August 2010 Archives

An accelerator experiment at Los Alamos National Laboratory in the late 1990s reported the observation of neutrino flavor oscillation on a laboratory length scale—tens of meters. All prior oscillation observations had been over much larger distances. The disquieting implication of the LANL claim was that so small an oscillation length required the existence of a “sterile” neutrino flavor impervious to the weak interactions, which would clutter the attractively neat prevailing theory. So experimenters designed the MiniBoone facility at Fermilab to confirm the LANL result or lay it to rest. In 2007, the MiniBoone collaboration reported that its results were incompatible with the LANL claim. But now a new MiniBoone result suggests that the sterile neutrino’s obituary was premature. The new result looks a lot like the original LANL data. Like the LANL experiment, it was based on a beam of muonic antineutrinos. The 2007 refutation, however, was based on a muonic neutrino beam. The standard theory assumes that neutrinos and antineutrinos oscillate identically. But the new result, though not yet statistically robust, appears to show that antineutrinos, unlike neutrinos, do indeed oscillate on a distance scale that implies one or more sterile neutrino states. In fact, theorists are already considering how interference between two sterile states of different mass might explain such a neutrino–antineutrino asymmetry. The photo shows the photomultiplier tubes in the oil-filled MiniBoone detector that discern the flavors of neutrinos by recording the Cherenkov light of the charged leptons they create in collisions with nucleons. (A. Aguilar-Arevalo et al., http://arxiv.org/abs/1007.1150.)—Bertram Schwarzschild.

Zinc ions and some other metal ions can bind to three or four organic molecules at once. If those molecules are long and attach to zinc at both ends, it's possible to create a metal–organic framework (MOF), an open sheet of linked molecules with ions at the vertices. And if those sheets bind to each other and stack in register, the result is a material whose columnar pores can store, catalyze, or otherwise usefully process small molecules. Matthew Rosseinsky and his coworkers at the University of Liverpool in the UK have made a MOF material, but with a new twist. For its linker, the Liverpool team used a dipeptide—that is, two peptide-bonded amino acids (glycine and alanine; see figure). The team made two versions of the material, one incorporating a solvent (a mix of water and methanol) and one not. X-ray diffraction and nuclear magnetic resonance spectroscopy revealed that adding the solvent caused the dipeptide linkers to straighten, widening the pores to accommodate the solvent ions. Glycine, alanine, and the 18 other naturally occurring amino acids are characterized by side chains that are polar, nonpolar, positively charged, or negatively charged. Given that variety, the Liverpool experiment suggests that peptide-based MOF materials might find uses as expandable sponges for a wide range of molecules. (J. Rabone et al., Science 329, 1053, 2010.)—Charles Day

Under the hot summer sun, the ocean’s surface waters become warmer than the atmosphere above them. As the heat is transferred to the atmosphere, it can strengthen low-pressure disturbances and drive the characteristic weather phenomena known in the Atlantic region as hurricanes and in the Pacific as typhoons or tropical cyclones (see the Quick Study on hurricane formation by Kerry Emanuel in Physics Today, August 2006, page 74). A new model from a research collaboration led by Anand Gnanadesikan at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in New Jersey shows how strongly correlated the sea surface temperature (SST) is to the ocean’s color. The image depicts average concentrations (in mg/m³) of chlorophyll—the green pigment in phytoplankton—from 1997–2000 in the Pacific Ocean, where more than half of the reported typhoon-force winds (greater than 32 m/s) occur. Considering an extreme scenario, the researchers set the concentration of chlorophyll to zero and then studied the evolution of cyclones in the North Pacific Ocean. Without chlorophyll to absorb much of the solar radiation, SSTs drop. Air over the colder water sinks, drying the atmosphere and increasing wind shear, which quenches typhoon formation. Although typhoon frequency increased by 20% along the equator, the simulation predicted an overall drop in the region—up to 70% for areas beyond 15 degrees north of the equator—and a decrease in frequency of the most intense typhoons. (A. Gnanadesikan et al., Geophys. Res. Lett., in press.)—Jermey N. A. Matthews

Children frolicking in a sandbox probably don’t think about the drag forces exerted on their limbs as they displace grains of sand. But physicists Nick Gravish and Daniel Goldman (Georgia Tech) and Paul Umbanhowar (Northwestern University) do think about such forces. Now they have conducted a systematic study of how the drag force on a vertical plate partially submerged in sand-sized glass beads depends on the beads’ packing fraction ϕ. Their study reveals a surprising phenomenon: For a dense packing—that is, when ϕ exceeds a critical value ϕ_c—the drag force oscillates as the plate moves horizontally. The crucial physics, argue the authors, hinges on the phenomenon of dilatancy: densely packed beads can become less dense when sheared. Dragging a plate through a dense packing creates a “shear plane” that runs from the bottom edge of the plate to the surface of the beads and makes an angle θ with the horizontal. Particles near the shear plane tend to move parallel to it, toward the surface; particles beyond the plane hardly move at all (see the figure). Shear forces arising at the plane cause the local packing fraction to decrease, which makes it easier to move the plate. When the packing fraction dips to ϕ_c, the shear plane remains stationary at the surface even as its bottom edge moves with the plate; thus θ increases, which causes the drag force to also increase. Once the drag force is high enough, a new low-θ, high-ϕ shear plane forms, and the cycle repeats. Sandboxes, it seems, have pleasures to offer physicists and children alike. (N. Gravish, P. B. Umbanhowar, D. I. Goldman, Phys. Rev. Lett., in press.)—Steven K. Blau

X-ray diffraction has long been an important tool for finding crystal structures by mapping their electron densities. In recent decades, time-resolved x-ray diffraction has probed ever-faster structural changes in single crystals, including atomic motions on the femtosecond time scale. But many materials of interest, such as the transition metal complexes used in organic photovoltaic cells, can’t easily be made into crystals of sufficient size and quality. Now Michael Woerner, Thomas Elsaesser, and colleagues at the Max Born Institute in Berlin have demonstrated femtosecond x-ray powder diffraction, in which the sample is an ensemble of randomly oriented microcrystals of ammonium sulfate, (NH₄)₂SO₄, and the diffraction pattern is composed of concentric rings rather than discrete peaks. The innovation was in engineering the x-ray source—a laser-driven plasma source that produced an ultrafast x-ray pulse from an equally brief optical pulse—to operate stably and at high repetition rate for long enough to reveal small changes in the diffraction ring intensities. From those changes, the researchers calculated the change in the sample’s electron density. As shown in the figure, which depicts the equilibrium electron density and the resulting changes over one slice through the crystal, electrons briefly pool (red blobs) where no nucleus exists in the equilibrium structure—so a nucleus, specifically a proton, must have migrated there. Ultrafast IR spectroscopy confirmed that NH₄⁺ ions were reversibly breaking apart; surprisingly, the observed structural change bears no resemblance to either of ammonium sulfate’s known phase transitions. (M. Woerner et al., J. Chem. Phys., in press.)—Johanna Miller

Separating isotopes is difficult business: Since an element’s various isotopes share similar size and shape, separation methods such as thermal diffusion and centrifugation tend to be time and energy intensive. Now, however, a team led by Suresh Bhatia (University of Queensland, Australia) has shed new light on what may prove an attractive alternative—nanoporous materials known as molecular sieves. Normally, molecular sieves aren’t particularly effective isotope separators. Take diatomic hydrogen and its isotopic relative deuterium. As one would expect, the lighter H₂ molecules diffuse through the porous molecular sieve faster than the heavier D₂, but the difference is slight. That picture changes, though, if the temperature is low enough—and the pores small enough—for quantum effects to set in. If the pore size is on the order of the molecules’ de Broglie wavelength, the molecules' zero-point energy becomes the important barrier for pore diffusion and small mass becomes a disadvantage. Not only does D₂ then diffuse faster than H₂, it can do so by a substantial margin. Using quasi-elastic neutron scattering and carbon molecular sieves with 3-Å-diameter pores, Bhatia and company directly measured those diffusivities, which differed by nearly an order of magnitude at the coldest temperatures. Although quantum sieve effects—first predicted nearly 15 years ago—had been previously seen in equilibrium adsorption experiments, the research team’s findings represent the first microscopic observations of the kinetic phenomenon. (T. X. Nguyen, H. Jobic, S. K. Bhatia, Phys. Rev. Lett., in press.)—Ashley G. Smart

During the collapse of a cavitation bubble, the gas inside it can reach temperatures of more than 15 000 K—as hot as the surface of a star—and the energy can be released in the form of shock waves, heat, light, turbulent vortices, and high-speed jets of fluid. For decades, medical researchers have worked to harness that energy for therapeutic applications, such as the disintegration of cancerous tumors using focused ultrasound (see the article by Gail ter Haar in Physics Today, December 2001, page 29) and the delivery of drugs or genes into living cells (see the article by Detlef Lohse in Physics Today, February 2003, page 36). Although studies have demonstrated that microbubbles can rupture nearby cells, control over the bubble–cell interaction has remained difficult. Duke University researchers led by Pei Zhong have now demonstrated an approach to puncturing a cell’s membrane that entails carefully manipulating the fluid dynamics around it. The high-speed sequence of photographs captures the process: Two laser pulses, offset in space by 40 μm and time by 4 μs, create two bubbles (B₁ and B₂) that act in concert. The rapid expansion of the second bubble causes the collapse of the first bubble by pressing against it; the interaction deforms the shape of both. The bubbles’ asymmetric collapse gives rise to two localized microjets—one toward the cell between 6 and 7 μs, one away from it 2 μs later. The researchers can control the microjets’ impact by adjusting the bubbles’ position, spacing, and orientation relative to the cell. (G. N. Sankin, F. Yuan, P. Zhong, Phys. Rev. Lett., in press.)—R. Mark Wilson

Observations of distant supernovae have revealed that the expansion of the universe is accelerating. Cosmologists seek to confirm that result, and perhaps learn about the dark energy causing the acceleration, by mapping the universe’s large-scale structure. Mass-density patterns, formed when the cosmos was filled with dense plasma that supported sound waves, are visible today in the distribution of galaxies; tracking those patterns over cosmic time could provide constraints on the expansion. Galaxy redshift surveys can reveal large-scale structure but are time-consuming to conduct over large enough volumes. A potentially more efficient strategy, called intensity mapping, is to detect atomic hydrogen by its RF spin-flip transition. That 21-cm emission is an isolated spectral line, so matter at different redshifts can be mapped simultaneously without resolving the component galaxies. Now, Tzu-Ching Chang (Taiwan’s Academia Sinica and University of Toronto), Ue-Li Pen (University of Toronto), and Kevin Bandura and Jeffrey Peterson (both at Carnegie Mellon University) have shown that intensity mapping can work at redshifts near 1—looking back to the time when, models indicate, the expansion started to accelerate. From 15 hours of radio observations on two square-degree patches that were part of the DEEP2 galaxy redshift survey, the researchers removed the terrestrial radio signals and astronomical synchrotron radiation to uncover the much weaker hydrogen emission. Their resulting signal was correlated with the DEEP2 galaxy density—evidence that the technique did in fact detect hydrogen. The team is now observing larger regions of sky and hopes to build a dedicated intensity-mapping telescope. (T.-C. Chang et al., Nature 466, 463, 2010.)—Johanna Miller

Willis Lamb’s 1947 measurement of the tiny splitting between the 2s and 2p states of atomic hydrogen gave a crucial impetus to the development of quantum electrodynamics (QED). That “Lamb shift” from the Dirac hydrogen spectrum is a 4-μeV increase in the 2s energy level due primarily to vacuum fluctuations of the electromagnetic field. Now Randolf Pohl (Max Plank Institute for Quantum Optics, Garching, Germany) and coworkers at the Paul Scherrer Institute (PSI) in Switzerland have finally measured the analogue of the Lamb shift in the muonic H atom—a proton orbited by a μ⁻ instead of an e⁻. Muons live only microseconds, but they are 200 times heavier than electrons, and their atomic orbits are correspondingly tighter. The muonic Lamb shift is about 200 meV, and its precise value is particularly sensitive to the proton’s finite size. The PSI experiment was accomplished with precision laser excitation of μ⁻ p atoms created by an intense μ⁻ beam stopping in a small volume of H₂ gas at very low pressure. The team measured the muonic Lamb shift to a part in 10⁵ and compared it with elaborate QED calculations that parameterize the proton’s finite size with an effective charge radius R_p. They find an R_p about 4% smaller than that measured, with less precision, by conventional H spectroscopy and e⁻–p scattering experiments. The discrepancy is 5 standard deviations. Either the proton really is smaller than previously thought, argue Pohl and company, or there’s something wrong with the QED calculations or their input constants. But the proton is a quark composite whose size and shape are quantum-chromodynamic manifestations beyond the purview of QED. Several QCD theorists suggest that at the extraordinary precision achieved by the PSI experiment, it may not be possible to describe proton-size effects adequately with a single length parameter. (R. Pohl et al., Nature 466 , 213, 2010.)—Bertram Schwarzschild