High critical current density and enhanced irreversibility field in superconducting MgB2 thin films

The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.     

LDL-receptor-related protein 6 is a receptor for Dickkopf proteins

Wnt glycoproteins have been implicated in diverse processes during embryonic patterning in metazoa. They signal through frizzled-type seven-transmembrane-domain receptors to stabilize beta-catenin. Wnt signalling is antagonized by the extracellular Wnt inhibitor dickkopf1 (dkk1), which is a member of a multigene family. dkk1 was initially identified as a head inducer in Xenopus embryos but the mechanism by which it blocks Wnt signalling is unknown. LDL-receptor-related protein 6 (LRP6) is required during Wnt/beta-catenin signalling in Drosophila, Xenopus and mouse, possibly acting as a co-receptor for Wnt. Here we show that LRP6 (ref. 7) is a specific, high-affinity receptor for Dkk1 and Dkk2. Dkk1 blocks LRP6-mediated Wnt/beta-catenin signalling by interacting with domains that are distinct from those required for Wnt/Frizzled interaction. dkk1 and LRP6 interact antagonistically during embryonic head induction in Xenopus where LRP6 promotes the posteriorizing role of Wnt/beta-catenin signalling. Thus, DKKs inhibit Wnt co-receptor function, exemplifying the modulation of LRP signalling by antagonists.     

Non-Fermi-liquid behaviour in La4Ru6O19

Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Transition-metal oxides have proved to be particularly fruitful in this regard, especially for those materials with the perovskite structure, where the special characteristics of transition-metal-oxygen orbital hybridization determine their properties. Ruthenates have recently emerged as an important family of perovskites because of the unexpected evolution from high-temperature ferromagnetism in SrRuO3 to low-temperature superconductivity in Sr2RuO4 (refs 1, 2). Here we show that a ruthenate in a different structural family, La4Ru6O19, displays a number of highly unusual properties, most notably non-Fermi-liquid behaviour. The properties of La4Ru6O19 have no analogy among the thousands of previously characterized transition-metal oxides. Instead, they resemble those of CeCu6-xAux-a widely studied f-electron-based heavy fermion intermetallic compound that is often considered as providing the best example of non-Fermi-liquid behaviour. In the ruthenate, non-Fermi-liquid behaviour appears to arise from just the right balance between the interactions of localized electronic states derived from Ru-Ru bonding and delocalized states derived from Ru-O hybridization.     

Disruption of neurotransmission in Drosophila mushroom body blocks retrieval but not acquisition of memory

Surgical, pharmacological and genetic lesion studies have revealed distinct anatomical sites involved with different forms of learning. Studies of patients with localized brain damage and work in rodent model systems, for example, have shown that the hippocampal formation participates in acquisition of declarative tasks but is not the site of their long-term storage. Such lesions are usually irreversible, however, which has limited their use for dissecting the temporal processes of acquisition, storage and retrieval of memories. Studies in bees and flies have similarly revealed a distinct anatomical region of the insect brain, the mushroom body, that is involved specifically in olfactory associative learning. We have used a temperature-sensitive dynamin transgene, which disrupts synaptic transmission reversibly and on the time-scale of minutes, to investigate the temporal requirements for ongoing neural activity during memory formation. Here we show that synaptic transmission from mushroom body neurons is required during memory retrieval but not during acquisition or storage. We propose that the hebbian processes underlying olfactory associative learning reside in mushroom body dendrites or upstream of the mushroom body and that the resulting alterations in synaptic strength modulate mushroom body output during memory retrieval.     

Discovery of X-rays from the protostellar outflow object HH2

Herbig-Haro (HH) objects have been known for 50 years to be luminous condensations of gas in star-forming regions, but their underlying physical nature is still being elucidated. Previously suggested models encompass newborn stars, stellar winds clashing with nebular material, dense pockets of interstellar gas excited by shocks from outflows, and interstellar 'bullets' (ref. 6). Recent progress has been made with the jet-induced shock model, in which material streams out of young stellar objects and collides with the surrounding interstellar medium. A clear prediction of this model is that the most energetic Herbig-Haro objects will emit X-rays, although they have not hitherto been detected. Here we report the discovery of X-ray emission from one of the brightest and closest Herbig-Haro objects, HH2, at a level consistent with the model predictions. We conclude that this Herbig-Haro object contains shock-heated material located at or near its leading edge with a temperature of about 106 K.