Shear Waves for Medical Imaging, The Mass of a Top Quark, A Wormhole for Electrons

APS Physics Tip Sheet – Jul 15, 2014

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Shear Waves for Medical Imaging

​Magnetic fields and electric currents can generate shear waves in biological tissue—an effect that could lead to novel medical imaging schemes.

The combined application of magnetic fields and electric currents could lead to a new method for imaging biological tissues, as suggested by a study carried out by a team of researchers in France. The group investigated a scheme in which a current is injected in a tissue placed in a magnetic field. Thanks to the Lorentz force (the force arising when charged particles move in a magnetic field), the electrons moving in the tissue generated shear waves that can be detected with ultrasound imaging techniques. The intensity of the observed waves can be related to the stiffness of the material, revealing, for instance, structures that differ from the surrounding tissue, such as tumors.

* Pol Grasland-Mongrain (contact author) et al., “Imaging of shear waves induced by Lorentz force in soft tissues,” Physical Review Letters (expected publication date: Jul 18)

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The Mass of a Top Quark

Researchers at Fermilab have reported the most precise measurement to date of the top quark’s mass.

The top quark, with a mass roughly two hundred times larger than a proton’s, is the heaviest elementary particle. Determining the top quark mass is considered one of the best tests of the standard model of particle physics, so even small refinements of its measured value can be used to constrain or rule out alternative theories. Researchers have now analyzed a large dataset previously collected by the now-shut-down Tevatron particle accelerator at Fermilab. The new value has an uncertainty of about 0.43% -- the most precise value from a single measurement -- and exceeds the precision of the current world average, which is based on several independent experiments.

* The D0 Collaboration (contact author: Oleg Brandt), “Precision measurement of the top quark mass in lepton+jets final states,” Physical Review Letters (expected publication date: Jul 17)

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A Wormhole for Electrons

A team of researchers from Portugal and the U.S. has shown how sheets of graphene can be used to build a new material that acts as a “wormhole” for electrons: a perfect tunnel that connects two regions of space, as if the in-between region did not exist. The scheme may have far reaching implications for graphene-based electronics.

* D.E. Fernandes, N. Engheta, and Mario G. Silveirinha (contact author), “Wormhole for electron waves in graphene,” Physical Review B (published Jul 11)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Matteo Rini, PhD 

Deputy Editor, Physics 

mrini@aps.org  

http://physics.aps.org

Simple Molecules Make Accurate Clocks, The Physics of Drop Impact, Building a Gamma-Ray Laser with Antimatter

APS Physics Tip Sheet – Jul 8, 2014

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Simple Molecules for Accurate Clocks

A team of researchers from Germany, Bulgaria and Russia has suggested that some of the simplest molecules, such as molecular hydrogen, could be used to build a new class of ultraprecise clocks. In the proposed scheme, the rate at which the clock ticks would be determined by a combination of molecular transitions whose shifts due to external perturbations would cancel out, yielding a very stable frequency. The authors’ calculations suggest a potential order-of-magnitude improvement compared to the best existing schemes based on atoms.

* Stephan Schiller (contact author), D. Bakalov, V.I. Korobov, “The simplest Molecules and Clocks,”Physical Review Letters (published Jul 8)

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The Physics of Drop Impact

​When a drop hits a surface, the resulting outcome depends critically on the surrounding gas.

A drop falling onto a surface may either spread in a smooth blob, or splash and disintegrate into smaller droplets, but the factors determining which of these two outcomes occurs are not entirely clear. Researchers in Spain used high-speed cameras to film droplets of different liquids as they fell onto a solid surface at varying speed. The analysis of the experiments allowed the authors to derive criteria for predicting when a drop splashes. The researchers conclude that a so-far under-appreciated factor—the gas surrounding the droplet—plays a crucial role because it determines the aerodynamic lift that propels the sheet of impacting liquid upwards until it ruptures. The model may find a wide range of applications, from inkjet printing to surface coating technologies.

* G. Riboux, José Manuel Gordillo (contact author), “Experiments of drops impacting a smooth solid surface: A model of the critical impact speed for drop splashing,” Physical Review Letters (expected publication date: Jul 11)

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Building a Gamma-Ray Laser with Antimatter

Calculations suggest that an efficient gamma-ray laser could be built using a Bose-Einstein condensate of electron-antielectron pairs.

Recent proposals have suggested that gamma ray lasers could be built if a Bose-Einstein condensate of positronium—atoms made of an electron and an antielectron (positron)—became available. In such schemes, gamma rays would be generated by the annihilation of electrons with antielectrons and amplified in a cavity formed by two mirrors. But the lack of mirrors that can reflect gamma rays would still pose a key obstacle to the realization of such lasers. Researchers in Armenia have now reported calculations suggesting that with the appropriate parameter choice lasing would be so efficient that an intense gamma-ray beam could be generated in a single pass, without requiring mirrors. Gamma ray lasers could have a number of important applications, from high-resolution imaging to new ways of controlling nuclear reactions.

* Hamlet K. Avetissian (contact author), A.K. Avetissian, G.F. Mkrtchian., “Self-amplified gamma-ray laser on positronium atoms from Bose-Einstein condensate,” Physical Review Letters (expected publication date: Jul 10)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Matteo Rini, PhD 

Deputy Editor, Physics 

mrini@aps.org  

http://physics.aps.org

APS Physics Tip Sheet – Jul 1, 2014

In this issue: A Switch for Sound, A Missing Piece for the Proton Spin Puzzle, Smarter Pulse Shaping for Fiber Optics, Spins That Control Heat Flow

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A Switch for Sound

​A novel material made of rubber and metal absorbs or transmits sound waves depending on how it is squeezed.

Acoustic metamaterials are composite materials that can be used to control sound waves in ways impossible with natural materials, with potential applications in noise reduction and in “stealth” technology. Researchers at Harvard University have designed a material whose response to sound waves changes when it is squeezed. The material consists of tiny copper circles embedded in rubber. Since the circles vibrate at different frequencies when the rubber is bent or buckled, the acoustic response is tunable: Experiments show that the structure behaves like a switch that blocks or lets pass a certain band of sound frequencies, depending on the applied pressure. 

* P. Wang, Katia Bertoldi (contact author) et al., “Harnessing buckling to design tunable locally resonant acoustic metamaterials,” Physical Review Letters (expected publication date: Jul 3)

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A Missing Piece for the Proton Spin Puzzle

A new analysis of proton-proton collision experiments suggests that gluons may provide a significant fraction of the proton's spin.

Physicists originally expected that the quarks that make up a proton determine the particle’s spin. But experiments in the 1980s showed that only a minor fraction of the proton's spin comes from its quarks—a problem known as the “proton spin crisis”. The rest has to come from gluon spins and/or the orbital motion of quarks and gluons inside the proton. A team of researchers from Argentina, Germany and the US has analyzed recent proton-proton collision experiments carried out at Brookhaven National Lab’s Relativistic Heavy Ion Collider (RHIC). The comprehensive analysis delivers the first clear evidence that the gluon spin polarization is not zero, and suggests that gluons may carry as much as half of the spin of the proton.

* D. de Florian, Marco Stratmann (contact author) et al., “Evidence for polarization of gluons in the proton,” Physical Review Letters (expected publication date: Jul 2)

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Smarter Pulse Shaping for Fiber Optics

Researchers propose a new way to eliminate signal distortions that limit the speed of data transmission in an optical fiber. 

Optical nonlinearity (the phenomenon by which a light pulse changes the index of refraction of the medium in which it propagates) is an important factor that causes the distortion of light pulses in an optical fiber, limiting how fast data can be transmitted. Because nonlinear effects are mathematically complex, it is difficult to design schemes that correct them. But a team of researchers from the UK, Israel and the US might have found a mathematical trick to simplify the problem. The researchers’ calculations showed that signals could be encoded in certain waveforms that behave like sinusoidal waves in a fiber with no nonlinearities, allowing distortion-free transmission over a 2000-kilometer fiber.

* Yaroslav E. Prilepsky (contact author) et al., “Nonlinear inverse synthesis and eigenvalue division multiplexing in optical fiber channels,” Physical Review Letters (published Jul 1)

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Spins That Control Heat Flow

The Peltier effect, used in many electrically controlled refrigerators, creates a temperature difference between two materials when a current passes through them. Researchers from the Netherlands, Japan and France have now observed the “spin-Peltier effect”, in which the flow of heat is controlled by the injection of a spin current into the junction between a magnetic insulator and a metal. The effect opens up new possibilities for solid-state heat pumps and temperature-controllers that could offer alternatives to bulky processes like vapor compression. 

* Joost Flipse (contact author), et al., “Observation of the spin Peltier effect for magnetic insulators,” Physical Review Letters (expected publication date: Jul 7)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Matteo Rini, PhD 

Deputy Editor, Physics 

mrini@aps.org  

http://physics.aps.org

Gamma-ray Bursts Guide Search for Gravitational Waves, A Crystalline Undulator, The Shape of Cosmic Voids, Can Bilayer Graphene Compete With Silicon?

APS Physics Tip Sheet – Jun 24, 2014

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Gamma-ray Bursts Guide Search for Gravitational Waves

A five-year search for gravitational waves by two large-scale experiments results in no detection—but suggests an observation may soon be within reach.

Two large international collaborations, the Laser Interferometer Gravitational-Wave Observatory (LIGO), running observatories in Louisiana and Washington, and Virgo, with an interferometer in Italy, have reported on their five-year search for gravitational waves. The experiments were guided by data from the International Satellite Network, a group of satellites measuring gamma ray bursts (GRBs) from all directions of the sky: Since GRBs are associated with events that might also release gravitational waves (like the merger of a neutron star with a black hole), the researchers focused their measurements on the arrival time and direction of GRBs. While the observed GRB sources were too distant to generate detectable GWs, the analysis suggests that GW might soon be observed: with GW sources at comparable distances, improved LIGO and Virgo detectors, plan to be operational in 2015, should be able to spot the elusive waves.

* LSC - Virgo Collaboration - IPN Collaboration (contact author: Nicolas Leroy), “Search for gravitational waves associated with γ-ray bursts detected by the interplanetary network,” Physical Review Letters (expected publication date: Jun 30)

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A Crystalline Undulator

An undulator made of a crystal, rather than a periodically arranged set of magnets, can generate very short wavelengths in the gamma-ray regime.

Undulators are periodic arrangements of magnets used as insertion devices at synchrotron light sources to generate x-ray beams. They can produce light with wavelengths as short as a tenth of an angstrom. But generating even shorter wavelengths, which could penetrate deeper into materials and image them with greater resolution, is problematic: it would require undulators with very short periods, which are impossible to build with current technologies. Researchers from Denmark and Germany have now realized a “crystalline undulator,” in which the periodicity is not provided by magnets but by the extremely strong internal electromagnetic field of a crystal. The scheme could lead to compact light sources operating in previously inaccessible hard x-ray and gamma-ray regimes.

* Tobias Nyholm Wistisen (contact author), et al., “Experimental realization of a new type of crystalline undulator,” Physical Review Letters(published Jun 24)

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The Shape of Cosmic Voids

Cosmic voids are regions of the universe with very low densities (less than one-tenth of the average matter density typical of the observable Universe). Researchers in France and the US have carried out calculations to predict how such voids are distributed on cosmic scales. Assuming standard cosmological models, they found that matter has a foamy distribution in which bubble-like voids have a surprisingly uniform average shape. The authors suggest that the combination of their detailed predictions with future void observations could provide a powerful alternative to conventional cosmology based on the observation of matter, helping to test cosmological, dark matter and gravity models.

* Nico Hamaus (contact author), P. M. Sutter,  B.D. Wandelt, “A Universal Density Profile for Cosmic Voids,” Physical Review Letters (expected publication date: Jun 27)

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Can Bilayer Graphene Compete With Silicon?

Time-resolved experiments image the motion of electrons in bilayer graphene, confirming the material behaves as a semiconductor that has potential for electronics applications. 

Graphene—the one-atom-thick layer of pure carbon discovered in 2004—could be used to build logic circuits that, in principle, could be much smaller than their silicon counterparts. Yet a key obstacle blocks the development of graphene-based circuits: graphene isn’t a a semiconductor and cannot be used to build transistors. Researchers have previously explored a possible solution based on bilayer graphene, which can be turned into a semiconductor by the application of an electric field. But transistors built with bilayer graphene haven’t performed well in tests. A team of researchers from Denmark, Switzerland, the UK, Germany and Italy has now used an advanced form of electron spectroscopy to study how electrons move in bilayer graphene, shedding light on its conductivity properties and suggesting growth strategies that might finally enable the construction of technologically viable transistors.

* S. Ulstrup, Philip Hoffman (contact author) et al., “Ultrafast dynamics of massive Dirac fermions in bilayer graphene,” Physical Review Letters (expected publication date: Jun 25)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Matteo Rini, PhD 

Deputy Editor, Physics 

mrini@aps.org  

http://physics.aps.org

A Magnetic Hose, A Soft Neutron Skin, Charged-up DNA Helps Viruses

APS Physics Tip Sheet – Jun 10, 2014

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A Magnetic Hose

​Researchers have built a magnetic “hose” that can channel a magnetic field from a source to a target.

Many modern technologies, from fiber communications to power grids, rely on the ability to transmit electromagnetic waves with minimal loss of energy and information. But compared to light or electric fields, static magnetic fields are much more difficult to channel over long distances. A team of researchers from Spain, Austria and Germany has designed and tested a magnetic “hose” that can channels the magnetic field from a source to a distance several centimeters away. The scheme, consisting of roll made of alternating layers of a magnetic material and a superconductor, could transfer a field over a distance of fourteen centimeters. This ability to transfer magnetic fields without degradation could be used to increase the storage density of magnetic memories, or to create high-resolution magnetic images of materials and devices.

* C. Navau, Alvaro Sanchez (contact author) et al., “Long-distance Transfer and Routing of Static Magnetic Fields,” Physical Review Letters(expected publication date: Jun 23)

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The Soft Skin of Neutrons

The outer layer of a lead nucleus is mostly made of neutrons, and is a soft and wispy halo, rather than a hard and well-defined surface

In heavy nuclei, the distribution of neutrons can extend further out in radius than that of protons, producing a neutron rich outer layer called the “neutron skin.” While proton distributions have been accurately measured with electron scattering, probing the neutron distribution is more challenging. Previous experiments have estimated the neutron skin thickness by bombarding nuclei with protons and other particles. Now a collaboration of scientists from Europe, the US and Russia, working at the Mainz Microtron (MAMI) facility in Germany has, for the first time, used photons to characterize the outer nuclear layer. By firing a beam of high-energy photons (hundreds of mega-electron-volts) on a lead target, the researchers were able to obtain a detailed characterization of the neutron skin, showing that it is not a sharply defined surface but rather a "soft" spherical halo. The result may help refine theoretical models of nuclear structure.

* The Crystal Ball at MAMI and A2 Collaboration (contact author: Daniel P. Watts), “Neutron skin of 208-Pb from coherent pion photoproduction,” Physical Review Letters (expected publication date: Jun 18)

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Charged-up DNA Helps Viruses

Researchers have studied how viruses pack DNA as they replicate, showing how DNA self-repulsion makes packing more efficient.

As a virus produces a copy of itself, it uses certain “molecular motor” proteins to pull a strand of DNA from the infected host cell into its newly constructed shell. Since DNA is negatively charged the packaging motor has to push against an increasing pressure as more DNA collects inside the virus, so researchers assumed that DNA self-repulsion acted as an obstacle to the process. But a US research team has carried out experiments showing that some degree of repulsion helps the motors work faster, perhaps because the repulsion can prevent DNA from becoming entangled with itself, like a long strip of sticky tape. The results were obtained using optical tweezers to track the motion of a DNA strand as it was "reeled in" by an assembling virus. The finding provides a clearer understanding of how viruses operate, but could also inform biotechnologies that enclose long polymers within nanoscale devices. 

* Nicholas Keller, Douglas E. Smith (contact author), et al., “Repulsive DNA-DNA interactions accelerate viral DNA packaging in phage phi29,”Physical Review Letters (published June 17)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Matteo Rini, PhD 

Deputy Editor, Physics 

mrini@aps.org  

http://physics.aps.org