March 2010 Archives

Aurélien Roux of the Curie Institute in Paris and his coworkers in Patricia Basssereau's group have cleared up one of the mysteries of how the protein dynamin helps form synaptic vesicles. Synaptic vesicles are lipid-wrapped nanoscale packets that contain neurotransmitters, the molecules that convey signals between neurons. Signaling begins when synaptic vesicles fuse with the signaling neuron's membrane, thereby releasing their contents. The neurotransmitters then diffuse across the tiny gap to the receiving neuron and bind to the neuron's surface. Once they've delivered their message, they unbind and make their way back to the signaling neuron, which retrieves them by budding fresh synaptic vesicles from its cell membrane. The protein clathrin initiates the budding by forming a curved coat on the membrane's interior surface (see figure); dynamin ties off and ultimately severs the vesicle. In solution, dynamin molecules polymerize into a spiral whose inside radius matches that of the dynamin monomers. In vivo, the spiral is seen to wrap around the necks of clathrin-coated vesicles. Roux and his team set out to determine whether the polymerization is triggered by the neck's curvature. Their experiment, which used artificial vesicles, optical tweezers, and fluorescently tagged dynamic molecules, showed that dynamin will readily polymerize around tubes whose outside radius matches dynamin's inside radius. More important, Roux and company also found that when dynamin's concentration is high enough, it will polymerize on fatter tubes and that the act of polymerizing can apply the few-piconewton force needed to squeeze a neck. (A. Roux et al., Proc. Natl. Acad. Sci. USA 107, 4141, 2010.)—Charles Day

In materials, as the axiom goes, structure follows function: A metal’s tightly bonded atomic crystal lattice gives it strength, and a polymer’s mesh of macromolecular chains makes it elastic. Medical implants, electronic components, and other similar devices call for multifunctional materials that are both strong and stretchy. One such material is the shape-memory alloy (SMA), a polycrystalline arrangement of assorted metals that, when stressed, undergoes a structural phase transition from high to low symmetry. The transition is reversible, and above a critical temperature SMAs are superelastic—they fully recover after being stretched well beyond the reversible-deformation strain values of pure metals. Now, materials scientists at Tohoku University in Sendai, Japan, have presented evidence for an iron-based SMA that is 35 times as elastic as pure metals. The new alloy, which also features nickel, cobalt, aluminum, tantalum, and boron, has an elastic strain of 13%, as shown in the figure, almost double the value of the more expensive commercial-standard nickel–titanium alloy. Furthermore, the material’s yield strength, 800 MPa, is 1.5 times that of the nickel–titanium SMA. The researchers say that microstructured precipitates similar in composition to the bulk matrix and interspersed through it are a key to the improved mechanical strength. The greater elastic strain and strength could be exploited for mechanical damping in building materials. Also, the ferrous SMA’s magnetism is phase dependent, which makes it potentially useful for electromechanical sensing applications. (Y. Tanaka et al., Science 327, 1488, 2010.)—Jermey N. A. Matthews

According to cosmological theory, the expanding universe has no preferred direction. Thus, the cosmos may be likened to a rising loaf of raisin bread, with the raisins playing the roles of galaxies. Viewed from Earth (or anywhere else), the motion of a distant galaxy should be determined by the overall cosmic expansion. Now, following on their earlier work presented in 2008, Alexander Kashlinsky of NASA’s Goddard Space Flight Center and colleagues report that superimposed on the cosmic expansion is a universal flow along the line from Earth to the Centaurus and Hydra constellations. The “dark flow,” as the authors call it, was revealed in the cosmic microwave background by minuscule temperature fluctuations that arise when x-ray-emitting gas from galaxy clusters scatters off CMB photons. A catalog of more than 1000 x-ray-luminous galaxy clusters told Kashlinsky and company where in the Wilkinson Microwave Anisotropy Probe’s five-year data set they should look for those fluctuations. The researchers had to average over ensembles of clusters to see evidence for the dark flow, which persisted unabated to the furthest measurable reaches, 2.5 billion light-years away. It’s as if—and this is a literal possibility—matter beyond the edge of the visible universe is pulling the entire cosmos toward it. (A. Kashlinsky et al., Astrophys. J. Lett. 712, L81, 2010.) —Steven K. Blau

Much of the light emitted from stars and other astrophysical objects is absorbed by dust and reemitted at far-IR or submillimeter wavelengths—radiation that is notoriously difficult to detect. Last year researchers from the Jet Propulsion Laboratory proposed a new type of detector for that regime, with an eye toward future, more sensitive space missions. The team has now built a prototype microdevice (see figure), called a quantum capacitance detector (QCD), which would be one pixel in an eventual array. The detection chain goes like this: Photons are received at an antenna and fed into a superconducting absorber where they break Cooper pairs and generate quasiparticles. A superconducting island, called a single Cooper-pair box (SCB), is connected to the absorber in such a way that, at most, one quasiparticle at a time can tunnel onto it; that changes the island’s capacitance, which is so small that the charging energy of a single electron has a large effect. With a resonant circuit, the physicists monitor the frequency of capacitance changes from which they can determine the density of quasiparticles in the absorber and thus the photon flux at the antenna. The device’s performance is already comparable to that of other superconducting detectors. The advantage of the QCD, say the researchers, is the ease with which it can be read out from an array of detectors. For example, each pixel detector could be fabricated with a different resonance and simultaneous readout could be done with a frequency comb. (J. Bueno et al., Appl. Phys. Lett. 96, 103503, 2010.) —Stephen G. Benka

One of the hallmarks of lasing is a dramatic narrowing of the light's frequency spread. In 1958 Arthur Schawlow and Charles Townes deduced that the laser linewidth is fundamentally limited by unavoidable spontaneous emission. (Thanks to other sources of noise, a real laser's linewidth is usually considerably broader.) Semiconductor diode lasers required a revision of the intrinsic linewidth formula to account for additional inherent broadening, but quantum cascade lasers (described in Physics Today, May 2002, page 34) had been thought to obey the original limit. Now Saverio Bartalini and colleagues at Italy's National Institute of Optics–CNR, the European Laboratory for Non-linear Spectroscopy, and the Second University of Naples have confirmed a recent theory predicting that QCLs can in fact beat the Schawlow–Townes limit and yield significantly improved spectral purity. Key to the 2008 theory by Masamichi Yamanishi and coworkers at Hamamatsu Photonics was the recognition that nonradiative transitions in QCLs strongly suppress spontaneous emission. To test the prediction, the Italian researchers tuned their IR QCL to be halfway down a carbon dioxide absorption peak at 4.33 µm (69.3 THz). Thanks to the steep slope of the absorption curve there, frequency fluctuations were converted into detectable intensity variations. That technique enabled the team to measure the noise spectrum over seven decades of frequency and to extract the intrinsic QCL linewidths for various pump currents. The obtained widths, in the range of 500 Hz, agreed well with the new theory and were three orders of magnitude smaller than predicted by the venerable Schawlow–Townes formula. (S. Bartalini et al., Phys. Rev. Lett. 104, 083904, 2010.) —Richard J. Fitzgerald

At the center of many an active galaxy lies an exceedingly powerful engine that, among other things, shoots out collimated jets of fast-moving plasma. Such jets can extend well beyond the galaxy's luminous boundary, ending in vast lobes that light up the intergalactic medium in the radio band. Closer to home, our sun's atmosphere has many a plasma-filled magnetic loop, the dynamics of which are somewhat mysterious. In February, at the joint meeting of the American Physical Society and the American Association of Physics Teachers, Paul Bellan (Caltech) reported on his group's recent experiments that shed light on both systems. The experimenters used the large currents and magnetic fields of spheromak technology to create plasma jets in a very large vacuum chamber, which ensured that the plasma configurations were unaffected by walls. With a preexisting magnetic field "frozen in," the physicists puffed some gas through an electrode, switched on a current, and watched as a plasma jet formed, self-collimated, underwent a kink instability, and then detached when the electric current was strong enough. In a different magnetic-field geometry, the figure shows counterpropagating collimated plasma jets—red hydrogen from the cathode and green nitrogen from the anode—colliding head-on within an arched magnetic loop, much like those seen in the Sun's corona. Bellan also developed a physical model for the self-collimation and a dusty-plasma dynamo mechanism suitable for generating actual astrophysical jets. (P. M. Bellan et al., invited APS/AAPT talk H3.2, 2010. Preprint available from the author.) —Stephen G. Benka

Laser radiation can induce cooling not only in dilute gases of atoms but also in certain transparent solids. The left panel of the figure shows the basic scheme: A laser photon excites a transition from an upper level of one state to a lower level of another, and a higher-energy photon is emitted, with phonons making up the difference. Now Mansoor Sheik-Bahae (University of New Mexico), Mauro Tonelli (University of Pisa), and colleagues have cooled a solid to 155 K, a new temperature record, using a laser-based system with no moving parts. The previous record, 208 K, was set in 2005 in the ytterbium-doped glass ZBLAN (composed of zirconium, barium, lanthanum, aluminum, and sodium fluorides). The Yb³⁺ ions’ lowest two energy levels are split by the surrounding atoms into seven sublevels, as shown in the right panel. ZBLAN’s appeal was that it had been synthesized to high purity for use in optical fibers. But its potential for cooling is limited: Its amorphous structure broadens the Yb³⁺ energy levels, so the peak absorption is weak. The new record was set using Yb-doped yttrium lithium fluoride (Yb:YLF), whose regular crystal structure makes the Yb³⁺ sublevels much sharper and the resonant absorption much stronger. But synthesis of high-purity Yb:YLF is relatively undeveloped, and existing high-power lasers fall just short of the Yb³⁺ E4–E5 transition’s 1020-nm peak. The researchers speculate that improvements in those areas should allow cooling to 77 K—the boiling point of liquid nitrogen. (D. V. Seletskiy et al., Nat. Photonics 4, 161, 2010.) —Johanna Miller

Since industrialization, the amount of mercury circulating in the air, water, and living organisms has roughly tripled. But the physical and chemical processes involved in that Hg cycle are poorly understood. In 2007 Bridget Bergquist and Joel Blum of the University of Michigan in Ann Arbor discovered that Hg’s two stable magnetic isotopes, ¹⁹⁹Hg and ²⁰¹Hg, sometimes exhibit slightly different behavior from that of the five stable nonmagnetic isotopes. The difference is attributed mostly to coupling between nuclear and electronic spins, which facilitates electron spin flips in certain photoinitiated reactions. Consequently, the varying isotopic compositions of natural samples can provide information about the Hg cycle and sources of Hg pollution. (See Physics Today, December 2008, page 25.) Now Blum, Laura Sherman, and colleagues have studied Hg isotope fractionation in Arctic snow. The Hg is deposited seasonally: Shortly after the annual polar sunrise, sunlight induces precipitation of almost all the Hg in the polar atmosphere. How and to what extent Hg leaves the snow before the snow melts is uncertain, but the researchers found that of the snow samples they collected, those that had been exposed to more sunlight contained less ¹⁹⁹Hg and ²⁰¹Hg. They conclude that Hg is emitted from the snow via a photochemical reaction that favors the odd-numbered isotopes. And they suggest that any Hg that enters Arctic ecosystems from the melted snow could be identified by its isotopic composition. (L. S. Sherman et al., Nat. Geosci. 3, 173, 2010.) —Johanna Miller

The binding energy of a nucleus—almost 1% of its mass—provides important information about its configuration of protons and neutrons. Theorists particularly want to know the binding energies of transuranic nuclear species approaching the so‑called island of stability predicted to lie not far beyond the most massive element yet discovered—with atomic number Z = 118. Despite the name, the island’s denizens would not be truly stable, just significantly longer‑lived than their offshore neighbors. But until now the masses, and therefore the binding energies, of transuranics have been determined only indirectly, by measuring the energies of α particles released in long α‑decay chains down to nuclei of well‑measured mass. Such indirect determinations can suffer from significant uncertainties and limitations. Now however, Michael Block and coworkers at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, have reported the first direct measurement of transuranic masses. The team measured the masses of three short‑lived isotopes of nobelium (Z = 102) created by the fusion of calcium ions from GSI’s heavy‑ion accelerator with target lead nuclei. The least stable of the three has a half‑life of two seconds. Their masses were measured to within a few parts in 10⁸ in a precision Penning trap, an electromagnetic device that confines charged particles in a small cavity and determines their masses by measuring their cyclotron‑orbit frequencies in the trap’s strong magnetic field. What makes the technique particularly challenging for those transuranics whose creation requires fusion is primarily their painfully slow production rates. (M. Block et al., Nature 463, 785, 2010.) —Bertram Schwarzschild