To reject heat or to generate "cold", refrigerators typically rely on the vapor-compression cycle of fluids such as the chlorofluorocarbon Freon. Now emerging is a new class of solid-state materials known as magnetocalorics; under an applied magnetic field or with changing temperature, they experience a large and sharp magnetic entropy change with the first-order paramagnetic-ferromagnetic phase transition. That entropy change, known as the magnetocaloric effect, makes the material an effective heat sink. In fact, magnetocalorics have already been exploited for cryogenic cooling in research settings. Recently discovered magnetocaloric metal alloys containing either gadolinium or arsenic have shown large MCE values at ambient temperatures; however, the high cost of the former and the undesirable toxicity of the latter present challenges to their commercial deployment. Inspired by scientists at the Beijing University of Technology who replaced arsenic with the relatively benign germanium, an international collaboration has now measured the germanium-containing material's MCE under varying temperatures and magnetic fields. The team also conducted neutron-scattering studies at NIST to examine the material's crystal structure. Diffraction measurements revealed that the ferromagnetic structure, when compared with the paramagnetic phase, was noticeably contracted, confirming that the phase transition is driven by structural changes. The team also discovered that in the 250- to 270-K range, a 5-tesla magnetic field induced an MCE value in the germanium compound that is considerably higher than the arsenic-containing magnetocalorics. The researchers say the MCE can be pushed even higher with further improvements in sample purity. (D. Liu et al., Phys. Rev. B. 79, 014435, 2009.) — Jermey N.A. Matthews
January 2009 Archives
About 10 years ago, observations of distant supernovas suggested that the universe is growing at an accelerating rate. Those stunning observations, and many corroborating studies since, determine the recessional velocity of objects as a function of the time when they emitted the light presently observed on Earth. Now Alexey Vikhlinin of the Harvard-Smithsonian Center for Astrophysics and colleagues have taken a completely different approach to probing the properties of the mysterious "dark energy" attributed with driving the cosmic acceleration. They have observed how the expansion of space retards the gravitational clumping together of mass to form clusters of galaxies. They began with detailed x-ray images, taken by the Chandra X-ray Observatory, of two sets of galaxy clusters. One set comprises clusters whose light was emitted roughly 5.5 billion years ago; the other includes nearby clusters. Recent advances in theoretical modeling enabled the group to compare cluster masses across the two sets. The mass observed in the nearby clusters, says Vikhlinin, is about one-fifth what it would be without the inhibiting effects of dark energy. Further, the Chandra observations allow the Vikhlinin team to estimate both the quantity of dark energy and the key equation of state parameter w. The Chandra-derived values agree well with those determined in earlier investigations. (A. Vikhlinin et al., Astrophys. J., in press; A. Vihlkinin et al., Astrophys. J., in press.) — Steven K. Blau
Rydberg blockade between neutral atoms held in traps several microns apart has now been demonstrated and exploited to create a quantum-entangled state. Both feats are considered significant steps in the quest for quantum computing with neutral atoms. Blockade refers to the inhibition of excitation in one part of a system by the prior excitation of another part. And the excitation in question is the raising of alkali atoms to high Rydberg states-—that is, states in which the valence electron is excited to a high principal quantum number. The atoms interact strongly enough at micron separations for Rydberg excitation of one to prevent the excitation of the other. A group at the University of Wisconsin–Madison has demonstrated Rydberg blockade between two rubidium atoms held in optical traps 10 μm apart. And a group at the Université Paris–Sud and the Institut d'Optique in France used Rydberg blockade between Rb atoms held 4 μm apart to create an entangled state of the kind one would need for a quantum logic gate. The figure shows that under laser excitation in the Paris experiment, the entangled two-atom state (blue curve) oscillated more rapidly than a lone atom (red curve) between ground and Rydberg states. (E. Urban et al., Nat. Phys., in press, doi:10.1038/nphys1178; A. Gaëtan et al., Nat. Phys., in press, doi:10.1038/nphys1183 . — Bertram Schwarzschild
X-ray crystallography routinely yields the structures of proteins with 0.1-nm resolution. But how does one take a detailed look at something far bigger―a chromosome, say, or a cell nucleus? Such objects don't crystallize, because no two individual examples have the same shape. And at a micron or so in size, they're too thick for electron microscopy. X rays, of course, pass through whole animals, not just single cells. For the past decade, the practitioners of a technique called x-ray diffraction microscopy have been steadily improving their ability to image single, biological samples. XDM's latest milestone is shown here. Made at the SPring-8 synchrotron in Sayo, Japan, this image of a pair of human chromosomes is the first from XDM in three dimensions. In general, determining a structure from a diffraction pattern requires obtaining the phases of the scattered photons. Those phases are lost when the diffraction pattern of a crystalline sample is detected; ingenious methods are needed to recover them. In XDM, the phases remain within the continuous diffraction pattern cast by the isolated sample. If the pattern is measured on a fine enough spatial scale, a computer algorithm can yield the structure by iterating between real space and diffraction space. The multiple exposures required to create this three-dimensional image damaged the sample and limited resolution to 120 nm. In principle, further improvements could push the resolution down to 10 nm. (Y. Nishino et al., Phys. Rev. Lett. 102, 018101, 2009.) ― Charles Day
Gas bubbles in a liquid driven by acoustic waves can be used in a variety of contexts—for example, as contrast agents for ultrasound imaging, as a delivery system for therapeutic drugs, as catalysts for sonochemical reactions, and as scrubbers of surfaces. The bubbles' compressibility, which allows their volume to oscillate in response to the varying sound pressure, accounts for the wide applicability. Unfortunately, spatial gradients in the pressure field can make it nearly impossible to control the bubbles' oscillations and position simultaneously. Researchers at Nanyang Technological University in Singapore have now found a simple recipe to dress the bubbles in a 1-μm-thick coat of magnetite nanoparticles. The coat, which self assembles in solution, stabilizes the bubbles (typically 40–350 μm in diameter) without sacrificing their compressibility; they remain intact for more than 6 months in a light-tight drawer and their position in solution can be controlled with a simple household magnet. The figure here illustrates an example: The oscillation of a single 130-μm-wide bubble, subject to sound at a frequency close to its resonant frequency, sets up eddies (signified by arrows) in the surrounding fluid. After the acoustic field is turned off, a permanent magnet nudges the bubble to the right. (For an animated version, see http://www1.spms.ntu.edu.sg/~cdohl/mubbles.html.) The authors expect magnetic bubbles to serve as remotely controlled microfluidic mixers and pumps, and, more generally, as tools to test fundamental fluid mechanics concepts. (X. Zhao, P. A. Quinto-Su, C.-D. Ohl, Phys. Rev. Lett., in press.) — R. Mark Wilson
In the late 1940s, Hendrik Casimir proposed that two perfectly conducting parallel plates should feel a feeble attractive force between them, due to the zero-point energy of the surrounding electromagnetic field and its dependence on the plates' positions. (See the article by Steve Lamoreaux, Physics Today, February 2007, page 40.) About a decade later, Evgeny Lifshitz and colleagues generalized Casimir's work to real conductors and dielectrics and found that the force persisted. In most cases the proposed force was still attractive, but for some configurations—a high-permittivity material and a low-permittivity material separated by a medium of intermediate permittivity—it could be repulsive. In fact, the repulsive Casimir-Lifshitz force is responsible for liquid helium's tendency to climb the walls of its container: The container repels the ambient vapor, and the liquid rises to fill the gap. Now, a group of researchers led by Harvard University's Federico Capasso have observed a repulsive Casimir-Lifshitz interaction between two solid objects, a silica surface and a 40-μm-diameter gold-coated sphere, immersed in bromobenzene. To monitor the force, they attached the sphere to an atomic force microscope cantilever and measured the cantilever's deflection using a light beam and a split-quadrant photodetector, as shown in the figure. A repulsive force of a few tens of piconewtons was measurable when the objects were brought within 40 nm of each other, and it increased as their separation decreased. The researchers suggest that the force they observed could levitate a solid within a liquid, which may lead to very low-friction sensors of force and torque. (J. N. Munday, F. Capasso, V. A. Parsegian, Nature 457, 170, 2009.) —Johanna L. Miller
Several experiments are operating or being built to detect astrophysical neutrinos. Ranging up to about a cubic kilometer in size, those experiments are embedded in ice or in a liquid such as water, where they watch for telltale flashes of Cherenkov radiation. (See the article by Francis Halzen and Spencer Klein in Physics Today, May 2008, page 29.) But the highest-energy neutrinos, with energies of an exaelectron volt (1EeV = 1018 eV) or higher, are so scarce that installations spanning 100 km3, along with massive numbers of expensive photomultiplier tubes, would be needed to collect adequate event statistics in a reasonable time. So other detection schemes are being explored, one of which involves acoustics: When a very–high-energy neutrino interacts with water or ice, a sudden localized thermal expansion occurs and the resulting wave propagates farther than the light flashes. To explore that method, the Aachen Acoustic Laboratory was set up in late 2007 and its first experiment made a precise measurement of the speed of sound in ice that is entirely devoid of bubbles and cracks. The Aachen physicists carefully positioned an array of sensors—six detectors and one emitter—in a 3-m3 water tank (shown here) equipped with a freeze-control unit and a degassing system. The difference in arrival times of an acoustic pulse at adjacent receivers determined the speed of sound. Between 0 °C and −17 °C, where they took measurements, the speed ranged from about 3840 m/s to 3890 m/s, agreeing well with earlier laboratory experiments. The team is also part of SPATS (the South Pole Acoustic Test Setup), which is currently obtaining complementary in situ measurements. (C. Vogt, K. Laihem, C. Wiebusch, J. Acoust. Soc. Am. 124, 3613, 2008.) —Stephen G. Benka
