Planning Decisions
This paper shows that robustness analysis is a technique with a potential for aiding decision makers in choosing transportation investment projects. In this paper, it has been demonstrated that it can be successfully used in urban transportation planning in conjunction with urban travel demand software. The robustness analysis procedure emphasizes the need, under conditions of uncertainty, to make early decisions in a time-phased sequence, while preserving future options that currently seem attractive. The results of the robustness analysis from the case study used in this paper indicate that the method is simple to understand, easy to use, minimizes future surprises in terms of expected future events not happening, and provides the flexibility required in typical urban planning problems where decision making is needed to be taken under conditions of uncertainty. A general framework to be used in such cases is proposed
High-contrast spectroscopy of exoplanet atmospheres
More than 5,000 exoplanets have been found over the past couple of decades. These exoplanets show a tremendous diversity, ranging from scorching hot Jupiters, common super-Earths, to widely separated super-Jupiters on the planet/brown dwarf boundary. We have now moved into the era of exoplanet atmospheric characterisation. Two crucial techniques for characterizing these exoplanets from the ground are high-contrast imaging and high-resolution spectroscopy. In the first part of the thesis, existing facilities are used to characterize the atmospheres of two of the most accessible types of planets: An ultra-hot Jupiter (WASP-76b) and a young supter-Jupiter (beta Pictoris b). The second part of the thesis develops instrumental concepts that are required to push exoplanet characterization towards smaller and closer-in planets. It shows how we can choose between spectral resolution, bandwidth, and field-of-view in developing an instrument for exoplanet detection, and how one can design an nearly optimal wavefront sensor for adaptive optics. Finally, it is demonstrated how machine learning techniques can help us improve the performance of these adaptive optics systems such that we can reach deeper contrasts for exoplanet imaging and characterization.High Energy Astrophysic
Design Strategy for the PFC in DEMO Reactor (KIT Scientific Reports ; 7637)
The performance of the plasma facing components (PFC) and materials in fusion reactor DEMO are fundamental issues affecting the ultimate technological and economic feasibility of fusion power. Many factors influence the choice of a functional and structural material in a fusion reactor. Component lifetime is mainly limited by radiation damage, disruptions, and sputtering erosion. Our design strategy is to determine the structure and coating thicknesses, which maximize component lifetime against all life limitations
Development of Multiple Growth Strategies for Use in Developing Traffic Forecasts: A Robustness Approach
Decisions that may be based on misleading forecasts may lead to a misallocation of funds and to under-performing projects during construction and operation. Poor projections of demographic and socioeconomic data are usually cited as the major source of poor traffic assignment projections and hence, unfavorably conceived planning and construction of street and highway infrastructure facilities.
This report evaluated the accuracy of long range projections by using a transportation study done the in 1970s, projecting transportation demand 20 years into the future. The projected travel model inputs were compared with what actually happened after the horizon year had been reached and also compared the projected traffic volumes versus the actual ground counts at the same horizon year. The results of this study show that there is a poor correlation between what was forecasted and what actually happened in terms of socioeconomic and demographic data, which are the major inputs used by travel demand models to forecast future traffic volumes on road links. The projected traffic volumes were poorly correlated with the actual ground traffic counts for the same road links in the network. However, the end results of these projections, the estimated number of lanes required to accommodate the resulting traffic, were not adversely affected. It was found that 98 percent of the major streets had the number of lanes correctly estimated based on the 1994 Highway Capacity Manual (HCM) planning level of service (LOS) criteria.
Robustness analysis is a technique with the potential in aiding decision makers in choosing transportation investment projects that more closely correlate to actual future development. In this report it has been demonstrated that robustness analysis can be successfully used in urban transportation planning in conjunction with urban travel demand software. The robustness analysis procedure emphasizes the need, under conditions of uncertainty, to make early decisions in a time-phased sequence, while preserving many future options until the choices are more definitive. The results of the robustness analysis indicate that the method is simple to understand, easy to use, minimizes future surprises in terms of expected future events not happening, and provides the flexibility required in typical urban planning problems where decision making has to be done under conditions of uncertainties. A general framework to be used in such cases is proposed
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