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51.
在计算机辅助设计(CAD)中,针对大量的表格和线图数据的传输与查询,提出了利用FoxBASE和Autolisp与ArtoCAD的直接传输方法,此方法简单实用,易于实现。  相似文献   
52.
53.
Persistence of soil organic matter as an ecosystem property   总被引:65,自引:0,他引:65  
Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for millennia whereas other SOM decomposes readily--and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming.  相似文献   
54.
Sawano F  Terasaki I  Mori H  Mori T  Watanabe M  Ikeda N  Nogami Y  Noda Y 《Nature》2005,437(7058):522-524
Thyristors are a class of nonlinear electronic device that exhibit bistable resistance--that is, they can be switched between two different conductance states. Thyristors are widely used as inverters (direct to alternating current converters) and for the smooth control of power in a variety of applications such as motors and refrigerators. Materials and structures that exhibit nonlinear resistance of this sort are not only useful for practical applications: they also provide systems for exploring fundamental aspects of solid-state and statistical physics. Here we report the discovery of a giant nonlinear resistance effect in the conducting organic salt theta-(BEDT-TTF)2CsCo(SCN)4, the voltage-current characteristics of which are essentially the same as those of a conventional thyristor. This intrinsic organic thyristor works as an inverter, generating an alternating current when a static direct-current voltage is applied. Whereas conventional thyristors consist of a series of diodes (their nonlinearity comes from interface effects at the p-n junctions), the present salt exhibits giant nonlinear resistance as a bulk phenomenon. We attribute the origin of this effect to the current-induced melting of insulating charge-order domains, an intrinsically non-equilibrium phenomenon in the sense that ordered domains are melted by a steady flow.  相似文献   
55.
Stomatal pores, formed by two surrounding guard cells in the epidermis of plant leaves, allow influx of atmospheric carbon dioxide in exchange for transpirational water loss. Stomata also restrict the entry of ozone--an important air pollutant that has an increasingly negative impact on crop yields, and thus global carbon fixation and climate change. The aperture of stomatal pores is regulated by the transport of osmotically active ions and metabolites across guard cell membranes. Despite the vital role of guard cells in controlling plant water loss, ozone sensitivity and CO2 supply, the genes encoding some of the main regulators of stomatal movements remain unknown. It has been proposed that guard cell anion channels function as important regulators of stomatal closure and are essential in mediating stomatal responses to physiological and stress stimuli. However, the genes encoding membrane proteins that mediate guard cell anion efflux have not yet been identified. Here we report the mapping and characterization of an ozone-sensitive Arabidopsis thaliana mutant, slac1. We show that SLAC1 (SLOW ANION CHANNEL-ASSOCIATED 1) is preferentially expressed in guard cells and encodes a distant homologue of fungal and bacterial dicarboxylate/malic acid transport proteins. The plasma membrane protein SLAC1 is essential for stomatal closure in response to CO2, abscisic acid, ozone, light/dark transitions, humidity change, calcium ions, hydrogen peroxide and nitric oxide. Mutations in SLAC1 impair slow (S-type) anion channel currents that are activated by cytosolic Ca2+ and abscisic acid, but do not affect rapid (R-type) anion channel currents or Ca2+ channel function. A low homology of SLAC1 to bacterial and fungal organic acid transport proteins, and the permeability of S-type anion channels to malate suggest a vital role for SLAC1 in the function of S-type anion channels.  相似文献   
56.
渗硼层的制备及其在锌液中的耐蚀性   总被引:8,自引:1,他引:8  
为提高低碳钢的抗锌蚀能力,采用固态渗硼工艺在低碳钢的表面形成了致密的铁硼化合物保护层.通过正交实验方法确定了最佳渗硼工艺参数,并对不同工艺条件下所获得的渗硼层的力学性能与耐锌液腐蚀性能进行了系统研究,成功制备了具有良好的抗锌蚀能力的渗硼层.  相似文献   
57.
本书涵盖了面向SoC(System on Chip,片上系统)处理器的集成综合器电路设计的论题,采取了一种更为全局的设计观念来考察电路级和体系结构级的设计空间。书中的论述十分广泛,而且包括电路理论和锁相环反馈控制理论的综述。在电路级方面,讨论包括深亚微米数字CMOS过程的低功耗模拟设计、供电噪声效应、设备噪声;在体系结构级方面的论述,涵盖了连续时间和离散时间模型的锁相环分析,以及锁相行为的细节分析。还有一些章节对特定的时钟生成器模块做了电路级和系统结构级的深入描述,其中包括高供电噪声屏蔽的锁相环电路、体系结构和数字锁相环体系结构,考察了为离散时间模拟部件产生低失真采样时钟的方法。这里所说的锁相环包括希格马.代尔塔N分锁相环、直接数字综合(DDS、Direct Digital Synthesis)技术和锁相环的非常规应用。本书讨论的面向测试的设计(Dvr、Design for Test),其中包括锁相环的精确测量滤波器方法和嵌人式测试(BIST、Built—in—self-test)技术。  相似文献   
58.
59.
Roos CF  Chwalla M  Kim K  Riebe M  Blatt R 《Nature》2006,443(7109):316-319
Entanglement is recognized as a key resource for quantum computation and quantum cryptography. For quantum metrology, the use of entangled states has been discussed and demonstrated as a means of improving the signal-to-noise ratio. In addition, entangled states have been used in experiments for efficient quantum state detection and for the measurement of scattering lengths. In quantum information processing, manipulation of individual quantum bits allows for the tailored design of specific states that are insensitive to the detrimental influences of an environment. Such 'decoherence-free subspaces' (ref. 10) protect quantum information and yield significantly enhanced coherence times. Here we use a decoherence-free subspace with specifically designed entangled states to demonstrate precision spectroscopy of a pair of trapped Ca+ ions; we obtain the electric quadrupole moment, which is of use for frequency standard applications. We find that entangled states are not only useful for enhancing the signal-to-noise ratio in frequency measurements--a suitably designed pair of atoms also allows clock measurements in the presence of strong technical noise. Our technique makes explicit use of non-locality as an entanglement property and provides an approach for 'designed' quantum metrology.  相似文献   
60.
The medaka draft genome and insights into vertebrate genome evolution   总被引:3,自引:0,他引:3  
Teleosts comprise more than half of all vertebrate species and have adapted to a variety of marine and freshwater habitats. Their genome evolution and diversification are important subjects for the understanding of vertebrate evolution. Although draft genome sequences of two pufferfishes have been published, analysis of more fish genomes is desirable. Here we report a high-quality draft genome sequence of a small egg-laying freshwater teleost, medaka (Oryzias latipes). Medaka is native to East Asia and an excellent model system for a wide range of biology, including ecotoxicology, carcinogenesis, sex determination and developmental genetics. In the assembled medaka genome (700 megabases), which is less than half of the zebrafish genome, we predicted 20,141 genes, including approximately 2,900 new genes, using 5'-end serial analysis of gene expression tag information. We found single nucleotide polymorphisms (SNPs) at an average rate of 3.42% between the two inbred strains derived from two regional populations; this is the highest SNP rate seen in any vertebrate species. Analyses based on the dense SNP information show a strict genetic separation of 4 million years (Myr) between the two populations, and suggest that differential selective pressures acted on specific gene categories. Four-way comparisons with the human, pufferfish (Tetraodon), zebrafish and medaka genomes revealed that eight major interchromosomal rearrangements took place in a remarkably short period of approximately 50 Myr after the whole-genome duplication event in the teleost ancestor and afterwards, intriguingly, the medaka genome preserved its ancestral karyotype for more than 300 Myr.  相似文献   
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