In this issue: Sliding Sand, A Tractor Beam Made of Sound, Neutron Star Goes Wobbly, Element Z=117 Confirmed, Remote Controlled Entanglement, The Arrow of Time in Physics and Psychology

APS Physics Tip Sheet – Apr 29, 2014

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Sliding Sand

​Adding a small amount of water to sand can significantly reduce the sliding friction.

Pouring some water—but not too much—on sand makes it easier to slide objects thanks to the formation of capillary bridges. This is the conclusion of a study by a team of researchers from the Netherlands, Iran, France, Germany and India. The authors tested the sliding friction of dry and wet sand when a weighted sled was pulled across the surface. As water was added, both the force needed to pull the sled and the friction coefficient were found to decrease below that of the dry sand, reaching a minimum before increasing again as more water was added. The authors explain that this is due to capillary liquid bridges that start to form between the sand grains when water is added, facilitating the sliding. But if too much water is added the capillary bridges merge and disappear. 

* A. Fall, Daniel Bonn (contact author) et al., “Sliding Friction on Wet and Dry Sand,” Physical Review Letters (published Apr 29)

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A Tractor Beam Made of Sound

Sound waves can be engineered to realize a tractor beam – a device able to pull centimeter-size objects.

A “tractor beam”—a term coined in the 1931 science-fiction novel Spacehounds of IPC by E.E. Smith—is a device that can attract one object from a distance. Researchers have had some success at a microscopic level, demonstrating optical tweezers that can pull nanoparticles opposite to the light propagation direction. Now a team of researchers from the UK and the US has reported a step towards a macroscopic device. The scheme is based on a sound beam that generates low-pressure zones towards which an object can be attracted. The authors were able to scale up the tractor beam to handle centimeter-size objects, a million times larger than those manipulated by previous schemes.

* Christine E.M. Démoré (contact author) et al., “Acoustic tractor beam,” Physical Review Letters (expected publication date: Apr 30)

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Neutron Star Goes Wobbly

The x-ray emission from a magnetar has revealed that the star’s huge magnetic field has distorted its shape, causing it to wobble.

Magnetars are neutron stars with a colossal magnetic field. Having  magnitudes up to 10¹¹ tesla, their fields are a billion times stronger than the most powerful magnets on Earth. Possible evidence of an even stronger field comes from recent x-ray observations of one of the brightest magnetars (4U 0142+61, located in the constellation Cassiopeia at 13000 light-years from Earth). A team of researchers from Japan and the US has reported the detection of a time-varying x-ray signal, indicating a wobble, or precession, in the magnetar’s rotation caused by an internal field of about 10¹² tesla. The authors suggest that a strong internal toroidal field deforms the magnetar into a prolate shape, like a football, which wobbles as it spins. 

* Kazuo Makishima (contact author) et al., “Possible evidence for free precession of a strongly magnetized neutron star in the magnetar 4U 0142+61,” Physical Review Letters (expected publication date: Apr 30)

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Element Z=117 Confirmed

​

An international collaboration has used an intense isotope beam provided by the GSI research facility in Darmstadt, Germany, and a target material of radioactive berkelium supplied by Oak Ridge National Lab in Tennessee to produce two atoms of the superheavy element with atomic number Z=117 (ununseptium). The result confirms the initial observation published in 2010 –an essential step towards the recognition of the discovery of the element. In the process, the researchers also discovered a new isotope (lawrencium-266). 

* J. Khuyagbaatar (contact author) et al., “Study of the ⁴⁸Ca+²⁴⁹Bk fusion reaction leading to element Z =117: long-lived-decaying ²⁷⁰Db and discovery of ²⁶⁶Lr,” Physical Review Letters (expected publication date: May 01)

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Remote Controlled Entanglement

Researchers from California and India have created entangled states of two superconducting qubits separated by more than a meter of coaxial cable. While it is now routine to entangle photons at a distance, the authors have managed to entangle two superconducting circuits using microwave radiation transported by 1.3 meters of coaxial cable. The result shows that quantum entanglement can be established between distant systems that interact only through a signal propagating along low-loss electrical wires. This functionality may be exploited in future quantum networks.

* Nicolas Roch (contact author) et al., “Observation of measurement-induced entanglement and quantum trajectories of remote superconducting qubits,” Physical Review Letters (published Apr 28)

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The Arrow of Time in Physics and Psychology

Researchers from Caltech and University of South California have presented a conjecture of why we perceive time as flowing from past to future—the same direction determined by the laws of thermodynamics. According to their theory, the psychological arrow of time is determined by a principle they call “generality”: this states that what defines a memory is the correlation with the things it remembers. A memory can thus be correlated with our past, but not with our future, since the correlation with the future would be contingent on different possible outcomes. This, suggest the authors, is what makes a prediction psychologically different from a true memory, forming the basis of our perception of time.  

* Leonard Mlodinow, Todd A. Brun (contact author), “On the Relation between the Psychological and Thermodynamic Arrows of Time,”Physical Review E (expected publication date: May 02)

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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

Sterile Neutrino as Dark-Matter Candidate, Amber Does Not Act Its Age, Stopping Light in a Waveguide, An Undulator Made of Microwaves

APS Physics Tip Sheet – Apr 22, 2014

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Sterile Neutrino as Dark-Matter Candidate 

A dark matter particle in the form of a so-called sterile neutrino could explain the recent detection of an x-ray emission line from galaxy clusters.

Recent astronomical observations of the Andromeda Galaxy and the Perseus Cluster have revealed an x-ray emission line whose origin cannot be explained. A researcher from UC Irvine has now suggested that such x rays could be generated through the decay of dark matter particles in the form of sterile neutrinos—hypothetical neutrinos that do not interact via any of the fundamental interactions except gravity. The researcher calculates that the decay of sterile neutrinos would produce x rays exactly at the observed wavelength. Further, he shows that a cosmological model based on this sterile-neutrino dark-matter particle would solve two key problems faced by the currently most popular dark-matter models: correctly predicting both the number of Milky Way satellite galaxies and their central densities.

* Kevork N Abazajian (contact author), “Resonantly-Produced 7 keV Sterile Neutrino Dark Matter Models and the Properties of Milky Way Satellites,” Physical Review Letters (expected publication date: Apr 24)

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Amber Does Not Act Its Age

110-million-year-old amber samples surprisingly retain the same thermodynamic properties as much younger glasses.

Amber is a unique example of a glass because it has “hyperaged,” that is, it has undergone thermodynamic stabilization for millions of years—a process that is impossible to replicate in a lab. This allows researchers to study how the properties of a glass change with age. A team of researchers from Spain has measured the specific heat and other thermodynamic properties of a 110-million-year-old amber sample, finding that such features are identical to those of younger samples. The result implies that the thermodynamic properties remain fossilized in the glass—much like a trapped insect—and are surprisingly unchanged by 110 million years of aging.

* T Pérez-Castañeda, RJ Jiménez-Riobóo, Miguel A Ramos (contact author), “Two-level systems and boson peak in 110-million-year-aged amber glass,” Physical Review Letters (published Apr 22)

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Stopping Light in a Waveguide

A new proposal suggests light could be stopped in a compact, solid-state device working under ambient conditions.

Researchers have recently demonstrated a variety of methods to stop light—a phenomenon that could be used, for instance, to process information in an optical network.  But most available techniques are based on complex setups using ultracold gases and can only stop light in a very narrow band of frequencies. A new method, theorized by a group at Imperial College, in London, could decelerate light pulses down to 20 meters per second inside a waveguide made of a silicon core surrounded by a metal layer. The scheme, which could lead to practical devices working under ambient conditions, could stop light for a time long enough for applications like fast optical switches, nanolasers, and solar cells. 

* Kosmas L Tsakmakidis (contact author) et al, “Completely Stopped and Dispersionless Light in Plasmonic Waveguides,” Physical Review Letters (expected publication date: Apr 25)

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An Undulator Made of Microwaves

Researchers at SLAC National Accelerator Laboratory have used a microwave cavity to build an undulator—a device that causes electrons to jiggle and emit a beam of intense and coherent x rays. Instead of using the field of fixed magnets as in conventional undulators, the scheme exploits the magnetic and electric fields of intense microwaves in a cavity. The new type of undulator may outperform conventional ones in the generation of x-ray radiation at very short wavelengths for high-resolution imaging and crystallography applications.

* S Tantawi, Muhammad Shumail (contact author) et al, “Experimental demonstration of a tunable microwave undulator,” Physical Review Letters (expected publication date: Apr 23)

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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

The Physics of Voting, Dark Matter as a Trigger for Comet Impacts, Neutrons Put Limits on Dark Matter and Energy, The Key to Thin-Film Solar-Cell Efficiency

APS Physics Tip Sheet – Apr 15, 2014

In this issue: The Physics of Voting, Dark Matter as a Trigger for Comet Impacts, Neutrons Put Limits on Dark Matter and Energy, The Key to Thin-Film Solar-Cell Efficiency

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The Physics of Voting

A computer simulation of voter behavior is able to reproduce and explain long-term electoral patterns observed in US presidential elections.

In recent years, researchers have developed a number of mathematical theories to analyze the behavior of voters in elections, but no model has been shown to reproduce real-world data. Now, a team of researchers from Poland and Spain tested a voter model with data from U.S. presidential election results. The model is based on the idea that the influence of one voter on another is similar to diffusion of particles in liquids. The simulations yielded a county-level map of electoral outcomes strikingly similar to actual U.S. presidential election results between 1980 and 2012, and could reproduce a number of observed statistical properties of the data. The authors suggest that, while the model cannot predict election outcomes, it explains what is behind who's winning. 

Juan Fernandez-Gracia (contact author) et al, “Is the voter model a model for voters?”, Physical Review Letters (expected publication date: Apr 18)

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Dark Matter as a Trigger for Comet Impacts

An increased likelihood of life-threatening comet impacts could occur when the Sun passes through a hypothetical dark matter disk in the plane of our galaxy.

Physicists have recently suggested that there's a thin, dense disk of dark matter running along the mid-plane of our Milky Way galaxy. Our solar system would then trace an up-and-down, wavy motion through that plane as it travels around the galaxy. According to a study by researchers at Harvard, the passage through such dark matter disk could disturb the path of certain comets or asteroids - and send them hurtling towards our planet, causing impacts of the kind that likely killed off the dinosaurs. The hypothesis is corroborated by the observation that comet impacts occur with a 35-million-year periodicity that closely matches the rate at which the Sun passes through the plane of the galactic disk.

* Lisa Randall and Matthew Reece (contact author), “Dark Matter as a Trigger for Periodic Comet Impacts”, Physical Review Letters (expected publication date: Apr 21)

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Neutrons Put Limits on Dark Matter and Energy

The behavior of a neutron bouncing in the gravitational field of the Earth can improve what we know about dark energy and dark matter. 

A team of researchers from Austria, France and Germany has studied the behavior of neutrons in the gravitational field of the Earth to put constraints on dark energy and dark matter scenarios. The authors made an “artificial atom” by confining neutrons in a thin cavity between two mirrors.  Similarly to light-induced electronic transitions in an atom, varying the distance between the mirrors can cause the neutrons to climb up and down the quantized energy levels in this gravitational box. The interaction of dark matter or dark energy particles could cause shifts in the energy levels. The lack of observation of such shifts allowed the authors to derive some of the most stringent limits on parameters of potential dark matter and dark energy particles.

* T Jenke, Hartmut Abele (contact author) et al, “Gravity resonance spectroscopy constrains dark energy and dark matter scenarios”, Physical Review Letters (expected publication date: Apr 16)

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The Key to Thin-Film Solar-Cell Efficiency

Thin-film solar cells are now a serious competitor to silicon cells for power generation, with comparable efficiencies and rapidly decreasing costs. Cadmium telluride (CdTe) is one of the most promising thin-film materials, with a record efficiency of 20%, obtained by including a stage during synthesis in which the material is treated with cadmium-chloride. But why such treatment leads to better efficiency has remained a mystery and progress has been driven by incremental trial and error. Now, a team of researchers from the UK and the US, through atomic imaging experiments and new calculations, showed the large photovoltaic efficiency is due to the polycrystalline nature of the films, which helps separating photogenerated electrons and holes and preventing unwanted recombination.

* Chen Li (contact author) et al, “Grain-boundary-enhanced carrier collection in CdTe solar cells”, Physical Review Letters (expected publication date: Apr 16)

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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

Sleep is Different if You Have Parkinson's Disease

The frequency of rolling over during sleep varies in a measurable way between healthy subjects and those with neurodegenerative disorders. A computer model produces similar results.

Below is the abstract for Phys. Rev. E 89, 032721, published 31 March:

 

Turnover is a typical intermittent body movement while asleep.

Exploring its behavior may provide insights into the mechanisms and

management of sleep. However, little is undertood about the dynamic

nature of turnover in healthy humans and how it can be modified in

disease. Here we present a detailed analysis of turnover signals that

are collected by accelerometry from healthy elderly subjects and

age-matched patients with neurodegenerative disorders such as

Parkinson's disease. In healthy subjects, the time intervals between

consecutive turnover events exhibit a well-separated bimodal

distribution with one mode at less than equal 10 s and the other at

greater than equal 100 s, whereas such bimodality tends to disappear

in neurodegenerative patients. The discovery of bimodality and fine

temporal structures (less than equal 10 s) is a contribution that is

not revealed by conventional sleep recordings with less time

resolution (approximately equal 30 s). Moreover, we estimate the

scaling exponent of the interval fluctuations, which also shows a

clear difference between healthy subjects and patients. We incorporate

these experimental results into a computational model of human

decision making. A decision is to be made at each simulation step

between two choices: to keep on sleeping or to make a turnover, the

selection of which is determined dynamically by comparing a pair of

random numbers assigned to each choice. This decision is weighted by a

single parameter that reflects the depth of sleep. The resulting

simulated behavior accurately replicates many aspects of observed

turnover patterns including the appearance/disappearance of bimodality

and leads to several predictions, suggesting that the depth parameter

may be useful as a quantitative measure for differentiating between

normal and pathological sleep. These findings have significant

clinical implications and may pave the way for the development of

practical sleep assessment technologies.

A Lens for Focusing Water Waves

From the authors of Phys. Rev. E 89, 023012, published 19 February:

Experiments show that features on the seafloor in shallow water can focus waves, just as glass lenses focus light waves.




Glass lenses can focus light at a focal point.  Here we show that small features added to the seafloor can result in a similar lens for overpassing oceanic waves. These seafloor features are typically in the shape of curved sandbars, and the effect is a result of a nonlinear interaction between surface waves and bottom undulations which is known as “Bragg Resonance”. Wave lensing may substantially contribute to the efficiency of ocean wave energy devices by providing localized high-energy wave zones. Therefore instead of many small and low-efficiency wave energy devices dispersed over a wide area, one (relatively large/high efficiency) device can be placed at the focal point receiving the entire energy of the initial area. This should be of interest of the environment and also the sea transportation as the covered surface of the sea is significantly reduced. Wave lensing can also be used to create artificial surf zones. It may have applications, by dispersing wave rays (a convex lens), in creating localized safe havens for fishermen and sailors in open seas, or if implemented in large scales to protect shores and harbors against strong storm waves. Quiet beaches and open-sea water parks are other potential application of the gravity wave lensing. The idea may be also used as a guideline for proper placement of (nearshore) facilities particularly in the areas with substantial bottom variations. The efficiency of the idea is particularly stronger over the shallower waters of continental shelves.

Videos:
1- Surface Gravity Wave Lensing, direct simulation:
http://www.youtube.com/watch?v=0XcPOEaEFWg
2-  Raw footage of the experiment at Richmond Field Station (above):
http://www.youtube.com/watch?v=6dHG7jNC_nc
3- Comparison of Numerical Simulation and Experiment:
http://www.youtube.com/watch?v=3ArPkKMN7aE

APS Physics Tip Sheet – Feb 11, 2014

In this issue: Biocompatible Carbon, Quantum Lego, A Microscopic View of Cracking

* Need an outside expert to comment on your science story? Check out the APS Science Expert Exchange:http://apsphysicsnewsticker.blogspot.com/2014/01/aps-science-expert-exchange.html

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Biocompatible Carbon 

Simulations show that lipid membranes called liposomes can be used as biocompatible solvents for carbon nanoparticles in medical applications. 

C₆₀ fullerenes, aka buckyballs, are being investigated for applications in nanomedicine, from drug delivery vectors to diagnostic contrast agents. Their use faces an important challenge: there are few solvents for fullerenes that are nontoxic and biocompatible. Recent work has shown that liposomes (vesicles made of lipid bilayers) may be an efficient fullerene solvent, but little is known on how to design optimal fullerene-carrying liposomes. Now, a research team in France has presented simulations that explain how liposomes dissolve fullerene clusters and suggest they may be regarded as biocompatible solvents that can be chemically tuned to specific medical applications.  

* J Barnoud, G Rossi, Luca Monticelli (contact author), “Lipid membranes as solvents for carbon nanoparticles”, Physical Review Letters (expected publication date: Feb 12)

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Quantum Lego

Theorists have proposed a modular design for quantum computers that can be used to assemble large numbers of qubits into a fault-tolerant device. 

A major challenge of quantum computing is the assembly of individual qubits into an actual working device: a large number of logic elements need to communicate over data channels while preserving the fragile entanglement between qubits that is the basis of any quantum advantage. A team of researchers from the US, Canada and China has confronted this problem with a proposed modular quantum computing design that is scalable to a large number of qubits while remaining fault tolerant. The scheme is based on building blocks made of arrays of trapped ions controlled with optical or microwave pulses and connected via optical fibers.

* C Monroe, Jungsang Kim (contact author), “Large Scale Modular Quantum Computer Architecture with Atomic Memory and Photonic Interconnects”, Physical Review A (expected publication date: Feb 13)

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A Microscopic View of Cracking

A new microscopic model describes how porous materials such as sandstone break, providing good agreement with the behavior observed in experiments.

Modeling the catastrophic failure of materials has a wide range of applications, from earthquake science to civil engineering. The description of such fracture processes is complicated by the fact that microscopic cracks can propagate all the way up to the size of mountains. As a consequence, most currently used models are too simplistic to connect to experiments. A team of researchers from Hungary and Scotland presents an improved model of cracking in a porous material (such as sedimentary rock), which better describes the microscopic material structure. They found that cracks spread through the material in cascading avalanches that happen at erratic intervals, separated by quiescent periods. Their model can reproduce well the complex behavior observed in experiments, including many features of cracking deduced from measurements of the sound waves that accompany the propagation of cracks. 

* Ferenc Kun (contact author), I Varga, S Lennartz-Sassinek, IG Main, Physical Review Letters (expected publication date:Feb 14)

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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