January 2010 Archives

Boron's next-door neighbor in the periodic table, beryllium, forms a simple metal lattice at 0 K. Boron's other next-door neighbor, carbon, forms another simple structure at 0 K, graphite. As for boron itself, its most stable form at 0 K is unknown. Compounding the mystery, the lowest-energy phase that experimenters have found, the β-rhombohedral phase, is stunningly complex and defect riddled: Each hexagonal unit cell has 423 atomic sites; on average only 320 of them are occupied. Now, Tadashi Ogitsu of Lawrence Livermore National Laboratory and his collaborators have explained why the stable β-rhombohedral phase has so many empty sites. If boron were simple, the defects—vacancies and interstitial atoms—would disappear as boron attained its perfect crystalline structure. But according to Ogitsu's calculations, which he carried out on a Livermore supercomputer, the defects actually stabilize the β-rhombohedral phase. It turns out the defect sites in the crystal are arranged in a particular geometric configuration, a double-layer expanded kagome lattice (see figure). Ogitsu and his collaborators realized that the problem of how boron atoms fill empty sites is essentially the same as another problem: how antiferromagnetically coupled spins align themselves on an expanded kagome lattice, whose ground state is degenerate and disordered. Like spin ices, and ordinary water ice, boron's β-rhombohedral phase is geometrically frustrated. Ogitsu notes that the hopping of defects between nearly degenerate configurations can also account for some of boron's peculiar and long-puzzling transport properties. (T. Ogitsu et al., Phys. Rev. B, in press.)—Charles Day

Biological and medical researchers have long sought to study or control cellular function by inserting biomolecular probes inside the cell. But those probes, which include peptides and nucleic acids, must first cross the cell’s highly selective membrane. Traditional approaches to breaching that barrier are to chemically modify the probe or membrane and to pack the probe into a virus, which fuses to a cell’s membrane before depositing its load; both methods induce unwanted side effects and are limited to delivering specific molecular cargo. Now a team of US and South Korean scientists, led by Harvard University’s Hongkun Park, has developed a minimally invasive delivery method that exploits the ability of silicon nanowires to physically penetrate the cell’s membrane. The researchers prepared vertically aligned nanowire arrays with a density of roughly 25 million nanowires/cm² and altered their surface chemistries to enable noncovalent binding of a broad spectrum of molecules. With the nanowire platform, they were able to simultaneously assay the intracellular effects of distinct molecular probes. In one experiment, the researchers layered human fibroblasts, shown green in the scanning electron microscope image, across the nanowires, shown in blue. Nearly all of the cells were impaled within one hour and received the bound probes within 24 hours. Impaled cells continued to grow for several weeks, albeit at a slightly slower rate. (A. K. Shalek et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0909350107.) — Jermey N. A. Matthews

Designers of transportation networks have to weigh the cost of serving customers against the need for an efficient, robust system. Natural organisms, too, confront tasks in which they need to balance competing desiderata. As it forages for food, for example, a slime mold must balance cost (that is, the amount of protoplasm it extrudes), efficiency, and the ability to withstand injury. Remarkably, as recently reported by Atsushi Tero and colleagues from Japan and the UK, the molds do as well as transportation engineers in balancing their analogous competing needs. Panel a of the figure re-creates a 17-cm-wide map of the principal cities served by the Tokyo railway system with a slime mold (yellow) at the location of Tokyo and food flakes (white) representing other cities. In about a day’s time, the slime mold finds where the nourishment is and generates a protoplasm network with the food flakes as nodes. Standard metrics for analyzing transportation networks reveal that the mold’s foraging network and the Tokyo railway system perform similarly. Perhaps more significantly, Tero and company imitated slime-mold networks in numerical simulations that don’t incorporate detailed biochemistry. Instead, they include a feedback step in which tubular links carrying a large protoplasm flux grow wider and flux-poor links contract. By tweaking their simulation parameters, the researchers could nudge the network toward, for example, greater cost efficiency. With optimal parameters, they could even improve upon the work of slime molds and human engineers. (A. Tero et al., Science 237, 439, 2010.) —Steven K. Blau

Light-colored (high-albedo) surfaces reflect more sunlight than dark surfaces and therefore have a lower surface temperature and are surrounded by cooler air. The proposal that painting a building's roof white can save energy for the occupant has been around for more than a decade. In recent years, region-wide modeling of so-called urban heat islands has included albedo effects. Keith Oleson (National Center for Atmospheric Research, Boulder, Colorado) and his colleagues have now gone global. They started with a dataset of urban extent and urban properties in 33 regions of the world, and a sophisticated model that includes factors like building heights, street widths, and thermal and radiative properties of roofs, walls, and streets. Next, they imposed interior building temperature ranges consistent with climate and socioeconomic conditions. Finally, they coupled the model to a global climate model and varied the roofs' albedos. All grid cells in the final model contained rural regions and some also had urban areas. The figure shows the average annual difference in the heat island due to white roofs. (White cells on the map included no urban areas.) The heat-island effect of cities is noticeably reduced. During the summer months, the use of air conditioning would also be reduced. Interestingly a closer look at data for the winter months showed a reversal at high latitudes, where the extra albedo effect prompts additional internal heating of buildings. (K. W. Oleson, G. B. Bonan, J. Feddema, Geophys. Res. Lett., in press.) —Stephen G. Benka

When Jean-Dominique Cassini discovered Iapetus in 1671, he was surprised to find it visible on just one side of its orbit around Saturn. The moon’s orbit had to be synchronous, he correctly inferred, with its leading hemisphere far darker than its trailing one. More than 300 years later, Voyager 2 revealed that the charcoal dark and frosty bright surfaces interleave, like two halves of a tennis ball. But the pattern and sharpness of the dark–bright boundaries remained mysterious. Using data collected by Cassini–Huygens since 2004, John Spencer (Southwest Research Institute), Tilmann Denk (Free University of Berlin), and colleagues have now confirmed a hypothesis first proposed in 1974. Micrometeorites swept up on the leading hemisphere during the moon’s orbit, they argue, darken it enough to trigger the thermal migration of ice: sublimation from dark, warmer patches centered around the equator and subsequent recondensation at bright, colder areas near the poles and trailing side. Visible-spectrum images such as the ones shown here indicate that the dust coating Iapetus’s leading side is redder than the dirt presumed intrinsic to the moon. Judging from IR data, the dust reaches temperatures up to 129 K during Iapetus’s 79-day rotation. Sublimed ice molecules can travel ballistically hundreds of kilometers before recondensing at cold (113 K) traps. An enormous gossamer ring around Saturn detected last year by the Spitzer Space Telescope is the putative source of reddish dust. (J. R. Spencer, T. Denk, Science, in press, doi:10.1126/science.1177132; T. Denk et al., Science, in press, doi:10.1126/science.1177088.)—R. Mark Wilson

Unlike the branches of a tree, the network of veins in a typical leaf is full of closed loops. Even after a visit by a hungry insect, no part of the leaf is cut off from the network, as shown in the top part of the figure. But is a leaf’s fractal-like form, with loops of various sizes, the best possible network for resisting that type of damage, or might a different loop-filled structure be better? And is the hierarchical structure the optimum for any other criterion? Marcelo Magnasco (the Rockefeller University, New York) and colleagues sought to find out. Using a mathematical model that assigns each vein segment a cost proportional to its capacity raised to a power γ, they looked for the networks with a given total cost that suffered the least average strain under two sets of circumstances. First, they looked at damage to a randomly chosen vein segment. Second, they considered the case of a fluctuating load, in which the amount of fluid to be delivered to each part of the network varied in time and space. (Real leaves do sometimes need to handle fluctuating loads. So, more obviously, do most human-built networks.) They found that for low values of γ (results for γ = 0.1 are shown in the figure), both cases yielded hierarchical networks of loops, qualitatively similar to real leaves. (E. Katifori, G. J. Szöllősi, M. O. Magnasco, Phys. Rev. Lett., in press.) —Johanna Miller

In most nuclei the protons and neutrons form a roughly spherical core of approximately uniform density. But along the edges—the so-called drip lines—of the chart of nuclides a handful of light nuclei have more nucleons than can be accommodated in the nuclear core. The excess, usually one or two neutrons, form a dilute distribution called a halo that extends far beyond the core. At the RIKEN Nishina Center for Accelerator-Based Science, a Japanese team has studied the reaction of heavy carbon nuclei with hydrogen and identified the extremely neutron-rich carbon-22, with its 6 protons and 16 neutrons, as a halo nucleus, the heaviest one yet found. Nuclear radii generally scale as the cube root of the total number of protons and neutrons, yet based on their cross-section data, the researchers calculated the radius of ²²C to be twice that of the much more common isotope ¹²C; indeed, at 5.4 fm it exceeds the radius of lead-208. The halo of ²²C comprises two valence neutrons; determining their distribution and other aspects of the halo structure will require experiments with different target nuclei and different beam energies. (K. Tanaka et al., Phys. Rev. Lett., in press.)—Richard Fitzgerald