318 research outputs found
Development of improved thermoelectric mater- ials for spacecraft applications final summary report, 29 jun. 1964 - 29 jun. 1965
Thermoelectric materials for spacecraft applications - optimization of bismuth-antimony alloys and ag-sb-fe-te-se system alloys for thermoelectric cooling in space environmen
An investigation of the reaction of titanium with hydrogen Summary report
Hydrogen gas reaction with titanium and titanium alloys determined at low temperatures and pressure
A Search for Energy Minimized Sequences of Proteins
In this paper, we present numerical evidence that supports the notion of minimization in the sequence space of proteins for a target conformation. We use the conformations of the real proteins in the Protein Data Bank (PDB) and present computationally efficient methods to identify the sequences with minimum energy. We use edge-weighted connectivity graph for ranking the residue sites with reduced amino acid alphabet and then use continuous optimization to obtain the energy-minimizing sequences. Our methods enable the computation of a lower bound as well as a tight upper bound for the energy of a given conformation. We validate our results by using three different inter-residue energy matrices for five proteins from protein data bank (PDB), and by comparing our energy-minimizing sequences with 80 million diverse sequences that are generated based on different considerations in each case. When we submitted some of our chosen energy-minimizing sequences to Basic Local Alignment Search Tool (BLAST), we obtained some sequences from non-redundant protein sequence database that are similar to ours with an E-value of the order of 10-7. In summary, we conclude that proteins show a trend towards minimizing energy in the sequence space but do not seem to adopt the global energy-minimizing sequence. The reason for this could be either that the existing energy matrices are not able to accurately represent the inter-residue interactions in the context of the protein environment or that Nature does not push the optimization in the sequence space, once it is able to perform the function
Quantum computing with defects
Identifying and designing physical systems for use as qubits, the basic units
of quantum information, are critical steps in the development of a quantum
computer. Among the possibilities in the solid state, a defect in diamond known
as the nitrogen-vacancy (NV-1) center stands out for its robustness - its
quantum state can be initialized, manipulated, and measured with high fidelity
at room temperature. Here we describe how to systematically identify other deep
center defects with similar quantum-mechanical properties. We present a list of
physical criteria that these centers and their hosts should meet and explain
how these requirements can be used in conjunction with electronic structure
theory to intelligently sort through candidate defect systems. To illustrate
these points in detail, we compare electronic structure calculations of the
NV-1 center in diamond with those of several deep centers in 4H silicon carbide
(SiC). We then discuss the proposed criteria for similar defects in other
tetrahedrally-coordinated semiconductors.Comment: 31 pages, 7 figures, 2 table
Semi-automated creation of accurate FEM meshes of heritage masonry walls from point cloud data
The structural analysis of buildings requires accurate spatial models. Additionally, pathologies such
as settlement-induced damages are paramount in the assessment of heritage assets. This spatial information is
used as a basis for Finite Element Method (FEM) meshes to evaluate the stability of the structure. Traditional
data acquisition approaches rely on manual measurements which are labor intensive and error prone. Therefore,
major simplifications are made to document structures efficiently. The goal of this research is to provide faster
and more accurate procedures to capture the spatial information required by a FEM.
This paper presents a semi-automated approach to create accurate models of complex heritage buildings for the
purpose of structural analysis. By employing non-destructive techniques such as terrestrial laser scanning and
photogrammetry, a complex mesh of the structure is created. Also, a methodology is proposed to capture crack
information. A stepwise approach is elaborated to illustrate how the spatial information is adapted towards a
FEM mesh. The results show a significant difference between the geometry our model and a traditional wire-
frame model. Not only does accurate modelling result in deviating loads, it also affects the behavior of the
object. Through the proposed approach, experts can develop highly accurate FEM meshes to assess the stability
of the structure up to as-built conditionsPostprint (published version
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Automated nondestructive evaluation method for characterizing ceramic and metallic hot gas filters.
In advanced coal-fired power generation, one technology under development to clean up hot gases before their use as fuel for gas turbines is rigid ceramic candle filters. These porous filters are typically 1.5 m long and 60 mm in diameter and are made of various ceramic materials, including clay-bonded SiC. The high costs of downtime in a large utility demands that nondestructive evaluation/characterization (NDE/C) methods be available. At shutdowns, data from such analysis are needed to decide which filters are still usable and which need to be replaced, and if possible, to estimate the remaining lifetimes. Thus our objective was to develop reliable low-cost NDE technology for these filters. Our approach was to develop NDE/C technology, referred to as acousto-ultrasonics (AU), for application to hot gas filters. Lamb waves generated by the AU method were analyzed to derive a stress wave factor (SWF). This technology was tested by comparing SWF data with the measured strength for a variety of rigid ceramic filters and was shown to work on iron-aluminide filters as well but no strength data have been obtained on the iron-aluminides at this time
Quantum Information Processing with Ferroelectrically Coupled Quantum Dots
I describe a proposal to construct a quantum information processor using
ferroelectrically coupled Ge/Si quantum dots. The spin of single electrons form
the fundamental qubits. Small (<10 nm diameter) Ge quantum dots are optically
excited to create spin polarized electrons in Si. The static polarization of an
epitaxial ferroelectric thin film confines electrons laterally in the
semiconductor; spin interactions between nearest neighbor electrons are
mediated by the nonlinear process of optical rectification. Single qubit
operations are achieved through "g-factor engineering" in the Ge/Si structures;
spin-spin interactions occur through Heisenberg exchange, controlled by
ferroelectric gates. A method for reading out the final state, while required
for quantum computing, is not described; electronic approaches involving single
electron transistors may prove fruitful in satisfying this requirement.Comment: 10 pages, 3 figure
Residual Kondo effect in quantum dot coupled to half-metallic ferromagnets
We study the Kondo effect in a quantum dot coupled to half-metallic
ferromagnetic electrodes in the regime of strong on-dot correlations. Using the
equation of motion technique for nonequilibrium Green functions in the slave
boson representation we show that the Kondo effect is not completely suppressed
for anti-parallel leads magnetization. In the parallel configuration there is
no Kondo effect but there is an effect associated with elastic cotunneling
which in turn leads to similar behavior of the local (on-dot) density of states
(LDOS) as the usual Kondo effect. Namely, the LDOS shows the temperature
dependent resonance at the Fermi energy which splits with the bias voltage and
the magnetic field. Moreover, unlike for non-magnetic or not fully polarized
ferromagnetic leads the only minority spin electrons can form such resonance in
the density of states. However, this resonance cannot be observed directly in
the transport measurements and we give some clues how to identify the effect in
such systems.Comment: 15 pages, 8 figures, accepted for publication in J. Phys.: Condens.
Mat
A climate-sensitive forest model for assessing impacts of forest management in Europe
FORMIT-M is a widely applicable, open-access, simple and flexible, climate-sensitive forest management simulator requiring only standard forest inventory data as input. It combines a process-based carbon balance approach with a strong inventory-based empirical component. The model has been linked to the global forest sector model EFI-GTM to secure consistency between timber cutting and demand, although prescribed harvest scenarios can also be used. Here we introduce the structure of the model and demonstrate its use with example simulations until the end of the 21st century in Europe, comparing different management scenarios in different regions under climate change. The model was consistent with country-level statistics of growing stock volumes (R-2=0.938) and its projections of climate impact on growth agreed with other studies. The management changes had a greater impact on growing stocks, harvest potential and carbon balance than projected climate change, at least in the absence of increased disturbance rates.Peer reviewe
Consensus of German Transplant Centers on Hematopoietic Stem Cell Transplantation in Fanconi Anemia
Allogeneic hematopoietic stem cell transplantation (HSCT) is currently the only curative therapy for the severe hematopoietic complications associated with Fanconi anemia (FA). In Germany, it is estimated that 10–15 transplants are performed annually for FA. However, because FA is a DNA repair disorder, standard conditioning regimens confer a high risk of excessive regimen-related toxicities and mortality, and reduced intensity regimens are linked with graft failure in some FA patients. Moreover, development of graft-versus-host disease is a major contributing factor for secondary solid tumors. The relative rarity of the disorder limits HSCT experience at any single center. Consensus meetings were convened to develop a national approach for HSCT in FA. This manuscript outlines current experience and knowledge about HSCT in FA and, based on this analysis, general recommendations reached at these meetings
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