Nanoscale Dominoes: Magnetic Moments Topple Over in Rows
S. Lounis, P. Dederichs, and S. Blugel
Physical Review Letters (forthcoming)
Physicists at the Institut für Festkörperforschung in Germany have discovered a type of domino effect in rows of individual manganese atoms on a nickel surface. They determined that the magnetic arrangement of these nanowires varies depending on their lengths.
Using computer simulations and statistical models, the physicists found that if only a single atom is added or taken away, the magnetic structure of the nanowire changes entirely. Specifically, when the number of atoms is odd, the magnetic moments (a measure of how well the nanowire acts as a magnet) are neatly aligned in opposite directions. When the number of atoms is even, the moments line up randomly, in a jumbled mix of different positions. Adding an atom at the end of the nanowire or taking one away causes the magnetic moments to topple over like a row of dominoes. But unlike dominoes, the effect can be completely reversed.
This new quantum mechanical effect makes magnetic switches possible on an atomic scale, and could one day be applied to the transporting and storing of magnetic information onto extremely small spaces. Their work is importance for the design of high speed, high storage capacity, and energy saving computing devices. The authors hope that it will be proven experimentally in the near future. -NR
The Price of Anarchy on the Road
Hyejin Youn, Michael T. Gastner, and Hawoong Jeong
Physical Review Letters (forthcoming)
Shutting down select streets could improve traffic in congested cities, according to a study by a collaboration of Korean and US physicists, and building additional roads to reduce congestion can potentially make things much worse. The paradoxical situations arises because the benefit to individual drivers that comes from choosing the quickest route sometimes ends up slowing traffic as a whole.
The physcists illustrate the conundrum by considering a starting point and a destination linked by two routes; a short but narrow bridge and a long, wide freeway. The combined travel time of all the drivers is minimized in the hypothetical example if half the drivers choose the bridge and half choose the highway. But potential highway travelers realize that switching to the bridge would shorten their commute, leading to lots of drivers clogging the short route. If the bridge gets so crowded that the trip is slower than taking the highway, some of the drivers switch back to the highway. Eventually, the traffic settles into an equilibrium solution such that no individual driver could reduce their commute time by switching to the other route. Unfortunately, the sum total of all the drivers' commute times is substantially longer than it would be if exactly half of the drivers stuck to each path.
Although the bridge/highway example is hypothetical and highly simplified, the physicists performed analyses of roadways in Boston, New York, and London that showed that the paradox is very likely slowing traffic flow in real life. In one case, they studied the time it takes to travel from Harvard Square to Boston Common. The conflict between individual and the collective commuter benefits causes up to 30% longer commute times overall than the optimal traffic distribution should be able to achieve.
The physicists call the inefficiency that comes from the conflict of interest the Price of Anarchy (PoA). The good news is that the PoA can be reduced by simply shutting down a few streets. If properly selected, eliminating travel options can bring the best interests of individual drivers closer to the interests of the collective commuters. In addition, the study explains why roadway construction specifically intended to enhance traffic flow can inadvertently, and counter-intuitively, make things worse instead of better. -JR
When Lightning Strikes, Spark Branches Reconnect
A. Luque, U. Ebert, and W. Hundsdorfer
Physical Review Letters (15 August 2008)
Bolts of lightning often resemble the forked, branches of trees. Similar to tree branches, lightning sparks typically spread apart. Recently, physicists at Centrum voor Wiskunde un Informatica and Eindhoven University of Technology in the Netherlands have for the first time determined the conditions that allow for spark branches to reconnect, by overcoming the electrostatic repulsion that usually causes them to separate.
These branches, known as streamers, are the building blocks of sparks and lightning. They are actually made of air that has been converted into long, thin, twisty rods of plasma. These streams of plasma serve as a medium or path for electricity to travel through.
The physicists recreated their own lightning environment using sophisticated computer simulations. They studied the dynamics of 3-dimensional streamers in nitrogen-oxygen mixtures like air. They found that with varying air composition and pressure, streamers can either repel or attract each other unexpectedly. Under the right conditions, the streamers can either branch out or recombine, like two rivers coming to together. They concluded that branches reconnect because of a process called photo-ionization, where a cloud of electrons is created between two streamers that eventually makes the coalesce into a single, wider one.
Their study offers new a perspective into understanding lightning's variable behavior. -NR
50 Years of PRL
Martin Blume
Physical Review Letters turns 50 this year. Martin Blume is celebrating the green journal's birthday by summarizing the most intriguing papers to appear in PRL each year since 1958. To see past editions of visit Marty's Milestone PRL project.
This week, Marty is taking a look at a milestone paper from 1985 that led to the 1997 Nobel Prize for the creation of "optical molasses."
Three-dimensional viscous confinement and cooling of atoms by resonance radiation pressure
Steven Chu, L. Hollberg, J. E. Bjorkholm, Alex Cable, and A. Ashkin
Phys. Rev. Lett. 55, 48 (1985)
This Letter reported the demonstration of a powerful method for slowing down (cooling) atoms with the use of an array of six laser beams -- two opposed beams in each of three orthogonal directions. In the experiment, sodium atoms were used and the laser beams were all slightly redshifted with respect to the characteristic wavelength absorbed by stationary sodium atoms. This meant that due to the Doppler shift for the moving atoms, an atom could absorb only photons from laser beams opposed to that atom's motion; this absorption would give the atom momentum in the direction of the laser beam, opposite to the atom's motion, thus slowing it. Though the atom would later emit a photon, this would be in a different direction, so that after repeated absorption and emission its forward motion would be slowed. The environment of the laser-beam array which thus slowed motion of the atoms in any direction was dubbed "optical molasses." With this technique, the atoms were cooled to a temperature of 240 degrees K, thought at the time to be the limiting temperature for cooling these atoms.
The development of laser-cooling techniques made possible important progress in atomic physics, including substantially improved atomic clocks and the discovery of Bose-Einstein condensation in atomic gases.
The 1997 Nobel Prize in physics was awarded jointly to Steven Chu, William D. Phillips, and Claude Cohen-Tannoudji "for development of methods to cool and trap atoms with laser light." For further information, see the Nobel press release (http://nobelprize.org/nobel_prizes/physics/laureates/1997/press.html) and Chu's Nobel lecture [Rev. Mod. Phys. 70, 685 (1998)].
Nadia Ramlagan and James Riordon contributed to this Tip Sheet.
