16,293 research outputs found
Surface stress of Ni adlayers on W(110): the critical role of the surface atomic structure
Puzzling trends in surface stress were reported experimentally for Ni/W(110)
as a function of Ni coverage. In order to explain this behavior, we have
performed a density-functional-theory study of the surface stress and atomic
structure of the pseudomorphic and of several different possible 1x7
configurations for this system. For the 1x7 phase, we predict a different, more
regular atomic structure than previously proposed based on surface x-ray
diffraction. At the same time, we reproduce the unexpected experimental change
of surface stress between the pseudomorphic and 1x7 configuration along the
crystallographic surface direction which does not undergo density changes. We
show that the observed behavior in the surface stress is dominated by the
effect of a change in Ni adsorption/coordination sites on the W(110) surface.Comment: 14 pages, 3 figures Published in J. Phys.: Condens. Matter 24 (2012)
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Ab-initio simulation and experimental validation of beta-titanium alloys
In this progress report we present a new approach to the ab-initio guided
bottom up design of beta-Ti alloys for biomedical applications using a quantum
mechanical simulation method in conjunction with experiments. Parameter-free
density functional theory calculations are used to provide theoretical guidance
in selecting and optimizing Ti-based alloys with respect to three constraints:
(i) the use of non-toxic alloy elements; (ii) the stabilization of the body
centered cubic beta phase at room temperature; (iii) the reduction of the
elastic stiffness compared to existing Ti-based alloys. Following the
theoretical predictions, the alloys of interest are cast and characterized with
respect to their crystallographic structure, microstructure, texture, and
elastic stiffness. Due to the complexity of the ab initio calculations, the
simulations have been focused on a set of binary systems of Ti with two
different high melting bcc metals, namely, Nb and Mo. Various levels of model
approximations to describe mechanical and thermodynamic properties are tested
and critically evaluated. The experiments are conducted both, on some of the
binary alloys and on two more complex engineering alloy variants, namely,
Ti-35wt.%Nb-7wt.%Zr-5wt.%Ta and a Ti-20wt.%Mo-7wt.%Zr-5wt.%Ta.Comment: 23 pages, progress report on ab initio alloy desig
An investigation of some thermal and mechanical properties of a low-density phenolic-nylon ablation material Final report
Thermal and mechanical properties of phenolic nylon ablation material
Characterization and design of C2H2 zinc finger proteins as custom DNA binding domains
As the storage medium for the source code of life, DNA is fundamentally linked to all cellular processes. Nature employs hundreds of sequence-specific DNA binding proteins as transcription factors and repressors to regulate the flow of genetic expression and replication. By adapting these DNA-binding domains to target desired genome locations, they can be harnessed to treat diseases by regulating genes and repairing diseased gene sequences.
The C2H2 zinc finger motif is perhaps the most promising and versatile DNA binding framework. Each C2H2 zinc finger domain (module) is capable of recognizing approximately three adjacent nucleotide bases in standard B form DNA. Through directed mutagenesis, novel zinc finger modules (ZFMs) can be selected for most of the 64 possible DNA triplets. By assembling multiple ZFMs with the appropriate linkers, zinc finger proteins (ZFPs) can be generated to specifically bind extended DNA sequence motifs.
Several methods of varying complexity are currently available for ZFP engineering. ZFPs generated from the relatively simple modular design method often fail to function in vivo. Those generated using the most reliable module subsets, those recognizing triplets with a 5\u27 guanine (GNN), only function successfully only an estimated 50% of the time, while modularly assembled ZFPs comprising primarily non-GNN modules rarely function in vivo. These low success rates are extremely problematic for applications requiring multiple ZFPs that target adjacent sequence motifs. More complex ZFP engineering approaches provide enhanced success rates, as compared to modular design, with the drawback that they are also more labor intensive and require additional biological expertise.
In this research we developed and engineered novel ZFPs, analyzed characteristics of functional custom zinc finger proteins and their targets, formulated algorithms predictive of ZFP success for both modular assembly and OPEN (Oligomerized Pool Engineering) selection methods, and generated a web-based server and software tools to aid others in the successful application of this technology
Wolf-Rayet stars in the Small Magellanic Cloud: I. Analysis of the single WN stars
Wolf-Rayet (WR) stars have a severe impact on their environments owing to
their strong ionizing radiation fields and powerful stellar winds. Since these
winds are considered to be driven by radiation pressure, it is theoretically
expected that the degree of the wind mass-loss depends on the initial
metallicity of WR stars. Following our comprehensive studies of WR stars in the
Milky Way, M31, and the LMC, we derive stellar parameters and mass-loss rates
for all seven putatively single WN stars known in the SMC. Based on these data,
we discuss the impact of a low-metallicity environment on the mass loss and
evolution of WR stars. The quantitative analysis of the WN stars is performed
with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical
properties of our program stars are obtained from fitting synthetic spectra to
multi-band observations. In all SMC WN stars, a considerable surface hydrogen
abundance is detectable. The majority of these objects have stellar
temperatures exceeding 75 kK, while their luminosities range from 10^5.5 to
10^6.1 Lsun. The WN stars in the SMC exhibit on average lower mass-loss rates
and weaker winds than their counterparts in the Milky Way, M31, and the LMC. By
comparing the mass-loss rates derived for WN stars in different Local Group
galaxies, we conclude that a clear dependence of the wind mass-loss on the
initial metallicity is evident, supporting the current paradigm that WR winds
are driven by radiation. A metallicity effect on the evolution of massive stars
is obvious from the HRD positions of the SMC WN stars at high temperatures and
high luminosities. Standard evolution tracks are not able to reproduce these
parameters and the observed surface hydrogen abundances. Homogeneous evolution
might provide a better explanation for their evolutionary past.Comment: 18+12 pages; 22+8 figures; accepted for publication in A&
A gobal fit to the anomalous magnetic moment, Higgs limit and b->s gamma in the constrained MSSM
New data on the anomalous magnetic moment of the muon together with the b->s
gamma decay rate and Higgs limits are considered within the supergravity
inspired constrained minimal supersymmetric model. We perform a global
statistical chi2 analysis of these data and show that the allowed region of
parameter space is bounded from below by the Higgs limit, which depends on the
trilinear coupling and from above by the anomalous magnetic moment.Comment: 3 pages, To appear in Proc. of SUSY01, Dubna (Russia
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