August 2009 Archives

At absolute zero, a two-dimensional array of identical particles will crystallize into a hexagonal close-packed lattice. At high temperature, the lattice melts. What happens in between has interested physicists for decades. In the 1970s, theorists predicted that 2D melting would proceed via a so-called hexatic state in which the crystal breaks up into patches of local orientational order. The hexatic state's existence has been inferred from changes in resistance and other sample-averaged quantities. Now, the whole melting process—from a crystal through a hexatic state to a liquid—has been directly imaged. Isabel Guillamón of the Autonomous University of Madrid, Spain, and her collaborators melted a lattice of superconducting vortices that forms in a thin film of tungsten under a magnetic field. For their imager, they used a scanning tunneling microscope, which can distinguish the non-superconducting vortex cores from the vortices themselves. As the movie shows, the vortices start off with hexagonal order. As the temperature increases, pentagonal (gold) and heptagonal (green) defects appear that cause dislocations (solid magenta lines). On further heating, the vortices become mobile. Just above 2 K, they move too quickly for the STM to track; they appear as white stripes, whose ordering resembles a liquid crystal's smectic phase. By 3 K, which is 1 K below the film's T_c, the lattice melts completely and the vortices move freely in a gray, undifferentiated blur. (I. Guillamón et al., Nat. Phys., in press.)—Charles Day

Almost 400 extrasolar planets have been found to date (see Physics Today, May 2009, page 46), but a new planet reported by Coel Hellier (Keele University) and colleagues stands out. Like many exoplanets, theirs, dubbed WASP-18b, is massive (10 times the mass of Jupiter) and has a small orbital radius (only 1/50th of Earth's). But its orbital period of only 0.94 day is the shortest for any "hot Jupiter" yet observed. Moreover, its large mass and small orbit are predicted to cause the strongest tidal interactions of any known star–planet system. According to current theory, the tidal bulge that the planet raises on its host star exerts a torque that will drain angular momentum from the planet and cause it to spiral inward. (For more on tidal interactions, see Physics Today, August 2009, page 11.) If the star's tidal dissipation rate is comparable to what's been measured for binary stars and for the gas giants in our own solar system, the infall will be quick: WASP-18b has less than a million years left in a lifetime, estimated from the age of its host star, of about a billion years. Over the next decade, WASP-18b's death spiral should produce a measurable shift in the planet's observed transit time. The absence of tidal decay—a notable possibility, given the rarity of finding a planet so close to the end of its life—would constitute direct evidence for a different class of tidal interactions in the host star and provide new constraints on models of stellar interiors. (C. Hellier et al., Nature 460, 1098, 2009.)—Richard J. Fitzgerald

Light sensors—photodetectors—have myriad uses in scientific, industrial, and consumer settings: Digital cameras, environmental monitors, remote controls, surveillance equipment, and biosensors are just a few applications. Most photodetectors are made from inorganic semiconductors and are sensitive in some limited waveband in the range between IR and UV. A new photodetector (PD), however, that uses a semiconducting polymer shows good responsiveness from UV (300 nm) to near-IR (1450 nm), as shown in the figure. The polymer is a small-bandgap semiconductor that exhibits photoinduced, ultrafast electron transfer to fullerenes—blended with the polymer in the form of PC₆₀BM. Sandwiched between two electrodes, the two materials form a phase-separated blend with interpenetrating donor and acceptor networks. Because the new photodetector covers nearly the entire solar spectrum at Earth's surface, the researchers—led by Alan Heeger of the University of California, Santa Barbara and CBrite Inc—note that it holds promise for photovoltaic cells. The next step is to make addressable arrays of these broadband, high-detectivity photodetectors. (X. Gong et al., Science, in press, doi:10.1126/science.1176706.) —Stephen G. Benka

When a person’s head strikes, or is struck by, another object, it accelerates. As it begins to decelerate, the brain slams into the skull, then bounces off and oscillates until the impact energy dissipates. The resulting shear and compressive strains can lead to brain damage. But in battlefield explosions, just the acoustic waves alone can cause soldiers traumatic brain injuries. To better understand that process, Lawrence Livermore National Laboratory's William Moss and Michael King and the University of Rochester’s Eric Blackman compared numerical simulations of a head colliding with a wall to one being struck by an explosion’s blast waves. Despite accelerating the head at less than half the rate of the wall collision, the simulated blast produced on the brain surprisingly comparable pressure spikes—ranging up to 3 bars—and even larger pressure gradients. Apparently, those mechanical loads are delivered by the skull, which ripples under the pressure of blast waves—the rippling motion is indicated in the image by velocity vectors. The researchers confirmed the role of the skull’s elasticity by making it 1000 times stiffer in their simulations and observing a fivefold drop in the pressure spikes. The simulations also revealed that helmets in contact with the head can impart an additional mechanical load to the skull and helmets that allow for an air cushion geometrically focus and increase the magnitude of blast waves. (W. C. Moss et al., Phys. Rev. Lett., in press.)—Jermey N. A. Matthews

In conventional photography, photons bounce off an object and imprint its shape onto film. In ghost imaging, an object’s shape is revealed after interrogating two light beams, only one of which interacts with the object. Ghost imagers have been practicing their craft for more than 10 years. Some of their schemes are based on photon entanglement, but others use classical light sources. Now Barry Jack of the University of Glasgow and colleagues have reported experiments in which quantum entanglement is manifest; their ghost images violate a generalized Bell inequality—that is, a condition on correlations that can arise classically. The accompanying figure presents one of their runs. The object to be imaged introduces a π phase difference between a disk (turquoise) and the surroundings (red). A smaller reference object (inset) introduces the phase difference on either side of a diagonal. After bouncing entangled photons, separated in space, off the object and reference, Jack and company measured photon coincidence counts. The black-and-white ghost image shown here maps those counts, with brighter regions corresponding to a greater coincidence rate. The brightest sections appear along those portions of the disk’s bounding circle parallel to the reference bisector. The variation in the coincidence rate along the bounding circle violates the Bell inequality, thus demonstrating the quantum nature of Jack’s system. Evidently, the ghost imaging relies on spooky action at a distance, accepted nowadays but so troubling to Albert Einstein decades ago. (B. Jack et al., Phys. Rev. Lett., in press.) —Steven K. Blau

Dispersed in the brains of Alzheimer's patients are disk-shaped lesions, about 100 µm across. Whether those lesions, or plaques, are a cause or a consequence of Alzheimer's disease is controversial, but their composition is clear. The plaques are made from fibrous aggregates—amyloid—of protein or their shorter cousins, peptides. Once sequestered in amyloid, a protein or a peptide can no longer perform its function. Even if amyloid does not directly cause Alzheimer's and other diseases, it seems at best a useless, dead-end repository of proteinaceous material. But as a new paper exemplifies, a less malign view of amyloid is emerging. Roland Riek of ETH Zürich and his collaborators have demonstrated that our bodies exploit amyloid as a temporary storage medium for a wide range of peptide hormones. Riek suspected a hormone–amyloid connection when he found that a stress hormone formed amyloid fibrils. He and his collaborators then subjected 41 other peptide hormones to a battery of biochemical, biophysical, and crystallographic tests. The finding: 75% of the peptide hormones form amyloid; and, as befits a storage medium, the amyloid can also disaggregate to release the peptides. In a final test, the team stained slices of mouse brain with hormone-sensitive and amyloid-sensitive dyes. The stained regions coincided. Riek's discovery adds to the modest but growing list of examples of so-called functional amyloid that perform useful tasks in living organisms. Evidently, amyloid is not always pernicious. (S. K. Maji et al., Science 325, 328, 2009.)—Charles Day

The Arctic Ocean’s floating sea-ice cover naturally waxes and wanes with the seasons. But the decline in area covered by perennial ice—that which survives the summer melt—has been accelerating in recent years. The nonlinear trend, no doubt, reflects the Arctic’s response to a warming climate: Thanks to the ice albedo–ocean feedback, a drop in ice cover increases the absorption of solar radiation, which warms the ocean, prolongs the melting, and reduces ice cover yet further. For quantitative details on the ice pack’s changing mass and heat capacity, though, researchers need more than areal measurements. Ronald Kwok and colleagues at NASA and the University of Washington have now published what may be the most comprehensive thickness maps of the entire Arctic basin. The lidar system they use aboard NASA’s ICESat can precisely distinguish height differences between the sea surface and ice floes. After measuring that “freeboard,” Kwok and company use Archimedes’ principle to calculate the portion that’s underwater. Because much of the brine drains from sea ice as it ages, making it more reflective to microwaves, the researchers are also able to distinguish between young, seasonal ice and older, perennial ice. The surveys taken by ICESat over five years reveal that the Arctic has thinned by about 0.7 m and lost 1.5 × 10⁶ km² of perennial ice—more than twice the area of Texas. Moreover, the volume of perennial ice shrank by 57%—so much that seasonal ice has become dominant for the first time on record. (R. Kwok et al., J. Geophys. Res. 114, C07005, 2009, doi:10.1029/2009JC005312.)—R. Mark Wilson

Atmospheric aerosols affect climate: The particles scatter, absorb, and emit radiation, and they also induce cloud formation. (See Physics Today, May 2004, page 24.) Much of the aerosol mass is produced by oxidation of organic compounds emitted into the atmosphere through human activity and from the biosphere. But many aspects of aerosol formation are poorly understood. It’s thought that biogenic aerosols might be formed from isoprene, a light hydrocarbon given off in large quantities by certain trees. Isoprene itself is volatile—at sea level it boils at 34 °C—so it must undergo a series of chemical reactions before it can form long-lived aerosol particles. Now Caltech’s Paul Wennberg, graduate student Fabien Paulot, and their colleagues offer some insight into what those reactions are. In laboratory experiments designed to replicate atmospheric conditions far from any human pollution, they found that isoprene reacts several times with OH radicals to ultimately form large amounts of dihydroxyepoxide, a newly identified airborne molecule and a likely aerosol precursor. Its hydroxyl groups make it hydrophilic, so it should be readily taken up by existing aqueous aerosol particles. And epoxides under acidic conditions can form low-volatility polymers; a similar reaction is used in epoxy adhesive. Knowing the reactants and their mechanisms will help researchers improve atmospheric models to better predict the consequences of human activity such as deforestation and pollution. (F. Paulot et al., Science 325, 730, 2009.) —Johanna Miller

In 1927, during the formative years of quantum mechanics, Friedrich Hund posed a paradox: Why is a chiral molecule found in either its left-handed or right-handed isomeric forms and not in a superposition of the two? After all, both isomers are equally likely. At first glance, the answer seems clear. If the tunneling time between the two isomers is long, their superposition is unlikely to arise. That answer might hold for a sugar, protein, or other large chiral molecule, whose tunneling time may exceed the age of the universe, but it fails for small molecules. Nor can it explain why the habitual states of a molecule, large or small, are its left-handed and right-handed isomers and not its energy or parity eigenstates. Now, Klaus Hornberger and Johannes Trost of Ludwig-Maximilians University in Munich have resolved Hund's venerable paradox. The two theoreticians analyzed the case of one of the smallest chiral molecules, deuterium disulphide (shown here), tumbling in and buffeted by a monoatomic gas. The calculation uncovered a surprisingly large phase-dependence in the scattering amplitude that distinguishes the two isomers. Thanks to the phase difference, the ambient gas atoms can pick out the states that correspond to the molecule's left-handed and right-handed isomers far more readily than the molecule’s other states. When the first few atoms strike a molecule, it's knocked into either its left-handed or right-handed configurational state. Further atomic bombardment acts on the molecule like repeated quantum measurements, keeping it in its chiral state. (J. Trost, K. Hornberger, Phys. Rev. Lett. 103, 023202, 2009.) —Charles Day