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.
Matteo Rini, PhD
Deputy Editor, Physics
