This Week in Science

Science  08 Jul 2005:
Vol. 309, Issue 5732, pp. 217
  1. Probing Coupling Between Quantum Dot Pairs

    CREDIT: HATANO ET AL.

    Manipulating exchange coupling between two electrons in coupled two-dot system is a fundamental concept in a spin-based quantum computing. Placing an electron on one dot affects the charging energy, and therefore the population dynamics of the other dot. However, these energies have not been well studied for realistic double-dot devices. Hatano et al. (p. 268) describe experiments and theory of electron tunneling in parallel through a hybrid vertical-lateral double-dot device. Depending on the alignment of the electronic states in the left and right dots, which can be tuned with gates, the additional electron can be localized in either dot or delocalized between the two. The two-quantum-dot system presented here should provide useful information for realistic implementations of quantum information processing using coupled quantum dots.

  2. Nanophases and Electron Correlations

    Some of most interesting condensed matter phenomena, such as high-temperature superconductivity and colossal magnetoresistance in transition metal oxides, occur in materials that have strongly correlated electrons. In addition, these materials often exhibit nanoscale phases that are spatially inhomogeneous. Dagotto (p. 257) reviews recent research in strongly correlated systems and argues that such materials are similar to other complex systems where new behavior emerges from the interaction of competing phases. Understanding these interactions and controlling the complex pattern formation in these materials will enable the emergence of novel functional properties.

  3. Tiny Glass Engineers

    Nature often has to make use of less than ideal construction materials because they are the only ones at hand. To compensate, organisms develop tricks to overcome the inherent weaknesses of these materials. Aizenberg et al. (p. 275; see the cover and the Perspective by Currey) have looked at the mineral-based skeleton of a deep-sea, sediment-dwelling sponge that is primarily made of glass. Euplectella uses a myriad of engineering tricks to overcome the brittle nature of glass and shows seven levels of hierarchical structure that span from the nanometer to the micrometer scale.

  4. A Bite Out of C60

    The initially surprising stability of C60 has been justified by the precise arrangement of five- and six-membered rings in the framework. Although larger clusters, such as C70, have been prepared, most smaller structures would require expanded rings, such as heptagons, in the skeleton. The associated strain has kept efficient synthesis of such compounds out of reach. Troshin et al. (p. 278) used a fluorinating agent, based on a cesium lead oxyfluoride salt, and synthesized milligram quantities of the elusive C58 clusters on heating with C60. Two stable isolated compounds, C58F18 and C58F17CF3, were characterized by mass spectrometry and by infrared and nuclear magnetic resonance spectroscopy. The data support a closed framework containing a seven-membered ring.

  5. Moving Electrons Locally

    The electrical conductivity of metals is understood in terms of delocalized band structures, but an alternative conductivity model, proposed by Pauling and modified by Anderson, suggests that conductivity can also arise in some materials in a localized way by the formation of resonating valence bond (RVB) structures that alternate between neutral species and ionic pairs. Pal et al. (p. 281) prepared a molecular solid based on the spirobiphenalenyl molecules that are neutral free radicals. The material has a high conductivity (0.3 siemens per centimeter), and extended Hückel calculations and magnetic susceptibility measurements indicate that the materials are metallic and have no band gap. However, the conductivity is slightly activated, and electronic spectra show an energy gap of 0.34 electron volt. The authors argue that these properties are best explained by viewing the material as a Mott insulator whose conductivity arises through an RVB ground state, unlike ion radical organic conductors.

  6. Australian Entry Evidence

    Long climate and environmental records have been difficult to obtain from Australia. Humans arrived there about 50,000 years ago, just at the limit of radiocarbon dating. Whether their arrival led to the demise of much of Australia's distinct megafauna has been debated. Miller et al. (p. 287; see the Perspective by Johnson) have now obtained a 140,000-year record of the paleovegetation from three distinct sites in Australia based on the stable carbon isotope ratios of emu eggshells and wombat teeth. This record shows that shortly after the proposed human arrival, the emus and wombats were forced to eat more shrubs instead of grasses.

  7. A Warning from Warmer Oceans

    Observations have shown that the upper parts of all of the oceans of the world have become warmer during the past 50 years, and such warming could only have been caused by the absorption of huge amounts of heat. Barnett et al. (p. 284, published online 2 June 2005; see the Perspective by Hegerl and Bindoff) examine the patterns of warming on an ocean-by-ocean basis, as a function of amount, location, and time, and discuss the physics responsible for the observed trends. The patterns of warming can be reproduced accurately by two different climate models only if radiative forcing caused by increases in atmospheric greenhouse gases is included.

  8. Expanding the Professional Cell Staff

    Antigen-presenting cells (APCs) chew up proteins and offer the resulting fragments of peptide, along with a suite of stimulatory molecules, to cells of the γδ T cell receptor (TCR) lineage to produce activated T cells armed and ready to clear the corresponding infection. Few cell types are known to be potent “professional” APCs, and at the very top of the stack are dendritic cells (DCs). Brandes et al. (p. 264, published online 2 June 2005; see the Perspective by Modlin and Sieling) now expand this realm to include a subset of nonconventional human T cells bearing the TCR. These cells react vigorously to microbial stimulation and when induced to do so in cell culture, became extremely efficient at presenting different types of antigen to their γδ T cell counterparts. The cells appeared to traffic antigen to the same cellular compartments as DCs and up-regulated an equivalent array of stimulatory and homing molecules. As well as contributing directly to innate immunity, T cells may also represent important instigators of adaptive immune responses.

  9. Controlling the Layout

    CREDIT: SHPAK ET AL.

    Successful adaptation and evolution of land plants relied on the acquisition of the stomatal complex, which allows efficient gas exchange for photosynthesis and respiration while minimizing water loss. In the epidermis of higher plants, stomatal complexes differentiate nonrandomly from precursor cells through rounds of asymmetric division. Shpak et al. (p. 290) now find that three Arabidopsis ERECTA family leucine-rich repeat receptor-like kinases, which are known to promote cell proliferation and organ growth, play overlapping but distinct roles to control stomatal patterning. The complexity of this signaling pathway illustrates how the interplay of moderate effects can lead to different outcomes in a developmental process.

  10. When It's Spring Again

    How does the plant know when its springtime? Imaizumi et al. (p. 293) now add some of the molecular details to the fascinating subcellular signaling process involved as plants respond to increases in daylength. As the days lengthen, so does the window of opportunity through which one protein, expressed in a daily cyclical pattern, can degrade its target. With longer days, the target suffers increasing degradation, removing its repression of the protein CONSTANS, thus allowing flowering to proceed.

  11. Slip Sliding Away

    Eukaryotic cells contain organized microtubule arrays that orchestrate polarized cellular behaviors. Fission yeast cells grow longitudinally and require a polarized distribution of their interphase microtubules along the long, growing axis of the cell. Carazo-Salas et al. (p. 297) describe how cytoplasmic microtubular arrays are arranged via microtubule sliding during interphase. An evolutionarily conserved, minus-end-directed molecular motor kinesin Klp2 is responsible for this sliding. The mechanism plays an important role in generating the highly polarized microtubules in fission yeast, and similar mechanisms may be exploited by other eukaryotes.

  12. Poetry in Motion

    The cooperative organization of dynamic biological processes often requires coordination via chemical signaling. Riedel et al. (p. 300) found that when attached to a surface, a critical number of sperm cells self-organized into a hexagonally packed array of rotating vortices where each vortex consisted of about 10 hydrodynamically synchronized cells forming a quantized rotating wave. This spatial-temporal pattern of entrained sperm cells formed in the absence of chemical cell-cell signaling, leading to a new coordination concept of cooperative cilia and flagella. Thus, single cells and microorganisms can be hydrodynamically coordinated without the need for chemical signaling.

  13. Sensing Friend or Foe

    Ants secrete and recognize specific blends of hydrocarbons in the cuticle, which enable them to display aggressive behavior toward non-nestmates. This identification process is thought to occur at a higher neural level. Ozaki et al. (p. 311, published online 9 June 2005) have found chemosensory sensilla in the ant antenna that respond to cuticle hydrocarbon blends from non-nestmates, and identify a protein that may carry the compounds to sensory receptors in the sensilla. This finding suggests that chemical information is also processed peripherally.

  14. Tunable Superconductivity

    Integrating superconducting and semiconducting materials to form hybrid devices offers a huge possibility for an enriched device technology. Doh et al. (p. 272) report low-temperature transport measurements through InAs semiconductor nanowires positioned between superconducting contacts. Below 1 kelvin, a supercurrent flows through the nanowires due to the proximity effect. Moreover, by varying the carrier density in the nanowires by application of a gate voltage, they modified the strength of the proximity effect, effectively switching superconductivity on and off.

  15. What Are miRNAs Good For?

    MicroRNAs (miRNAs) are small ~22-nucleotide noncoding RNAs that play a role in posttranscriptionally regulating target gene expression. Many miRNA genes have been found in both plant and animal genomes, and recent work has suggested that miRNAs may be involved in cell differentiation and morphogenesis, rather than in fate specification. Wienholds et al. (p. 310, published online 26 May 2005) examined the in situ expression patterns of 115 miRNAs in zebrafish embryos, miRNAs that are also conserved in mice and humans. In many cases, the patterns were highly tissue- and cell-specific, consistent with a general role for miRNAs in cell differentiation. Expression was not seen before segment formation, as expected if miRNAs do not promote tissue establishment.

  16. Beyond Subjective Judgments

    Although nuclear magnetic resonance (NMR) spectroscopy is an established technique for the determination of three-dimensional molecular structures, problems remain in translating the data into atomic coordinates. The data is usually insufficient to uniquely define a structure, and subjective choices in data treatment and parameter settings make it difficult to judge the precision of NMR structures. Rieping et al. (p. 303) introduce a probabilistic method to calculate structures from NMR data. They view structure determination as an inference problem, and use a Bayesian approach to derive a probability distribution that represents the calculated structure and its precision. The method is not only more objective and general than those presently used, but also gives better quality structures.

  17. Memory Transfer Across the Senses

    Is there an interaction between different sensory modalities during the acquisition of memories? Guo and Guo (p. 307) investigated crossmodal synergism and memory transfer in Drosophila. Simultaneous presentation of visual and olfactory cues during reinforcement learning conditioning facilitated memory acquisition at a threshold level that had been shown to be ineffective for conditioning of only one sensory input. Bimodal conditioning also lowered the threshold for memory retrieval of either sensory modality. Bimodal preconditioning and subsequent unimodal conditioning with either olfactory or visual induced crossmodal memory transfer.

  18. Making Muscle from Test Tube to Mouse

    Bone marrow stromal cells are generally known as being the source of skeletal tissue. Dezawa et al. (p. 314) have now shown how to efficiently direct the bone marrow stromal cells of human and rat toward muscle cell fate. The defined cell culture conditions include modifying the cells to express a portion of the signaling molecule Notch. The treated cells fuse to form myotubes in culture and can be incorporated into the muscles of dystrophic mice and into the degenerated muscles of rats.

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