1,452 research outputs found
Shakedown Analysis of Framed Structures: Strong Duality and Primal-dual Analysis
AbstractThe paper is aimed to illustrate the strong duality between the lower and upper bound formulations of shakedown analysis in a novel way. By the lower or upper bound theorem, shakedown analysis is a well-known direct method to evaluate the load carrying capacity of a structure subjected to cyclic loads. In the paper, the Hölder inequality is uniquely utilized to establish the upper bound formulation from the lower bound formulation. Accordingly, the strong duality between them is revealed by duality theorems. Following that, shakedown analysis is performed by the primal-dual algorithm provided by the computing tool MATLAB. Moreover, elastic-plastic analysis is also conducted for comparisons and validations using the commercial finite-element code ABAQUS. Finally, comparisons with good agreement validate the numerical results presented in the paper
Breaking a species barrier by enabling hybrid recombination
Hybrid sterility maintains reproductive isolation between species by preventing them from exchanging genetic material1. Anti-recombination can contribute to hybrid sterility when different species' chromosome sequences are too diverged to cross over efficiently during hybrid meiosis, resulting in chromosome mis-segregation and aneuploidy. The genome sequences of the yeasts Saccharomyces cerevisiae and Saccharomyces paradoxus have diverged by about 12% and their hybrids are sexually sterile: nearly all of their gametes are aneuploid and inviable. Previous methods to increase hybrid yeast fertility have targeted the anti-recombination machinery by enhancing meiotic crossing over. However, these methods also have counteracting detrimental effects on gamete viability due to increased mutagenesis2 and ectopic recombination3. Therefore, the role of anti-recombination has not been fully revealed, and it is often dismissed as a minor player in speciation1. By repressing two genes, SGS1 and MSH2, specifically during meiosis whilst maintaining their mitotic expression, we were able to increase hybrid fertility 70-fold, to the level of non-hybrid crosses, confirming that anti-recombination is the principal cause of hybrid sterility. Breaking this species barrier allows us to generate, for the first time, viable euploid gametes containing recombinant hybrid genomes from these two highly diverged parent species
Influence of Magnetism on Phonons in CaFe2As2 Via Inelastic X-ray Scattering
In the iron pnictides, the strong sensitivity of the iron magnetic moment to
the arsenic position suggests a significant relationship between phonons and
magnetism. We measured the phonon dispersion of several branches in the high
temperature tetragonal phase of CaFe2As2 using inelastic x-ray scattering on
single-crystal samples. These measurements were compared to ab initio
calculations of the phonons. Spin polarized calculations imposing the
antiferromagnetic order present in the low temperature orthorhombic phase
dramatically improve agreement between theory and experiment. This is discussed
in terms of the strong antiferromagnetic correlations that are known to persist
in the tetragonal phase.Comment: 4 pages, 3 figures; added additional information and references about
spin fluctuation
Synthesis, Pore Morphology, and Dielectric Property of Mesoporous Low-k Material PSMSQ using a Reactive High-Temperature Porogen, TEPSS
A high-temperature reactive porogen, triethoxy(polystyrene)silane (TEPSS) (M w =3,500 g/mole), suitable for late-porogen removal integration scheme has been synthesized in p-xylene via atom transfer radical polymerization. TEPSS was then grafted onto poly(methyl-silsesquioxane) (MSQ) matrix (k=2.9) to circumvent possible phase separation between matrix and porogen in the hybrid approach and porogen aggregation. Our results shows porous low-k MSQ films possess uniform pore size, 24 nm for porosity up to 40%, primarily due to low PDI and reactive porogen, and the dielectric constant is decreased to 2.37 at 40% porosity. In addition, less porogen aggregation was observed at porogen loading ~40 v%
Spin states of the first four holes in a silicon nanowire quantum dot
We report measurements on a silicon nanowire quantum dot with a clarity that
allows for a complete understanding of the spin states of the first four holes.
First, we show control of the hole number down to one. Detailed measurements at
perpendicular magnetic fields reveal the Zeeman splitting of a single hole in
silicon. We are able to determine the ground-state spin configuration for one
to four holes occupying the quantum dot and find a spin filling with
alternating spin-down and spin-up holes, which is confirmed by
magnetospectroscopy up to 9T. Additionally, a so far inexplicable feature in
single-charge quantum dots in many materials systems is analyzed in detail. We
observe excitations of the zero-hole ground-state energy of the quantum dot,
which cannot correspond to electronic or Zeeman states. We show that the most
likely explanation is acoustic phonon emission to a cavity between the two
contacts to the nanowire.Comment: 24 pages, 8 figures, both including supporting informatio
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Robust enzymatic saccharification of a Douglas-fir forest harvest residue by SPORL
Forest harvest residues can be a cost-effective feedstock for a biorefinery, but the high
lignin content of forest residues is a major barrier for enzymatic sugar production. Sulfite
pretreatment to overcome strong recalcitrance of lignocelluloses (SPORL) was applied to a
Douglas-fir (Pseudotsuga menziesii (Mirb) Franco var. menziesii) forest residue in a range of
sulfite and acid loadings at 165°C for 75 min with liquid to wood ratio of 3:1. Sodium
bisulfite and sulfuric acid charge as mass fraction of oven dry biomass of 12% and 2.21%,
respectively, was optimal in terms of enzymatic cellulose saccharification, sugar yield and
formation of hydroxymethylfurfural (HMF) and furfural. Enzymatic glucose yield was
345 g kg⁻¹, or equivalent to 82.3% of theoretical at a cellulase (CTec2) dosage of 15 filter
paper unit (FPU) per gram of glucan. HMF and furfural formation were low at approximately
2.5 g L⁻¹ each in the pretreatment hydrolyzate. Delignification was important to
achieve good cellulose saccharification efficiency, however, approximately 80-90% hemicellulose
removal is also required. Substrate enzymatic digestibility (SED) was found to
correlate to a combined parameter Z(CHF) of delignification and hemicellulose dissolution
well, suggesting that the combined hydrolysis factor (CHF) - a pretreatment severity
measure - can be used to predict saccharification of forest residue for scale-up studies to
reduce numbers of experiments.Keywords: Forest harvest residue, Pretreatment, Biofuel, Enzymatic hydrolysis/saccharification, Pretreatment severit
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Active Creation of Instrinsically Localized Vibrations in Uranium Using X-Ray and Neutron Scattering
In real materials, nonlinear forces cause the frequencies of vibrating atoms to depend on amplitude. As a consequence, a large-amplitude fluctuation on the scale of the atom spacing can develop a frequency that does not resonate with the normal modes, causing energy to become trapped in an intrinsically localized mode (ILM)--also called 'discrete breather' or 'lattice soliton'. As temperature is increased, entropy is expected to stabilize increased concentrations of these random hotspots. This mechanism, which spontaneously concentrates energy, has been observed in analogous systems on a larger scale, but direct sightings at the atomic scale have proved difficult. Two challenges have hampered progress: (1) the need to separate ILMs from modes associated with crystal imperfections, and (2) complications that arise at high temperatures, including feature broadening and multiphonon processes. Here we solve both of these problems by actively creating ILMs at low temperatures in {alpha}-uranium using high-energy inelastic x-ray and neutron scattering. The ILM creation excitation occurs at energies ten times higher than conventional lattice excitations, cleanly separating it from modes associated with crystal imperfections. The discovery of this excitation not only proves the existence of ILMs in uranium but also opens up a new route for finding ILMs in other materials and, in the process, a new area for spectroscopy
Security and Privacy Issues in Wireless Mesh Networks: A Survey
This book chapter identifies various security threats in wireless mesh
network (WMN). Keeping in mind the critical requirement of security and user
privacy in WMNs, this chapter provides a comprehensive overview of various
possible attacks on different layers of the communication protocol stack for
WMNs and their corresponding defense mechanisms. First, it identifies the
security vulnerabilities in the physical, link, network, transport, application
layers. Furthermore, various possible attacks on the key management protocols,
user authentication and access control protocols, and user privacy preservation
protocols are presented. After enumerating various possible attacks, the
chapter provides a detailed discussion on various existing security mechanisms
and protocols to defend against and wherever possible prevent the possible
attacks. Comparative analyses are also presented on the security schemes with
regards to the cryptographic schemes used, key management strategies deployed,
use of any trusted third party, computation and communication overhead involved
etc. The chapter then presents a brief discussion on various trust management
approaches for WMNs since trust and reputation-based schemes are increasingly
becoming popular for enforcing security in wireless networks. A number of open
problems in security and privacy issues for WMNs are subsequently discussed
before the chapter is finally concluded.Comment: 62 pages, 12 figures, 6 tables. This chapter is an extension of the
author's previous submission in arXiv submission: arXiv:1102.1226. There are
some text overlaps with the previous submissio
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Non-Equilibrium Creation of Intrinsically Localized Vibrations in Uranium Using X-Ray and Neutron Scattering
In an anharmonic crystal, large-amplitude vibrational fluctuations on the scale of the lattice spacing can develop frequencies that do not resonate with the normal modes, causing energy to become trapped in intrinsically localized modes (ILMs)--also called 'discrete breathers' or 'lattice solitons'. This mechanism has been observed in analogous systems on a larger scale, but unambiguous sightings in atomic lattice vibrations, where quantum mechanics may play a role, have proved difficult. Two challenges have hampered progress: (1) the need to separate ILMs from defect modes, and (2) complications that arise at high temperatures, including feature broadening and multiphonon processes. Here we solve these problems by using x-ray and neutron scattering to induce ILM-forming amplitude fluctuations in uranium at low temperatures, thereby creating nonequilibrium ILMs. Creation of ILMs occurs at a discrete energy, indicating an unexpected quantum character to ILM formation and greatly simplifying detection
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