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Aerogels are porous, very low density materials that have the appearance of frozen smoke. They are typically made from oxides and are thus insulators. Mohanan et al. (p. 397) have made analogous aerogels from metal chalcogenides (sulfides, selenides, and tellurides), which are materials commonly used for making semiconductor quantum dots. As a result, the aerogels retain semiconducting properties such as photoluminescence, and yet have a porous network structure with pores in the 2- to 50-nanometer-size range.
The addition of sulfur to many metals and alloys causes them to become brittle, but the reason for this weakening is not , - - —
edited by Stella Hurtley and Phil Szuromi ed to understand the tectonics of the region and the strength of the crust. Chevalier et al. (p. 411) estimated a rate of slip of about 11 milli-meters per year over about 20,000 to 140,000 years on one branch of the Karakorum based on offset moraines, which is consistent with the extrusion of western Tibet owing to the collision of India with Eurasia. This rate is higher than some geodetic estimates of recent slip over shorter time periods and suggests that slip rates on the fault have varied over time.
well understood. Yam- ¡ p aguchi et al. (p. 393, -published online 6 January 2005) modeled the embrittlement of nickel by progressively adding sulfur atoms to a grain boundary. First-princi- i ples calculations reveal that the weakening of the boundary is caused by the aggregation of sulfur atoms at the boundary, which repel each other. The sulfur atoms are forced into non-ideal bonding because the nickel-sulfur bonds are stronger than the sulfur-sulfur bonds.
The use of ultrashort, broadband laser pulses, or optical combs, was recently extended from being a reference standard for continuous wave lasers to being a way to probe the energy levels of atoms. The advantage of using the combs is that they combine the high temporal resolution needed to study dynamics with precise frequency measurement. Witte et al. (p. 400;see the Perspective by Udem) have now extended this method to the short-wavelength, deep ultraviolet region of the spectrum by creating a train of the pulses with the fourth harmonic of an optical laser. The authors measured a high-energy transition frequency in Kr atoms with an order of magnitude reduction in uncertainty from prior studies.
The sensitivity of magnetic resonance force microscopy (MRFM) is reaching the point where single spins can be detected. Making measurements on a small ensemble of localized spins created by microwave irradiation of silicon, Budakian et al. (p. 408) show that that MRFM cannot only detect spin fluctuations but can also be used to manipulate them. Pockets of ordered spin can be formed from a background bath of thermally fluctuating spins in the vicinity of the cantilever tip, and these pockets of ordered spin can be stored and read out. The technique itself should prove useful as a probe of the dynamics of nanoscale magnets, and the ability to create, store, and read out small pockets of ordered spin should prove useful in quantum computing.
The Karakorum fault is a major strike-slip fault trending northwest just north of the western Himalayan Mountain Range. The rate of slip on the fault is difficult to estimate, but these rates are need
Sudden Changes in Lions' Ranges
Population dynamics of social species can be highly complex because of the interplay of group-level factors and population-level factors. Packer et al. (p. 390;see the Perspective by Ranta and Kaitala) present long-term data from the Serengeti plains of East Africa which show how herbivore populations (wildebeest, buffalo, zebra, and gazelle) influence lion populations directly and indirectly through the herbivores' impact on vegetation. The herbivore population changes are smooth and gradual, but the lion populations show sudden shifts between alternative equilibria. A model that constrained the upper and lower limits of pride size gave rise to the observed patterns of sudden shifts. Thus, population trends cannot necessarily be understood solely on the basis of individual survival and reproduction.
Ring species, which are isolated species connected by intergraded populations, have long been thought to exemplify the occurrence of speciation in the presence of gene flow. However, some taxo-nomic and molecular evidence have cast doubt on this classic model. Irwin et al. (p. 414) conducted a genome-wide survey for the greenish warbler, whose territory encircles the Tibetan plateau. Two genetically distinct and reproductively isolated forms of the warble are indeed connected by a chain of populations through which genetic patterns change gradually.
Thiomargarita namibiensis is a colossus among bacteria (almost 1 millimeter in diameter) found off the Namibian coast. Schulz and Schulz (p. 416) show it accumulates intracellular polyphosphates under aerobic conditions and releases phosphate under anoxic conditions, thereby creating pore water supersaturated in phosphate that precipitates as phosphorite. Energy gained by breakdown of polyphosphate under anoxic conditions is used for intracellular accumulation of sulfide and acetate or other organic carbon. The sulfide is oxidized to elemental sulfur by using nitrate as an electron acceptor. The release of phosphate by these organ-
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All you need is a proper diet of fresh fruits and vegetables and get plenty of exercise and you'll be fine. Ever heard those words from your doctor? If that's all heshe recommends then you're missing out an important ingredient for health that he's not telling you. Fact is that you can adhere to the strictest diet, watch everything you eat and get the exercise of amarathon runner and still come down with diabetic complications. Diet, exercise and standard drug treatments simply aren't enough to help keep your diabetes under control.