Ultimately, the resolution of an electron microscope is limited by the electron's de Broglie wavelength. For the 300-keV electrons typical in scanning transmission electron microscopy, that limit is about 2 pm, or 1/25th of the radius of hydrogen's 1s orbital. But STEM images are formed by focusing a billion or so electrons per second onto a sample. The spherical aberration of the electromagnetic lenses and the finite size of the electron source cause the electrons to lose phase coherence, lowering the resolution to about 100 pm, or about twice the distance between atoms in many crystals. Now, a team from Lawrence Berkeley National Laboratory in California has halved the STEM resolution limit to 50 pm. The boost in performance comes from two novel components: an electron source that emits copious electrons from a region just 25 pm across and a hexapole corrector that can compensate for phase aberrations up to fifth order. Using their new microscope, the LBNL researchers looked at a piece of germanium foil. According to x-ray crystallography, Ge atoms are arranged in rows of dumbbell pairs aligned end-to-end. Ordinarily, the dumbbells are too small to be resolved with STEM. But, as the accompanying figure shows, the LBNL microscope could resolve the 47-pm separation between two paired atoms. The resolution is so fine that the thermal jiggling of the atoms during the room-temperature measurement acts as an additional source of blur. (R. Erni, M. D. Rossell, C. Kisielowski, U. Dahmen, Phys. Rev. Lett., in press.) — Charles Day
February 2009 Archives
The couplings between Earth's solid surface and atmosphere are a rich area for study. For example, it is known that soil "respiration" plays a large role in the global water cycle. Researchers have long assumed that diffusion is the dominant mechanism for transferring gases across the interface between air and soil or rock, enhanced somewhat by wind- and pressure-fluctuation-driven transport. But scientists working in Israel's Negev Desert have uncovered a surprisingly important new mechanism: In regions where Earth's porous surface has cracks, fractures, or other discontinuities, thermal convection can expel, on a daily basis, up to 200 times more gas than diffusion, depending on the surrounding conditions. The team from Ben Gurion and Oregon State Universities installed arrays of sensors in the large--2-cm-wide, 1-m-deep--crack in the foreground of the photograph: Temperature and relative-humidity data were acquired every 10 minutes for more than two years. Every day, the Sun's warmth propagates slowly down through the rock until, as the evening cools, the air in the crack becomes less dense than the overlying atmosphere, and convection sets in. Venting of warm, moist air from the rock surrounding the crack, along with the entrainment of cool dry atmospheric air, then continues until dawn. Convection takes place for up to 19 hours a day in the winter, 12 during the summer. The process, say the researchers, is natural and pervasive and could have a large impact on Earth-atmosphere gas exchange. Also, because water vapor, carbon dioxide, and other gases are involved, there may be implications for climate change studies. (N. Weisbrod et al., Geophys. Res. Lett. 36, L02401, 2009, doi:10.1029/2008GL036096 — Stephen G. Benka
Inspired by crabs, cockroaches, and other nimble creatures, engineers at the University of Pennsylvania have designed robotic vehicles to traverse complex terrain such as deserts and presumably the Martian landscape. Equipped with six spring-loaded and synchronously rotating C-shaped limbs, the robots outmaneuver current military and rescue vehicles over coarse but rigid terrain.
On granular media, however, that agility comes at a cost: The latest robot model, the 30-cm-long SandBot designed by Georgia Institute of Technology physicists in collaboration with the UPenn engineers, drops from a speed of 60 cm/s on a rigid surface to a crawl of 2 cm/s in a bed of poppy seeds (see image below).
Only when the researchers empirically tweak the limb-control parameters does the speed approach a respectable 30 cm/s. A team led by Daniel Goldman at Georgia Tech set out to determine how the robot's speed is influenced by the angular frequency of its limbs and the granular medium's packing fraction. The experiments revealed that the robot's legs sink into the fluid-like medium, then slip, before walking forward.
Below a critical packing fraction, and at high limb frequencies, a sharp transition from rotary walking to a slower swimming motion was observed.
The researchers say that understanding the physics associated with crater formation and collapse in granular media will lead to advances in limb geometry and robot locomotion. (C. Li et al., Proc. Nat. Acad. Sci. USA, doi:10.1073/pnas.0809095106.)—Jermey N. A. Matthews
The several approaches being pursued for fuel cells vary in their chemical reactions, materials, and optimal operating conditions, but they share a basic configuration (see Physics Today, November 1994, page 54, and October 2006, page 38). A fuel, often hydrogen, is oxidized at the anode, where it liberates electrons. The electrons travel through and power an external circuit and eventually reach the cathode, where oxygen is reduced. Meanwhile, to complete the redox reaction, ions travel through an electrolyte that separates the electrodes. In alkaline fuel cells, first developed for the Apollo space missions in the 1960s, oxygen combines with water and electrons at the cathode to form hydroxyl ions (OH-) that travel through an aqueous alkaline electrolyte to produce water by combining with protons from the anode. Commercialization of those fuel cells, however, has been limited by the high cost of the platinum used for the cathode. New work by Liming Dai of the University of Dayton and colleagues at the Air Force Research Laboratory and the University of Akron has shown that vertically aligned carbon nanotubes doped with nitrogen provide an efficient, lower-cost alternative for the cathode. Nitrogen-doped nanotubes have better long-term stability and, unlike Pt, are not harmed by the presence of carbon monoxide or any fuel molecules that cross the electrolyte from the anode to the cathode. The researchers attribute the catalytic performance to the relatively high positive charge density on the carbon atoms adjacent to the nitrogen atoms. (K. Gong et al., Science 323, 760, 2009.) — Richard J. Fitzgerald
A group of astronomers headed by Michael Mumma of NASA's Goddard Space Flight Center in Greenbelt, Maryland, has reported in substantial spatial and temporal detail the first definitive detection of methane (CH4) in the atmosphere of Mars. The Martian atmosphere consists overwhelmingly of oxidized gases such as carbon dioxide; reduced gases such as methane were known to be rare. In 2003 and 2006, using two telescopes on Mauna Kea in Hawaii, the group observed much of the planet's surface spectroscopically at IR wavelengths to confirm the presence of methane and determine its seasonal and "ariegraphic" distribution. (A Martian season is about six months long.) Mumma and company found extended plumes of methane that appear to emanate at substantial rates from three localized sources in the northern summer (see the figure). They suggest that the summer emergence results from the unfreezing of pores and fissures connecting with underground accumulations of the gas, and that oxidation in winter dust storms accounts for the very meager methane signal remaining at the start of spring. Is the methane biogenic? On Earth 90% of it is, the rest coming from inorganic geochemistry. If it is biogenic on Mars, it could be a vestige of life long extinct or a sign of ongoing life in warm precincts deep underground, perhaps energized by molecular hydrogen from the hydrolysis of water by radioactivity. Isotopic ratios measured by future IR missions to Mars should help determine the methane's origin. (M. J. Mumma et al., Science Express, doi:10.1126/science.1165243.) — Bertram Schwarzschild
Inorganic crystal aggregates known as biomorphs earn their name by virtue of a remarkable resemblance to the fossils of primitive organisms. But although the structures can be varied and complex—leaflike sheets, wormy ropes, and helical filaments, among others—biomorphs are exceedingly simple to make. They self assemble when an alkaline earth halide such as barium chloride is mixed with a silica-rich solution under high pH conditions at ambient pressure and temperature. As carbon dioxide from the air dissolves into solution, barium carbonate and silica precipitate out and produce the complex structures. A long-standing question is how. Juan Manuel García-Ruiz, his postdoc Emilio Melero-García (both at the University of Granada), and Stephen Hyde (Australian National University) now propose a mechanism for the morphogenesis. As the carbonate crystallizes, it lowers the pH of the local environment and triggers the precipitation of silica. The silica precipitation, in turn, raises the local pH, which prompts another round of carbonate formation. The sensitivity of silica and carbonate species to opposite trends in pH fluctuation creates a chemical feedback that eventually produces rodlike, carbonate particles, each coated with silica. Freed from the hexagonal symmetry restrictions imposed by carbonate growth, the silica-coated nanoparticles form clusters that can adopt various shapes. For reasons unexplained by their mechanism, the clusters align themselves on the micron scale and grow as two-dimensional sheets. The edges of those sheets can then curl like a scroll to create the sort of curved and twisted filaments captured here by optical microscopy. (J. M. García-Ruiz, E. Melero-García, S. T. Hyde, Science 323, 362, 2009.) — R. Mark Wilson
To understand how babies and children learn to process music and other sounds, it's important to know what they can do at birth. Researchers in Hungary and the Netherlands, led by István Winkler of the Hungarian Academy of Science in Budapest, have found that three-day-old infants can distinguish the downbeat in a musical rhythm — that is, the "one" in "one, two, three, four." They used electroencephalography — the detecting of electrical activity in the brain via electrodes affixed to the head, as shown in the photo — to monitor the babies' reactions to a repeating synthesized drum rhythm from which notes were sporadically left out. When the omitted sound was a downbeat, the electrodes picked up a strong discriminative response, but when a note in any other position was left out, the infants' response was much weaker. In music played by human musicians, the downbeat is often longer or louder than the surrounding notes, but that was not the case for the computer-generated sound sequence used in the experiment: The downbeat was distinguished by the arrangement of sounds alone. The result suggests that beat perception is either innate or learned in the womb. (I. Winkler et al., Proc. Nat. Acad. Sci. USA, in press; photo courtesy of Gábor Stefanics, Hungarian Academy of Sciences.) — Johanna L. Miller
