Enhancement of the high-magnetic-field critical current density of superconducting MgB2 by proton irradiation

Magnesium diboride, MgB2, has a relatively high superconducting transition temperature, placing it between the families of low- and high-temperature (copper oxide based) superconductors. Supercurrent flow in MgB2 is unhindered by grain boundaries, making it potentially attractive for technological applications in the temperature range 20-30 K. But in the bulk material, the critical current density (Jc) drops rapidly with increasing magnetic field strength. The magnitude and field dependence of the critical current are related to the presence of structural defects that can 'pin' the quantized magnetic vortices that permeate the material, and a lack of natural defects in MgB2 may be responsible for the rapid decline of Jc with increasing field strength. Here we show that modest levels of atomic disorder induced by proton irradiation enhance the pinning of vortices, thereby significantly increasing Jc at high field strengths. We anticipate that either chemical doping or mechanical processing should generate similar levels of disorder, and so achieve performance that is technologically attractive in an economically viable way.     

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.     

The size and albedo of the Kuiper-belt object (20000) Varuna

Observations over the last decade have revealed the existence of a large number of bodies orbiting the Sun beyond Neptune. Known as the Kuiper-belt objects (KBOs), they are believed to be formed in the outer reaches of the protoplanetary disk around the young Sun, and have been little altered since then. They are probably the source of short-period comets. The KBOs are, however, difficult objects to study because of their distance from earth, so even basic physical properties such as their sizes and albedos remain unknown. Previous size estimates came from assuming an albedo with the canonical value being 0.04. Here we report simultaneous measurements of the thermal emission and reflected optical light of the bright KBO (20000) Varuna, which allow us to determine independently both the size and the albedo. Varuna has an equivalent circular diameter of D = 900+129-145 km and a red geometric albedo of pR = 0.070+0.030-0.017. Its surface is darker than Pluto's, suggesting that it is largely devoid of fresh ice, but brighter than previously assumed for KBOs.     

'Inverse' melting of a vortex lattice

Inverse melting is the process in which a crystal reversibly transforms into a liquid or amorphous phase when its temperature is decreased. Such a process is considered to be very rare, and the search for it is often hampered by the formation of non-equilibrium states or intermediate phases. Here we report the discovery of first-order inverse melting of the lattice formed by magnetic flux lines in a high-temperature superconductor. At low temperatures, disorder in the material pins the vortices, preventing the observation of their equilibrium properties and therefore the determination of whether a phase transition occurs. But by using a technique to 'dither' the vortices, we were able to equilibrate the lattice, which enabled us to obtain direct thermodynamic evidence of inverse melting of the ordered lattice into a disordered vortex phase as the temperature is decreased. The ordered lattice has larger entropy than the low-temperature disordered phase. The mechanism of the first-order phase transition changes gradually from thermally induced melting at high temperatures to a disorder-induced transition at low temperatures.     

Evidence for ubiquitous strong electron-phonon coupling in high-temperature superconductors

Coupling between electrons and phonons (lattice vibrations) drives the formation of the electron pairs responsible for conventional superconductivity. The lack of direct evidence for electron-phonon coupling in the electron dynamics of the high-transition-temperature superconductors has driven an intensive search for an alternative mechanism. A coupling of an electron with a phonon would result in an abrupt change of its velocity and scattering rate near the phonon energy. Here we use angle-resolved photoemission spectroscopy to probe electron dynamics-velocity and scattering rate-for three different families of copper oxide superconductors. We see in all of these materials an abrupt change of electron velocity at 50-80 meV, which we cannot explain by any known process other than to invoke coupling with the phonons associated with the movement of the oxygen atoms. This suggests that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity.     

Fabry - Perot interference in a nanotube electron waveguide

The behaviour of traditional electronic devices can be understood in terms of the classical diffusive motion of electrons. As the size of a device becomes comparable to the electron coherence length, however, quantum interference between electron waves becomes increasingly important, leading to dramatic changes in device properties. This classical-to-quantum transition in device behaviour suggests the possibility for nanometer-sized electronic elements that make use of quantum coherence. Molecular electronic devices are promising candidates for realizing such device elements because the electronic motion in molecules is inherently quantum mechanical and it can be modified by well defined chemistry. Here we describe an example of a coherent molecular electronic device whose behaviour is explicitly dependent on quantum interference between propagating electron waves-a Fabry-Perot electron resonator based on individual single-walled carbon nanotubes with near-perfect ohmic contacts to electrodes. In these devices, the nanotubes act as coherent electron waveguides, with the resonant cavity formed between the two nanotube-electrode interfaces. We use a theoretical model based on the multichannel Landauer-Büttiker formalism to analyse the device characteristics and find that coupling between the two propagating modes of the nanotubes caused by electron scattering at the nanotube-electrode interfaces is important.     

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.     

Self-assembly of regular hollow icosahedra in salt-free catanionic solutions

Self-assembled structures having a regular hollow icosahedral form (such as those observed for proteins of virus capsids) can occur as a result of biomineralization processes, but are extremely rare in mineral crystallites. Compact icosahedra made from a boron oxide have been reported, but equivalent structures made of synthetic organic components such as surfactants have not hitherto been observed. It is, however, well known that lipids, as well as mixtures of anionic and cationic single chain surfactants, can readily form bilayers that can adopt a variety of distinct geometric forms: they can fold into soft vesicles or random bilayers (the so-called sponge phase) or form ordered stacks of flat or undulating membranes. Here we show that in salt-free mixtures of anionic and cationic surfactants, such bilayers can self-assemble into hollow aggregates with a regular icosahedral shape. These aggregates are stabilized by the presence of pores located at the vertices of the icosahedra. The resulting structures have a size of about one micrometre and mass of about 1010 daltons, making them larger than any known icosahedral protein assembly or virus capsid. We expect the combination of wall rigidity and holes at vertices of these icosahedral aggregates to be of practical value for controlled drug or DNA release.     

A freely diffusible form of Sonic hedgehog mediates long-range signalling

The secreted protein Sonic hedgehog (Shh) exerts many of its patterning effects through a combination of short- and long-range signalling. Three distinct mechanisms, which are not necessarily mutually exclusive, have been proposed to account for the long-range effects of Shh: simple diffusion of Shh, a relay mechanism in which Shh activates secondary signals, and direct delivery of Shh through cytoplasmic extensions, termed cytonemes. Although there is much data (using soluble recombinant Shh (ShhN)) to support the simple diffusion model of long-range Shh signalling, there has been little evidence to date for a native form of Shh that is freely diffusible and not membrane-associated. Here we provide evidence for a freely diffusible form of Shh (s-ShhNp) that is cholesterol modified, multimeric and biologically potent. We further demonstrate that the availability of s-ShhNp is regulated by two functional antagonists of the Shh pathway, Patched (Ptc) and Hedgehog-interacting protein (Hip). Finally, we show a gradient of s-ShhNp across the anterior-posterior axis of the chick limb, demonstrating the physiological relevance of s-ShhNp.