Chaotic electron diffusion through stochastic webs enhances current flow in superlattices

Understanding how complex systems respond to change is of fundamental importance in the natural sciences. There is particular interest in systems whose classical newtonian motion becomes chaotic as an applied perturbation grows. The transition to chaos usually occurs by the gradual destruction of stable orbits in parameter space, in accordance with the Kolmogorov-Arnold-Moser (KAM) theorem--a cornerstone of nonlinear dynamics that explains, for example, gaps in the asteroid belt. By contrast, 'non-KAM' chaos switches on and off abruptly at critical values of the perturbation frequency. This type of dynamics has wide-ranging implications in the theory of plasma physics, tokamak fusion, turbulence, ion traps, and quasicrystals. Here we realize non-KAM chaos experimentally by exploiting the quantum properties of electrons in the periodic potential of a semiconductor superlattice with an applied voltage and magnetic field. The onset of chaos at discrete voltages is observed as a large increase in the current flow due to the creation of unbound electron orbits, which propagate through intricate web patterns in phase space. Non-KAM chaos therefore provides a mechanism for controlling the electrical conductivity of a condensed matter device: its extreme sensitivity could find applications in quantum electronics and photonics.     

浅谈计算机网络会计在网络财务中所起的作用

通过与传统会计信息系统的比较，分析了计算机网络会计信息系统及网络财务所具有的特点，阐述了计算机网络会计信息系统在网络财务管理中所起的作用。     

圆柱蜗杆与蜗轮的加工测量

给出了圆柱蜗杆及蜗轮轮齿在任意半径处弦齿厚计算的精确公式，采用了圆球法测量蜗轮直径方向的两齿间距，为了计算上的需要，引进了蜗轮弦齿厚计算斜齿轮的概念和斜齿轮测量圆棒跨距法。     

Purification and unique properties of mammary epithelial stem cells

Elucidation of the cellular and molecular mechanisms that maintain mammary epithelial tissue integrity is of broad interest and paramount to the design of more effective treatments for breast cancer. Evidence from both in vitro and in vivo experiments suggests that mammary cell differentiation is a hierarchical process originating in an uncommitted stem cell with self-renewal potential. However, analysis of the properties and regulation of mammary stem cells has been limited by a lack of methods for their prospective isolation. Here we report the use of multi-parameter cell sorting and limiting dilution transplant analysis to demonstrate the purification of a rare subset of adult mouse mammary cells that are able individually to regenerate an entire mammary gland within 6 weeks in vivo while simultaneously executing up to ten symmetrical self-renewal divisions. These mammary stem cells are phenotypically distinct from and give rise to mammary epithelial progenitor cells that produce adherent colonies in vitro. The mammary stem cells are also a rapidly cycling population in the normal adult and have molecular features indicative of a basal position in the mammary epithelium.     

我国青贮玉米的研究现状及市场前景

青贮玉米作为饲料玉米既能大幅度地提高饲料产量，同时具有较高的营养价值，在国民经济和人民生活中占有越来越重要的地位。在借鉴欧美等发达国家对青贮玉米研究成果的基础上，分析了我国发展青贮玉米的优势和潜力，提出了青贮玉米的育种策略、育种目标和主要用于家畜饲养业的有效利用途径。     

Reproduction: widespread cloning in echinoderm larvae

Asexual reproduction by free-living invertebrate larvae is a rare and enigmatic phenomenon and, although it is known to occur in sea stars and brittle stars, it has not been detected in other echinoderms despite more than a century of intensive study. Here we describe spontaneous larval cloning in three species from two more echinoderm classes: a sea cucumber (Holothuroidea), a sand dollar and a sea urchin (Echinoidea). Larval cloning may therefore be an ancient ability of echinoderms and possibly of deutero-stomes - the group that includes echinoderms, acorn worms, sea squirts and vertebrates.     

Reach Envelope of Human Extremities

Significant attention in recent years has been given to obtain a better understanding of human joint ranges, measurement, and functionality, especially in conjunction with commands issued by the central nervous system. While researchers have studied motor commands needed to drive a limb to follow a path trajectory, various computer algorithms have been reported that provide adequate analysis of limb modeling and motion. This paper uses a rigorous mathematical formulation to model human limbs, understand their reach envelope, delineate barriers therein where a trajectory becomes difficult to control, and help visualize these barriers. Workspaces of a typical forearm with 9 degrees of freedom, a typical finger modeled as a 4-degree-of-freedom system, and a lower extremity with 4 degrees of freedom are discussed. The results show that using the proposed formulation, joint limits play an important role in distinguishing the barriers.     

Haplotype tagging for the identification of common disease genes

Genome-wide linkage disequilibrium (LD) mapping of common disease genes could be more powerful than linkage analysis if the appropriate density of polymorphic markers were known and if the genotyping effort and cost of producing such an LD map could be reduced. Although different metrics that measure the extent of LD have been evaluated, even the most recent studies have not placed significant emphasis on the most informative and cost-effective method of LD mapping-that based on haplotypes. We have scanned 135 kb of DNA from nine genes, genotyped 122 single-nucleotide polymorphisms (SNPs; approximately 184,000 genotypes) and determined the common haplotypes in a minimum of 384 European individuals for each gene. Here we show how knowledge of the common haplotypes and the SNPs that tag them can be used to (i) explain the often complex patterns of LD between adjacent markers, (ii) reduce genotyping significantly (in this case from 122 to 34 SNPs), (iii) scan the common variation of a gene sensitively and comprehensively and (iv) provide key fine-mapping data within regions of strong LD. Our results also indicate that, at least for the genes studied here, the current version of dbSNP would have been of limited utility for LD mapping because many common haplotypes could not be defined. A directed re-sequencing effort of the approximately 10% of the genome in or near genes in the major ethnic groups would aid the systematic evaluation of the common variant model of common disease.