February 2010 Archives

Plastics and other organic materials can be fashioned into bendy, stretchy sheets. Exploiting that flexibility for electronic devices entails finding organics that exhibit useful phenomena (see Physics Today, October 2008, page 18). Display panels in cell phones already make use of the semiconductivity and light emission of two organics, polyfluorene and poly(phenylene–vinylene). Now, Sachio Horiuchi of Japan's National Institute of Advanced Industrial Science and Technology, Yoshinori Tokura of the University of Tokyo, and their collaborators have found an organic material that exhibits another useful phenomenon, ferroelectricity—that is, the ability to switch polarization in response to an electric field. Ferroelectrics, and their cousins, piezoelectrics, can be used as nonvolatile memories, temperature sensors, and mechanical actuators. The new ferroelectric is the crystalline form of croconic acid. As the accompanying figure shows, croconic acid molecules consist of a pentagonal ring of carbon atoms bound to oxygen and hydroxyl groups. The carbon atoms' p orbitals merge to form π orbitals above and below the ring. The merging weakens the difference between the bonds holding the O and OH groups, thereby loosening the hydrogen atoms. In a croconic acid crystal, the H atoms form a network of hydrogen bonds that holds the crystal together. Crucially, the molecules are arranged in such a way that an applied electric field can shift the loosely attached H atoms along the hydrogen bonds to positions on either side of the molecules (see figure). That controlled displacement of charge is what makes croconic acid a ferroelectric. Fortunately for future applications, the ferroelectricity is robust and strong, and shows up at room temperature. (S. Horiuchi et al., Nature 463, 789, 2010.)—Charles Day

In a combustion engine, work is produced from heat liberated by burning the fuel. In hydrocarbon fuel cells, the fuel is directly converted into electricity. Both types of engines, however, waste heat and emit gas byproducts that are considered useless—or even pernicious, as in the case of the greenhouse gas carbon dioxide. But Martin Gellender, an environmental officer for the state government of Queensland, Australia, makes the case for exhaust gases as an energy source: In a conceptual paper, he explores the overlooked entropy increase that occurs when concentrated gases isothermally mix with air. As illustrated in the schematic, if an exhaust gas mixture containing, for example, CO₂ at a high concentration is separated from air by a piston-membrane barrier that selectively blocks CO₂ passage, the concentration gradient performs work on the piston until the CO₂ concentrations on both sides are equal. According to Gellender’s calculations, a secondary entropy engine could theoretically recover up to 7% of the fuel’s energy and could provide a power boost to the primary engine: up to 1.5% for combustion engines and up to 3.5% for fuel cells. He says that new fuel-cell designs and material advances could lead to a practical entropy engine that reduces the fuel consumption of power plants. (M. Gellender, J. Renew. Sust. Energy, in press.)—Jermey N. A. Matthews

Among its marvelous consequences, general relativity asserts that a stationary clock at Earth’s surface will run slower than one high in a tower where the gravitational potential is weaker; the phenomenon is called gravitational redshift (see the article by Neil Ashby, Physics Today, May 2002, page 41). Now Holger Müller (University of California, Berkeley and Lawrence Berkeley National Laboratory) and colleagues report that the redshift idea, first experimentally confirmed 45 years ago, has passed its strictest test yet. In its analysis, the group reanalyzed a 10-year-old experiment that used atom interferometry to determine the gravitational acceleration. In that earlier work, an upward-directed atom interacted with a pair of laser pulses that put it in a superposition of states with differing momentum. As the figure shows, the phase of the atomic wavefunction evolves along each of the two paths, but with a lower frequency along the bottom path. A second pair of pulses tweaked the atom so that the diverging paths would reconverge; an experimental measurement of the probability that the atom is observed at the convergence point yields the phase difference between the two paths. As Müller and company discuss, the earlier-measured phase difference receives contributions due to the relative motion of the atom in its different states and from the laser interactions, but the two effects cancel. The total phase difference is attributable to the redshift. And to better than one part in 10⁸, it is precisely what is predicted by general relativity. (H. Müller, A. Peters, S. Chu, Nature 463, 926, 2010.) —Steven K. Blau

The human body is an incredibly complex dynamical system, which makes it an increasingly appealing subject for nonlinear dynamicists. For example, even when we stand upright, we are not motionless—the body oscillates continuously at a low amplitude. But as we lean farther from the vertical, our response becomes more complicated. In the language of dynamical systems, the upright position is an attractor: As long as the body is in the vicinity of that position, it will be drawn upright. The various leaning positions from which one can right oneself constitute what's called the basin of attraction. The boundary of the basin separates the upright attractor from another attractor—the floor. If we lean too far, we fall down. Studying the self-righting ability of judokas and other elite athletes, María Zakynthinaki of Madrid's Institute for the Mathematical Sciences and colleagues at the Technical University of Madrid have shown that even when the basin of attraction of a person is rotated or skewed—due to athletic training, repetitive work motions, or injuries—the boundary of the basin can nevertheless be characterized by just four experimental parameters: the maximum lean from which one can regain balance in the general forward, backward, left, and right directions. Moreover, the researchers present a method for measuring and describing a person's basin geometry mathematically, which should prove useful for further modeling and simulation of balance. (M. S. Zakynthinaki et al., Chaos, two papers in press.)—Richard J. Fitzgerald

Natural light fields are threaded by lines of darkness. For monochromatic light, the phenomenon is familiar in laser speckle—the black points that appear in scattered light because of destructive interference. Those points, known as optical vortices, become lines in three dimensions; when multiple plane waves interfere, the vortices can form a complicated tangle throughout the volume of the light field. Using techniques from mathematics and optical wavefront engineering, Mark Dennis (University of Bristol), Miles Padgett (University of Glasgow), and their UK colleagues have now demonstrated that those lines of zero intensity can be shaped into linked loops and knots. The trick lay in finding the exotic solutions to Maxwell’s equations—or more precisely, the paraxial wave equation—that do the job. Last year Dennis and student Robert King realized they could exploit a topological technique known as Milnor mapping to construct a complex scalar function for a knot. To imprint the function into an optical field, Padgett and students Kevin O’Holleran and Barry Jack used a diffractive hologram to generate the proper destructive interference pattern onto an incoming laser beam. They spatially mapped out the resulting knot (shown here red) by measuring where the phases (different colors in the cross section) become singularities. One goal of the research is to better understand the role knotted vortices may play in systems as diverse as cold turbulent superfluids and hot magnetized plasmas. (M. R. Dennis et al., Nat. Phys. 6, 118, 2010.)—R. Mark Wilson

The partitioning of the continuous visible spectrum into a small number of basic colors is done differently in different languages. But the variation is less than would be expected by chance, as statistical analysis of the World Color Survey's data set has shown. Several computational approaches have been taken toward understanding how languages’ color categories develop. Among them is the work of Andrea Baronchelli (Polytechnic University of Catalonia, Barcelona, Spain) and his collaborators. They performed computer simulations in which individuals in a population, beginning with no words to describe colors at all, were tasked with describing different colors to one another. The individuals independently invented words and categories and, based on the success or failure of their communications, adjusted their categories and vocabularies to match those around them. Eventually, each population came to a near-consensus, as shown in two examples in the top panel of the figure. Now, the researchers have revised their model to include a real property of human vision, the “just noticeable difference” (JND; shown in the bottom panel), or wavelength resolution. In the new simulations, individuals were not required to distinguish between colors that a human couldn't tell apart. The categories produced by the JND-based simulations clustered together in color space to the same degree as the World Color Survey results did. The researchers hope that the quantitative agreement between their simple model and empirical data will pave the way for greater use of synthetic modeling in studying language development. (A. Baronchelli et al., Proc. Natl. Acad. Sci. USA, in press, doi/10.1073/pnas.0908533107.) —Johanna Miller

The brain localizes the source direction of a pure tone at low frequency by interaural phase difference (IPD), and at high frequency by interaural level difference (ILD), a logarithmic measure of the ratio of sound intensities at the two ears. (See Physics Today, November 1999, page 24.) Localization by IPD shuts off abruptly around 1 kHz, where phase ambiguity could cause a disastrous 180° mistake. But nature doesn’t protect us from all acoustic misinformation. At frequencies up to 4 kHz, wavelengths are still comparable to the size of the head, so diffraction around the head might be misleading. At much higher frequencies, where diffraction is negligible, the head casts a proper acoustic shadow and ILD is a reliable clue to how far the source is off to the left or right. A new paper by Eric Macaulay and coworkers in the Psychoacoustics Group at Michigan State University compares sound-localization attempts of test subjects at 1.5 kHz with wave-propagation calculations that predicted they should often be badly misled by a diffractive phenomenon analogous to Fresnel’s optical bright spot. And indeed they were. The acoustic bright spot is a diffractive enhancement in the middle of the shadow cast by the head. The MSU results show that the effect consistently misleads hearers by spoiling the monotonic growth of ILD with increasing departure of the source from the forward direction. The photo shows a tiny unobtrusive microphone being put in a subject’s ear in the group’s anechoic test room to measure ILDs and correlate them with his guesses about source location. (E. J. Macaulay, W. M. Hartmann, B. Rakerd, J. Acoust. Soc. Am., in press.) —Bertram Schwarzschild