Showing papers by "North Carolina State University published in 1996"

Nanomechanics of carbon tubes: Instabilities beyond linear response.

Abstract: Carbon nanotubes subject to large deformations reversibly switch into different morphological patterns. Each shape change corresponds to an abrupt release of energy and a singularity in the stress-strain curve. These transformations, simulated using a realistic many-body potential, are explained by a continuum shell model. With properly chosen parameters, the model provides a remarkably accurate ``roadmap'' of nanotube behavior beyond Hooke's law.

Firm Performance and Mechanisms to Control Agency Problems between Managers and Shareholders

Abstract: This paper examines the use of seven mechanisms to control agency problems between managers and shareholders. These mechanisms are: shareholdings of insiders, institutions, and large blockholders; use of outside directors; debt policy; the managerial labor market; and the market for corporate control. We present direct empirical evidence of interdependence among these mechanisms in a large sample of firms. This finding suggests that crosssectional OLS regressions of firm performance on single mechanisms may be misleading. Indeed, we find relationships between firm performance and four of the mechanisms when each is included in a separate OLS regression. These are insider shareholdings, outside directors, debt, and corporate control activity. Importantly, the effect of insider shareholdings disappears when all of the mechanisms are included in a single OLS regression, and the effects of debt and corporate control activity also disappear when estimations are made in a simultaneous systems framework. Together, these findings are consistent with optimal use of each control mechanism except outside directors.

Strain-related phenomena in GaN thin films

Abstract: Photoluminescence (PL), Raman spectroscopy, and x-ray diffraction are employed to demonstrate the co-existence of a biaxial and a hydrostatic strain that can be present in GaN thin films. The biaxial strain originates from growth on lattice-mismatched substrates and from post-growth cooling. An additional hydrostatic strain is shown to be introduced by the presence of point defects. A consistent description of the experimental results is derived within the limits of the linear and isotropic elastic theory using a Poisson ratio $\ensuremath{ u}=0.23\ifmmode\pm\else\textpm\fi{}0.06$ and a bulk modulus $B=200\ifmmode\pm\else\textpm\fi{}20$ GPa. These isotropic elastic constants help to judge the validity of published anisotropic elastic constants that vary greatly. Calibration constants for strain-induced shifts of the near-band-edge PL lines with respect to the ${E}_{2}$ Raman mode are given for strain-free, biaxially strained, and hydrostatically contracted or expanded thin films. They allow us to extract differences between hydrostatic and biaxial stress components if present. In particular, we determine that a biaxial stress of one GPa would shift the near-band-edge PL lines by 27\ifmmode\pm\else\textpm\fi{}2 meV and the ${E}_{2}$ Raman mode by 4.2\ifmmode\pm\else\textpm\fi{}0.3 ${\mathrm{cm}}^{\ensuremath{-}1}$ by use of the listed isotropic elastic constants. It is expected from the analyses that stoichiometric variations in the GaN thin films together with the design of specific buffer layers can be utilized to strain engineer the material to an extent that greatly exceeds the possibilities known from other semiconductor systems because of the largely different covalent radii of the Ga and the N atom.

A Brassinosteroid-Insensitive Mutant in Arabidopsis thaliana Exhibits Multiple Defects in Growth and Development

Abstract: Brassinosteroids are widely distributed plant compounds that modulate cell elongation and division, but little is known about the mechanism of action of these plant growth regulators. To investigate brassinosteroids as signals influencing plant growth and development, we identified a brassinosteroid-insensitive mutant in Arabidopsis thaliana (L.) Henyh. ecotype Columbia. The mutant, termed bri1, did not respond to brassinosteroids in hypocotyl elongation and primary root inhibition assays, but it did retain sensitivity to auxins, cytokinins, ethylene, abscisic acid, and gibberellins. The bri1 mutant showed multiple deficiencies in developmental pathways that could not be rescued by brassinosteroid treatment including a severely dwarfed stature; dark green, thickened leaves; males sterility; reduced apical dominance; and de-etiolation of dark-grown seedlings. Genetic analysis suggests that the Bri1 phenotype is caused by a recessive mutation in a single gene with pleiotropic effects that maps 1.6 centimorgans from the cleaved, amplified, polymorphic sequence marker DHS1 on the bottom of chromosome IV. The multiple and dramatic effects of mutation of the BRI1 locus on development suggests that the BRI1 gene may play a critical role in brassinosteroid perception or signal transduction.

Navigating the Bumpy Road to Student-Centered Instruction

Abstract: (1996). Navigating the Bumpy Road to Student-Centered Instruction. College Teaching: Vol. 44, No. 2, pp. 43-47.

Variation in Lignin Content and Composition (Mechanisms of Control and Implications for the Genetic Improvement of Plants).

Abstract: Lignin, a complex phenolic polymer, is important for mechanical support, water transport, and defense in vaseular plants. Compressive strength and hydrophobicity of xylem cell walls are imparted by the lignin polymer, which is deposited during the terminal differentiation of tracheids and other cell types. The resistance of xylem to compressive stresses imposed by water transport and by the mass of the plants is important to growth and development. In addition, the insolubility and complexity of the lignin polymer makes it resistant to degradation by most microorganisms. Therefore, lignin serves an important function in plant defense. Variation in lignin content, composition, and location is likely to affect these essential processes. The constraints on the amount, composition, and localization of lignin for normal xylem function and plant defense are not known. Lignin composition, quantity, and distribution also affect the agroindustrial uses of plant material. Digestibility and dietary conversion of herbaceous crops are affected by differences in lignin content and composition (Akin et al., 1986, 1991). Lignin is an undesirable component in the conversion of wood into pulp and paper; remova1 of lignin is a major step in the paper making process. Furthermore, the resistance of lignin to microbial degradation enhances its persistence in soils. Lignin is, therefore, a significant component in the global carbon cycle. The mechanisms of control of lignin composition and quantity have wide implications regarding the adaptation and evolution of land plants and provide a basis for improved genetic manipulation of lignin for agroindustrial end uses. In this Update, we will focus on the levels of control of lignin variation, including (a) metabolic control, (b) regulation of individual enzymes in the biosynthetic pathway, and (c) regulation of gene expression. These levels of regulation affect variation in lignin content, quality, and distribution. Finally, the implications of these regulatory mechanisms for the genetic improvement of lignin for

Molecular chaperones and protein folding in plants.

Abstract: Protein folding in vivo is mediated by an array of proteins that act either as ‘foldases’ or ‘molecular chaperones’. Foldases include protein disulfide isomerase and peptidyl prolyl isomerase, which catalyze the rearrangement of disulfide bonds or isomerization of peptide bonds around Pro residues, respectively. Molecular chaperones are a diverse group of proteins, but they share the property that they bind substrate proteins that are in unstable, non-native structural states. The best understood chaperone systems are HSP70/DnaK and HSP60/GroE, but considerable data support a chaperone role for other proteins, including HSP100, HSP90, small HSPs and calnexin. Recent research indicates that many, if not all, cellular proteins interact with chaperones and/or foldases during their lifetime in the cell. Different chaperone and foldase systems are required for synthesis, targeting, maturation and degradation of proteins in all cellular compartments. Thus, these diverse proteins affect an exceptionally broad array of cellular processes required for both normal cell function and survival of stress conditions. This review summarizes our current understanding of how these proteins function in plants, with a major focus on those systems where the most detailed mechanistic data are available, or where features of the chaperone/foldase system or substrate proteins are unique to plants.

Bioequivalence trials, intersection-union tests and equivalence confidence sets

Abstract: The bioequivalence problem is of practical importance because the approval of most generic drugs in the United States and the European Community (EC) requires the establishment of bioequivalence between the brand-name drug and the proposed generic version. The problem is theoretically interesting because it has been recognized as one for which the desired inference, instead of the usual significant difference, is practical equivalence. The concept of intersection-union tests will be shown to clarify, simplify and unify bioequivalence testing. A test more powerful than the one currently specified by the FDA and EC guidelines will be derived. The claim that the bioequivalence problem defined in terms of the ratio of parameters is more difficult than the problem defined in terms of the difference of parameters will be refuted. The misconception that size-$\alpha$ bioequivalence tests generally correspond to $100(1 - 2 \alpha)%$ confidence sets will be shown to lead to incorrect statistical practices, and should be abandoned. Techniques for constructing $100(1 - \alpha)%$ confidence sets that correspond to size-$\alpha$ bioequivalence tests will be described. Finally, multiparameter bioequivalence problems will be discussed.

Trends in power semiconductor devices

Abstract: This paper reviews recent trends in power semiconductor device technology that are leading to improvements in power losses for power electronic systems. In the case of low voltage ( 100 V) power rectifiers, the silicon P-i-N rectifier continues to dominate but significant improvements are expected by the introduction of the silicon MPS rectifier followed by the GaAs and SiC based Schottky rectifiers. Equally important developments are occurring in power switch technology. The silicon bipolar power transistor has been displaced by silicon power MOSFETs in low voltage ( 100 V) systems. The process technology for these MOS-gated devices has shifted from V-MOS in the early 1970s to DMOS in the 1980s, with more recent introduction of the UMOS technology in the 1990s. For the very high power systems, the thyristor and GTO continue to dominate, but significant effort is underway to develop MOS-gated thyristors (MCTs, ESTs, DG-BRTs) to replace them before the turn of the century. Beyond that time frame, it is projected that silicon carbide based switches will begin to displace these silicon devices.

Towards better uncertainty estimates for turbulence statistics

Abstract: Methods for calculating the statistical uncertainty associated with the sampling of random processes such as those which occur in turbulence research are given. In particular, formulas based on normal distribution assumptions and on any general distribution shape are given for means, variances, Reynolds stresses, correlation coefficients, homogeneous and mixed turbulent triple products and fourth order turbulence moments. In addition, two resampling algorithms, the “bootstrap” and “jackknife”, are presented and compared using actual turbulence data. The availability of these methods will allow turbulence data to be presented with statistical uncertainty error bars on all turbulence quantities.

cis-trans lycopene isomers, carotenoids, and retinol in the human prostate.

Abstract: An evaluation of the Health Professionals Follow-Up Study has detected a lower prostate cancer risk associated with the greater consumption of tomatoes and related food products. Tomatoes are the primary dietary source of lycopene, a non-provitamin A carotenoid with potent antioxidant activity. Our goal was to define the concentrations of lycopene, other carotenoids, and retinol in paired benign and malignant prostate tissue from 25 men, ages 53 to 74, undergoing prostatectomy for localized prostate cancer. The concentrations of specific carotenoids in the benign and malignant prostate tissue from the same subject are highly correlated. Lycopene and all-trans beta-carotene are the predominant carotenoids observed, with means +/- SE of 0.80 +/- 0.08 nmol/g and 0.54 +/- 0.09, respectively. Lycopene concentrations range from 0 to 2.58 nmol/g, and all-trans beta-carotene concentrations range from 0.09 to 1.70 nmol/g. The 9-cis beta-carotene isomer, alpha-carotene, lutein, alpha-cryptoxanthin, zeaxanthin, and beta-cryptoxanthin are consistently detectable in prostate tissue. No significant correlations between the concentration of lycopene and the concentrations of any other carotenoid are observed. In contrast, strong correlations between prostate beta-carotene and alpha-carotene are noted (correlation coefficient, 0.88; P < 0.0001), as are correlations between several other carotenoid pairs, which reflects their similar dietary origins. Mean vitamin A concentration in the prostate is 1.52 nmol/g, with a range of 0.71 to 3.30 nmol/g. We further evaluated tomato-based food products, serum, and prostate tissue for the presence of geometric lycopene isomers using high-performance liquid chromatography with a polymeric C30 reversed phase column. All-trans lycopene accounts for 79 to 91% and cis lycopene isomers for 9 to 21% of total lycopene in tomatoes, tomato paste, and tomato soup. Lycopene concentrations in the serum of men range between 0.60 and 1.9 nmol/ml, with 27 to 42% all-trans lycopene and 58 to 73% cis-isomers distributed among 12 to 13 peaks, depending upon their chromatographic resolution. In striking contrast with foods, all-trans lycopene accounts for only 12 to 21% and cis isomers for 79 to 88% of total lycopene in benign or malignant prostate tissues. cis Isomers of lycopene within the prostate are distributed among 14 to 18 peaks. We conclude that a diverse array of carotenoids are found in the human prostate with significant intra-individual variation. The presence of lycopene in the prostate at concentrations that are biologically active in laboratory studies supports the hypothesis that lycopene may have direct effects within the prostate and contribute to the reduced prostate cancer risk associated with the reduced prostate cancer risk associated with the consumption of tomato-based foods. The future identification and characterization of geometric lycopene isomers may lead to the development of novel agents for chemoprevention studies.

Crop Insurance, Moral Hazard, and Agricultural Chemical Use

Abstract: This study examines the relationship between chemical input use and crop insurance purchase decisions for a sample of Kansas dryland wheat farmers. Recent research by Horowitz and Lichtenberg indicated that, contrary to conventional wisdom, farmers that purchased insurance tended to use relatively more chemical inputs than farmers who did not insure. In contrast, our results confirm the conventional view that moral hazard incentives lead insured farmers to use fewer chemical inputs. Implications for the joint determination of insurance and input use decisions and appropriate estimation techniques are discussed. Copyright 1996, Oxford University Press.

Sediment and Chemical Load Reduction by Grass and Riparian Filters

Abstract: Vegetated filter strips help reduce non-point source pollution from agricultural areas. Even though they are an accepted and highly promoted practice, little quantitative data exist on their effectiveness under field conditions. The objective of this research was to determine the amount of nutrients and sediment removed by natural and planted filters. This was achieved by collecting and analyzing runoff at field edges and at various locations in vegetated buffers. Total weight of sediment and nutrients in runoff from North Carolina agricultural fields showed that the grass and riparian filter strips studied reduced runoff load by 50 to 80%. Total sediment decrease through the filters was about 80% for both grass and riparian vegetation. The reduction in the chemical load depended on the nutrient and its form. Filters reduced total P load by 50%, but 80% of the soluble PO 4 -P arriving at the field edge frequently passed through the filters. The filters retained 20 to 50% of the NH 4 and approximately 50% of the total Kjeldahl N and NO 3 . High-volume flows commonly overwhelmed both grass and riparian filters next to cultivated fields. Forested ephemeral channels had little vegetation and were effective sediment sinks during the dry season but were ineffective during large storm events because there was little resistance to flow. When possible, drainageways should be designed to hold sediment and to disperse the discharge into a riparian area.

A Sparse Approximate Inverse Preconditioner for the Conjugate Gradient Method

Abstract: A method for computing a sparse incomplete factorization of the inverse of a symmetric positive definite matrix $A$ is developed, and the resulting factorized sparse approximate inverse is used as an explicit preconditioner for conjugate gradient calculations. It is proved that in exact arithmetic the preconditioner is well defined if $A$ is an H-matrix. The results of numerical experiments are presented.

Gender Ideology and Perceptions of the Fairness of the Division of Household Labor: Effects on Marital Quality

Abstract: Under what circumstances will married women perceive inequalities in the division of household labor as unfair? This research develops and tests a model based on relative deprivation theory that suggests that gender ideology functions as a moderator variable in a process through which inequalities in the division of household labor come to be seen as inequities. Using data from the National Survey of Families and Households, three empirical tests of the model provide evidence that inequalities in the division of household labor are more strongly related to perceptions of inequity for egalitarian than for traditional wives, and that perceptions of inequity are more strongly related to perceived quality of the marital relationship for egalitarian than for traditional wives. The findings suggest that researchers studying the division of household labor need to shift their focus away from analyses of objective inequalities and toward the study of perceived inequity.

RFLP and Cytogenetic Evidence on the Origin and Evolution of Allotetraploid Domesticated Peanut, Arachis hypogaea (Leguminosae)

Abstract: Nuclear restriction fragment length polymorphism (RFLP) analysis was used to determine the wild diploid Arachis species that hybridized to form tetraploid domesticated peanut. Results using 20 previously mapped cDNA clones strongly indicated A. duranensis as the progenitor of the A genome of domesticated peanut and A ipaensis as the B genome parent A large amount of RFLP variability was found among the various accessions of A duranensis, and accessions most similar to the A genome of cultivated peanut were identified. Chloroplast DNA RFLP analysis determined that A duranensis was the female parent of the original hybridization event Domesticated peanut is known to have one genome with a distinctly smaller pair of chromosomes ("A"), and one genome that lacks this pair. Cytogenetic analysis demonstrated that A duranensis has a pair of "A" chromosomes, and A. ipaensis does not The cytogenetic evidence is thus consistent with the RFLP evidence concerning the identity of the progenitors RFLP and cytogenetic evidence indicate a single origin for domesticated peanut in Northern Argentina or Southern Bolivia, followed by diversification under the influence of cultivation

A Comparison of Poisson, Negative Binomial, and Semiparametric Mixed Poisson Regression Models: With Empirical Applications to Criminal Careers Data

Abstract: Specifications and moment properties of the univariate Poisson and negative binomial distributions are briefly reviewed and illustrated. Properties and limitations of the corresponding poisson and negative binomial (gamma mixtures of Poissons) regression models are described. It is shown how a misspecification of the mixing distribution of a mixed Poisson model to accommodate hidden heterogeneity ascribable to unobserved variables—although not affecting the consistency of maximum likelihood estimators of the Poisson mean rate parameter or its regression parameterization—can lead to inflated t ratios of regression coefficients and associated incorrect inferences. Then the recently developed semiparametric maximum likelihood estimator for regression models composed of arbitrary mixtures of Poisson processes is specified and further developed. It is concluded that the semiparametric mixed Poisson regression model adds considerable flexibility to Poisson-family regression models and provides opportunities for...

Call admission control schemes: a review

Abstract: Over the last few years, a substantial number of call admission control (CAC) schemes have been proposed for ATM networks. We review the salient features of some of these algorithms. Also, we quantitatively compare the performance of three of these schemes.

Multiple methods for the identification of polymorphic simple sequence repeats (SSRs) in sorghum [Sorghum bicolor (L.) Moench].

Abstract: Simple sequence repeats (SSRs), also known as microsatellites, are highly variable DNA sequences that can be used as markers for the genetic analysis of plants. Three approaches were followed for the development of PCR primers for the amplification of DNA fragments containing SSRs from sorghum [Sorghum bicolor (L.) Moench]: a search for sorghum SSRs in public DNA databases; the use of SSR-specific primers developed in the Poaceae species maize (Zea mays L.) and seashore paspalum grass (Paspalum vaginatum Swartz); and the screening of sorghum genomic libraries by hybridization with SSR oligonucleotides. A total of 49 sorghum SSR-specific PCR primer pairs (two designed from GenBank SSR-containing sequences and 47 from the sequences of genomic clones) were screened on a panel of 17 sorghum and one maize accession. Ten primer pairs from paspalum and 90 from maize were also screened for polymorphism in sorghum. Length polymorphisms among amplification products were detected with 15 of these primer pairs, yielding diversity values ranging from 0.2 to 0.8 with an average diversity of 0.56. These primer pairs are now available for use as markers in crop improvement and conservation efforts.

Comparison of eddy viscosity-transport turbulence models for three-dimensional, shock-separated flowfields

Abstract: An evaluation of four one-equation eddy viscosity-transport turbulence closure models as applied to three-dimensional shock wave/boundary-layer interactions is presented herein. Comparisons of two versions of the Baldwin-Barth model, an approach of Edwards and McRae, and a modified form of the Spalart-Allmaras model are presented for two test cases, one involving Mach 8 flow over a flat plate/sharp fin apparatus and the other involving Mach 3 flow over a cylinder-offset-cone geometry. Strengths and weaknesses of the one-equation approaches are highlighted through direct comparison with experimental data, and the effect of grid refinement is examined.

Costs and benefits of a predator-induced polyphenism in the gray treefrog hyla chrysoscelis

Abstract: The phenotypes of gray treefrog (Hyla chrysoscelis) tadpoles vary depending on whether predators are present in the pond. Tadpoles reared in ponds with predatory dragonfly larvae are relatively inactive compared with tadpoles in predator-free ponds, and have relatively large, brightly colored tailfins with dark spots along the margins. Models for the evolution of plasticity predict that induced phenotypes such as this should confer high fitness relative to the typical phenotype when in the presence of predators, but should be costly when the predator is absent. Our study tested for the predicted fitness trade-off in H. chrysoscelis by first rearing tadpoles in mesocosms under conditions that induce the alternate phenotypes, and then comparing the performance of both phenotypes in both environments. We generated the two phenotypes by rearing tadpoles in 600-liter outdoor artificial ponds that contained either two caged dragonflies (Anax junius) or an empty cage. Tadpoles from the two environments showed significantly different behavior, tail shape, and tail color within two weeks of exposure. We compared the growth and survival of both phenotypes over four weeks in ponds where there was no actual risk of predation. Under these conditions, both phenotypes grew at the same rate, but the predator-induced phenotype had significantly lower survival than the typical phenotype, indicating that induced tadpoles suffered greater mortality from causes other than odonate predation. We tested the susceptibility of both phenotypes to predation by exposing them to dragonflies in 24-h predation trials. The predator-induced phenotype showed a significant survival advantage in these trials. These results confirm that the predator-induced phenotype in H. chrysoscelis larvae is associated with fitness costs and benefits that explain why the defensive phenotype is induced rather than constitutive.

Judging Instrument Relevance in Instrumental Variables Estimation

Abstract: Recent research has emphasized the poor finite-sample performance of the instrumental variables (IV) estimator when the instruments are weakly correlated with the regressors. We show how the canonical correlations between regressors and instruments can provide a measure of instrument relevance in the general multiple-instrument-multiple-regressor case. However, our simulation results indicate that any such relevance measure probably has little practical merit, as its use may actually exacerbate the poor finite-sample properties of the IV estimator.

Real-space multigrid-based approach to large-scale electronic structure calculations

Abstract: We describe a set of techniques for performing large scale ab initio calculations using multigrid accelerations and a real-space grid as a basis. The multigrid methods provide effective convergence acceleration and preconditioning on all length scales, thereby permitting efficient calculations for ill-conditioned systems with long length scales or high energy cutoffs. We discuss specific implementations of multigrid and real-space algorithms for electronic structure calculations, including an efficient multigrid-accelerated solver for Kohn-Sham equations, compact yet accurate discretization schemes for the Kohn-Sham and Poisson equations, optimized pseudopotentials for real-space calculations, efficacious computation of ionic forces, and a complex-wave-function implementation for arbitrary sampling of the Brillouin zone. A particular strength of a real-space multigrid approach is its ready adaptability to massively parallel computer architectures, and we present an implementation for the Cray-T3D with essentially linear scaling of the execution time with the number of processors. The method has been applied to a variety of periodic and nonperiodic systems, including disordered Si, a N impurity in diamond, AlN in the wurtzite structure, and bulk Al. The high accuracy of the atomic forces allows for large step molecular dynamics; e.g., in a 1-ps simulation of Si at 1100 K with an ionic step of 80 a.u., the total energy was conserved within 27 \ensuremath{\mu}eV per atom. \textcopyright{} 1996 The American Physical Society.

Nitrogen Release from Grass and Legume Cover Crop Monocultures and Bicultures

Abstract: The use of grass-legume bicultures grown as winter annual cover crops may provide farmers with additional cover crop management options regarding the availability of cover crop residue N. A 2-yr field experiment was conducted to determine dry matter (DM) accumulation, chemical composition, and N release from grass and legume cover crops grown in monoculture (rye, crimson clover, and hairy vetch) and biculture (rye-crimson clover and rye-hairy vetch). Air-dried plant material was placed on the soil surface in 1-mm mesh nylon bags for 1, 2, 3, 4, 6, 8, and 16 wk. Following retrieval, mesh bag contents were analyzed for total N, C, cellulose, hemicellulose, and lignin concentrations. The 2-yr mean cover crop DM production was in the order of rye-hairy vetch > hairy vetch > rye-crimson clover > rye > crimson clover. The greatest cover crop N content (2-yr mean) occurred with hairy vetch monoculture (154 kg N ha -1 ), compared with a low of 41 kg N ha -1 for the rye monoculture. When grown in biculture with rye, hairy vetch accumulated more DM and biomass N compared with crimson clover, both as a proportion of the biculture and as DM yield. In general, the order of N release rates (rapid to slow) was hairy vetch > crimson clover = rye-hairy vetch > rye-crimson clover = rye. Estimates of N (kg ha -1 ) released from cover crop residue after 8 wk of field decomposition, averaged over 2 yr, were 24 for rye, 60 for crimson clover, 132 for hairy vetch, 48 for rye-crimson clover, and 108 for rye-hairy vetch. Results of this study demonstrate only slight reductions in N release from grass-legume bicultures compared with legume monocultures.