Anisotropic violation of the Wiedemann-Franz law at a quantum critical point

A quantum critical point transforms the behavior of electrons so strongly that new phases of matter can emerge. The interactions at play are known to fall outside the scope of the standard model of metals, but a fundamental question remains: Is the basic concept of a quasiparticle-a fermion with renormalized mass-still valid in such systems? The Wiedemann-Franz law, which states that the ratio of heat and charge conductivities in a metal is a universal constant in the limit of zero temperature, is a robust consequence of Fermi-Dirac statistics. We report a violation of this law in the heavy-fermion metal CeCoIn5 when tuned to its quantum critical point, depending on the direction of electron motion relative to the crystal lattice, which points to an anisotropic destruction of the Fermi surface.     

Deconfined quantum critical points

The theory of second-order phase transitions is one of the foundations of modern statistical mechanics and condensed-matter theory. A central concept is the observable order parameter, whose nonzero average value characterizes one or more phases. At large distances and long times, fluctuations of the order parameter(s) are described by a continuum field theory, and these dominate the physics near such phase transitions. We show that near second-order quantum phase transitions, subtle quantum interference effects can invalidate this paradigm, and we present a theory of quantum critical points in a variety of experimentally relevant two-dimensional antiferromagnets. The critical points separate phases characterized by conventional "confining" order parameters. Nevertheless, the critical theory contains an emergent gauge field and "deconfined" degrees of freedom associated with fractionalization of the order parameters. We propose that this paradigm for quantum criticality may be the key to resolving a number of experimental puzzles in correlated electron systems and offer a new perspective on the properties of complex materials.     

Imaging coherent electron flow from a quantum point contact

Scanning a charged tip above the two-dimensional electron gas inside a gallium arsenide/aluminum gallium arsenide nanostructure allows the coherent electron flow from the lowest quantized modes of a quantum point contact at liquid helium temperatures to be imaged. As the width of the quantum point contact is increased, its electrical conductance increases in quantized steps of 2 e(2)/h, where e is the electron charge and h is Planck's constant. The angular dependence of the electron flow on each step agrees with theory, and fringes separated by half the electron wavelength are observed. Placing the tip so that it interrupts the flow from particular modes of the quantum point contact causes a reduction in the conductance of those particular conduction channels below 2 e(2)/h without affecting other channels.     

Self-assembly at all scales

Self-assembly is the autonomous organization of components into patterns or structures without human intervention. Self-assembling processes are common throughout nature and technology. They involve components from the molecular (crystals) to the planetary (weather systems) scale and many different kinds of interactions. The concept of self-assembly is used increasingly in many disciplines, with a different flavor and emphasis in each.     

Swedish agriculture at a turning point

This paper gives an overview of the present state of Swedish agriculture and discusses some problems and experiences of general interest. After an introductory section on the characteristics of Swedish agriculture, a number of problems are described, such as social problems, market imbalances, environmental damage, changes in landscape amenities, and animal health. Possible causes of these problems are discussed. Recent attempts to solve the problems as well as the current Swedish debate on more radical changes in agricultural policy are also discussed. The main components of a suggested program are subsidies to farmers based on their acreage of grazed land or grassland and elimination of the present price supports and market regulations.     

无公害畜产品生产关键点控制初探

无公害畜产品是指产地环境、生产过程和产品质量符合国家有关标准和规范的要求,经认证合格获得认证证书并使用无公害农产品标志的未经加工或经初加工的食用畜产品。其生产涉及到产地饲养环境、饲料及饲料添加剂的使用、兽药使用、动物防疫、饲养管理及屠宰加工、包装运输等诸多方面和环节,因此必须在各环节中找到关键制约因素并加以控制。     

Universal signatures of fractionalized quantum critical points

Ground states of certain materials can support exotic excitations with a charge equal to a fraction of the fundamental electron charge. The condensation of these fractionalized particles has been predicted to drive unusual quantum phase transitions. Through numerical and theoretical analysis of a physical model of interacting lattice bosons, we establish the existence of such an exotic critical point, called XY*. We measure a highly nonclassical critical exponent η = 1.493 and construct a universal scaling function of winding number distributions that directly demonstrates the distinct topological sectors of an emergent Z(2) gauge field. The universal quantities used to establish this exotic transition can be used to detect other fractionalized quantum critical points in future model and material systems.     

强化实践性是增强政治理论课生命力的关键点

政治理论课教学缺乏生命力，缺少吸引力，其原因就在于理论教学本身缺少实效性，脱离实践，没有针对性，强化实践教学，使理论教学和社会实践和大学生思想实际紧密联系起来，是增强政治理论课生命力的关键点，强化政治理论课教学的实践性给任课教师提出了较高要求，对教学内容、教学方法、教学评价都提出了更高要求。     

Corridors increase plant species richness at large scales

Habitat fragmentation is one of the largest threats to biodiversity. Landscape corridors, which are hypothesized to reduce the negative consequences of fragmentation, have become common features of ecological management plans worldwide. Despite their popularity, there is little evidence documenting the effectiveness of corridors in preserving biodiversity at large scales. Using a large-scale replicated experiment, we showed that habitat patches connected by corridors retain more native plant species than do isolated patches, that this difference increases over time, and that corridors do not promote invasion by exotic species. Our results support the use of corridors in biodiversity conservation.     

Conductance quantization at a half-integer plateau in a symmetric GaAs quantum wire

We present data from an induced gallium arsenide (GaAs) quantum wire that exhibits an additional conductance plateau at 0.5(2e2/h), where e is the charge of an electron and h is Planck's constant, in zero magnetic field. The plateau was most pronounced when the potential landscape was tuned to be symmetric by using low-temperature scanning-probe techniques. Source-drain energy spectroscopy and temperature response support the hypothesis that the origin of the plateau is the spontaneous spin-polarization of the transport electrons: a ferromagnetic phase. Such devices may have applications in the field of spintronics to either generate or detect a spin-polarized current without the complications associated with external magnetic fields or magnetic materials.     

干红葡萄酒酿造过程中"关键质量控制点"的确定与控制

为了更好地控制葡萄酒的质量,满足消费者对产品质量的需求,结合HACCP体系过程及关键点控制与ISO9000族标准过程质量控制,对干红葡萄酒生产过程中影响质量的各环节进行分析,主要包括除梗破碎、浸渍发酵、分离和压榨、酒精发酵、苹果酸-乳酸发酵、老熟和陈酿、澄清、稳定和装瓶等工序,确定了葡萄酒酿造过程中的关键质量控制点,并建立了相应的预防和控制措施。     

Super ENSO and global climate oscillations at millennial time scales

The late Pleistocene history of seawater temperature and salinity variability in the western tropical Pacific warm pool is reconstructed from oxygen isotope (delta18O) and magnesium/calcium composition of planktonic foraminifera. Differentiating the calcite delta18O record into components of temperature and local water delta18O reveals a dominant salinity signal that varied in accord with Dansgaard/Oeschger cycles over Greenland. Salinities were higher at times of high-latitude cooling and were lower during interstadials. The pattern and magnitude of the salinity variations imply shifts in the tropical Pacific ocean/atmosphere system analogous to modern El Ni?o-Southern Oscillation (ENSO). El Ni?o conditions correlate with stadials at high latitudes, whereas La Ni?a conditions correlate with interstadials. Millennial-scale shifts in atmospheric convection away from the western tropical Pacific may explain many paleo-observations, including lower atmospheric CO2, N2O, and CH4 during stadials and patterns of extratropical ocean variability that have tropical source functions that are negatively correlated with El Ni?o.     

Community structure of corals and reef fishes at multiple scales

Distributions of numerical abundance and resource use among species are fundamental aspects of community structure. Here we characterize these patterns for tropical reef fishes and corals across a 10,000-kilometer biodiversity gradient. Numerical abundance and resource-use distributions have similar shapes, but they emerge at markedly different scales. These results are consistent with a controversial null hypothesis regarding community structure, according to which abundance distributions arise from the interplay of multiple stochastic environmental and demographic factors. Our findings underscore the importance of robust conservation strategies that are appropriately scaled to the broad suite of environmental processes that help sustain biodiversity.