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PT Sprunger L Petersen EW Plummer E Laegsgaard F Besenbacher 《Science (New York, N.Y.)》1997,275(5307):1764-1767
Large-amplitude electron density oscillations were observed on a Be(0001) surface by means of variable-temperature scanning tunneling microscopy. Fourier transforms of the images showed a ring of radius 2kF, where kF is the Fermi wave vector of the Be(0001) surface state. This wavelength was expected from Friedel oscillations caused by electronic screening of surface defects, but the amplitude of the waves for energies near the Fermi energy was anomalously large and inconsistent with the Friedel concept of screening. The enhanced amplitude of the waves must be a many-body effect, either in the electron gas (possibly an incipient charge density wave) or in the response of the lattice (electron-phonon coupling). 相似文献
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Block copolymers consist of two or more chemically different polymer segments, or blocks, connected by a covalent linkage. In solution, amphiphilic blocks can self-assemble as a result of energetic repulsion effects between blocks. The degree of repulsion, the lengths of the block segments, and the selectivity of the solvent primarily control the resultant assembled morphology. In an ideal situation, one would like to be able to alter the morphology that forms without having to change the chemistry of the block copolymer. Through the kinetic manipulation of charged, amphiphilic block copolymers in solution, we are able to generate different nanoscale structures with simple block copolymer chemistry. The technique relies on divalent organic counter ions and solvent mixtures to drive the organization of the block copolymers down specific pathways into complex one-dimensional structures. Block copolymers are increasingly used as templating materials; thus, the ability to control the formation of specific patterns and structures is of growing interest and applicability. 相似文献
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Investor ownership or social investment? Changing farmland ownership in Saskatchewan,Canada 总被引：1，自引：0，他引：1
Annette Aurélie Desmarais Darrin Qualman André Magnan Nettie Wiebe 《Agriculture and Human Values》2017,34(1):149-166
There is growing recognition that land grabbing is a global phenomenon. In Canada, investors are particularly interested in Saskatchewan farmland, the province where 40 % of country’s agricultural land is situated. This article examines how the changing political, economic, and legal context under neoliberalism has shaped patterns of farmland ownership in Saskatchewan, between 2002 and 2014. Our research indicates that over this time, the amount of farmland owned by investors increased 16-fold. Also, the concentration of farmland ownership is on the rise, with the share of farmland owned by the largest four private owners increasing six-fold. Our methodology addresses some of the criticisms raised in the land grabbing literature. By using land titles data, we identified farmland investors and determined very precisely their landholdings thus allowing us to provide a fine-grained analysis of the actual patterns of farmland ownership. Although the article analyzes changes to farmland ownership in a specific historical, cultural and legislative context, it serves as the basis for a broader discussion of the values and priorities that land ownership policies reflect. Namely, we contrast an ‘open for business’ approach that prioritizes financial investment to one based on a land sovereignty approach that prioritizes social investment. The latter has greater potential if the aim is ecological sustainability and food sovereignty. 相似文献
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PW MORCOMBE DF HIDE† GE YOUNG EW ARMSTRONG‡ LE MILLAR§ ARB HIGGS¶ 《Australian veterinary journal》1995,72(11):411-414
SUMMARY Insecticidal dipping fluid emulsions, mixed in vitro in dam water containing suspended clay particles and 1% w/v zinc sulphate, were analysed to determine rates of settling of diazinon, cyhalothrin and cypermethrin. Fifteen minutes after mixing, the concentration of the insecticides 5 cm below the surface had declined by 72.5%, 72.8% and 89.4%, respectively. On remixing, the concentration of insecticide in suspension was close to or greater than the initial concentration. In 2 trials, lice were eradicated from sheep showered with dip wash mixed in cloudy dam water to which 1% w/v of zinc sulphate was added. In 12 flock treatments in which 1000 to 2000 sheep were dipped with added zinc sulphate, the concentration of insecticide remained above the minimum lethal for susceptible strains of lice. However, lice were still present 6 months later in 8 of these flocks. When zinc sulphate is added to dip wash, agitation is needed to maintain the insecticide in suspension. 相似文献
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Samuel A. Wyffels Mark K. Petersen Darrin L. Boss Bok F. Sowell Janice G.P. Bowman Lance B. McNew 《Strength and Conditioning Journal》2019,72(6):878-887
Grazing dormant forage under low-input heifer development strategies typically exposes cattle to low-quality forage. Protein supplementation while grazing dormant range can enhance heifer growth and reproductive performance. We examined resource utilization of heifers and the effects of dormant season grazing on residual vegetation characteristics under two supplementation management strategies. Approximately 100 weaned composite heifer calves were randomly selected and placed into one of two supplementation treatments in each of 2 yr, one receiving a free access 62% crude protein self-fed concentrate and the other receiving a daily hand-fed 20% crude protein cake. Grazing occurred from December (2015 and 2016) through March (2016 and 2017). Thirty transects were randomly located within each pasture for measuring vegetation quality and structure before and after grazing. Daily space use and behavior was evaluated for 21 individuals within each treatment using global positioning system (GPS) collars and resource utilization functions. Heifers supplemented with concentrated protein spent more time grazing per day than heifers supplemented with cake (6.92 ± 0.18, 6.24 ± 0.17 h). Relative use by heifers in the cake treatment was negatively related to horizontal distance from the supplement delivery site early to midwinter ( = ? 0.41 ± 0.16, ? 0.53 ± 0.17). Both treatments selected grazing locations relative to standing biomass of perennial grasses ( = 0.0005 ± 0.00004) and crude protein ( = 0.12 ± 0.007). However, resource selection was highly variable among individuals for both supplementation treatments. We found no treatment effects on pre-post grazing differences in residual cover of litter, grass, forbs and shrubs (P > 0.24). However, the time period when grazing occurred had an effect on residual vegetation conditions (P < 0.01). Our results indicate high levels of variability in grazing site selection by heifers, suggesting future research should incorporate individual animal measurements in an attempt to account for individual animal variability. 相似文献
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We determined how ecophysiological characteristics of two juniper species, Juniperus occidentalis Hook. (western juniper) and Juniperus osteosperma (Torr.) Little (Utah juniper), changed along altitudinal and regional environmental gradients in the Great Basin of western North America. We obtained diurnal measurements of leaf gas exchange and xylem water potential (Psi) from plants at a low and a high altitude site within each of six mountain ranges during fall 1994, spring, summer, and fall 1995, and summer 1996. We also determined carbon isotope composition (delta(13)C) of leaf cellulose produced during the 1995 growing season. Overall, leaf gas exchange, Psi and delta(13)C did not differ significantly between species. Differences in daily (A(d)) and season-long (A(s)) carbon assimilation among mountain ranges suggested two groupings-a group of northern ranges and a group of southern ranges. Each group contained one mountain range with J. occidentalis and two with J. osteosperma. Differences in carbon assimilation based on this grouping were associated with two findings: (1) conductance of CO(2) from substomatal cavities to the site of carboxylation (g(m)) for junipers in the northern ranges averaged almost twice that of junipers in the southern ranges; and (2) physiological shifts occurred such that A(d) of junipers in the northern ranges was influenced more by Psi(pd), whereas A(d) of junipers in the southern ranges was influenced more by leaf temperature. Mean delta(13)C over all trees at a site was significantly correlated with annual precipitation. Significant differences in A(d) occurred between altitudes, but these differences were associated with differences in the timing of optimum leaf temperature for photosynthesis rather than with physiological acclimation to temperature, irradiance, or Psi. Most gas exchange parameters (e.g., assimilation, transpiration, stomatal conductance, and water use efficiency) varied seasonally, and the seasonal differences were strongly influenced by water stress. 相似文献
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The effects of the pancreatic hormones, insulin and glucagon, on rates of lipid biosynthesis in liver removed from rainbow trout, Oncorhynchus mykiss, were evaluated in vitro. Livers were removed from animals fasted for 30–36h, cut into ca. 1 mm3 pieces, and incubated in the presence of various concentrations of salmon insulin (sINS), bovine insulin (bINS), or a combination of BINS and bovine/porcine glucagon (GLU). Lipid synthesis was evaluated by total lipid concentration, 3H2O incorporation into total lipid, and by fatty acid synthetase activity. Both mammalian and sINS tended to increase tissue total lipid concentration in hepatic tissue incubated for 5h. Insulin also stimulated 3H2O incorporation into total lipid in a dose-dependent manner. Bovine INS (2 × 10?6 M) stimulated de novo synthesis nearly 6-fold over control rates; sINS (2 × 10?6 M) stimulated label incorporation more than 7-fold over control rates. Glucagon inhibited INS-stimulated 3H2O incorporation; whereas, GLU alone had no effect on lipid synthesis in liver pieces incubated 5h. Lipid class analysis indicated that bINS significantly stimulated 3H2O incorporation into phospholipids, fatty acids, and triacylglycerols. The greatest accumulation of label was in the triacylglycerol fraction, where incorporation was stimulated 17-fold over control levels. Hepatic enzymatic analysis indicated that bINS also significantly stimulated lipogenic enzyme activity 9-fold above control levels. These results indicate that INS is an important regulator of lipid synthesis in the liver of trout. 相似文献
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A stable phase of toroidal, or ringlike, supramolecular assemblies was formed by combining dilute solution characteristics critical for both bundling of like-charged biopolymers and block copolymer micelle formation. The key to toroid versus classic cylinder micelle formation is the interaction of the negatively charged hydrophilic block of an amphiphilic triblock copolymer with a positively charged divalent organic counterion. This produces a self-attraction of cylindrical micelles that leads to toroid formation, a mechanism akin to the toroidal bundling of semiflexible charged biopolymers such as DNA. The toroids can be kinetically trapped or chemically cross-linked. Insight into the mechanism of toroid formation can be gained by observation of intermediate structures kinetically trapped during film casting. 相似文献