Physics tip sheet #38 - November 3, 2003

November 03, 2003

1) Most efficient solar cells made possible by new material
K. Yu et. al.
Physical Review Letters (to appear)

A new semiconductor material could lead to solar cells with efficiencies up to 56%, a huge improvement over current solar cells, which convert only about 30% of light into power. In standard solar cells, light absorbed by the cell is converted to power by knocking loose an electron, allowing current to flow. Based on the properties of the particular semiconductor material used in the cell, the light must have a certain amount of energy, called the band gap, to knock an electron loose. Light with lower energy will not be absorbed; light with higher energy will be absorbed, but the extra energy will be wasted. The new material, a ZnMnTe crystal with added oxygen impurities, has three band gaps instead of one, and thus takes advantage of a much larger range of the solar energy spectrum. The researchers synthesized the material using a novel technique called oxygen ion implantation and pulsed laser melting. Adjusting the amount of oxygen in the material varies the band gap to optimize power conversion.

Journal article: Available to journalists on request

2) 1000 times faster magnetic memory devices
A. Melnikov et. al.
Physical Review Letters (to appear)

Magnetic data storage devices could soon speed up by a factor of 1000. While these devices have greatly increased in density in recent years, the speed at which they write information is still determined by the time required to switch a magnet. A new paper suggests achieved that time can be significantly reduced. The authors found that on Gadolinium, a ferromagnetic metal, atomic vibrations couple to magnetization almost instantaneously, allowing much faster data storage at up to THz frequencies.

Journal article: Available to journalists on request

3) Name-dropping and self-promotion pay off
M. Rosvall and K. Sneppen
Physical Review Letters (print issue: 24 October 2003)

Correctness is not as important as communication. That's a key finding from a new model of the Internet and other social and biological networks. The authors model society as a network of individuals, all trying to adjust their connections to get as close as possible to the center of the network. The authors find that where local communication is strong, the society tends to organize around a stable central hub, but when local communication is weak, chaos ensues. For an individual trying to be the center of attention, success comes from sharing the information one has about the position and connections of others, even if that information is incorrect. Surprisingly, having correct information barely improved an individual's status at all.

Journal article:

4) Quantum Mirrors
D. Clougherty
Physical Review Letters (to appear)

A cold atom can never get close enough to a surface to stick. This strange "quantum reflection" effect, explored in a new paper, is a result of the wave nature of the atom. While classical mechanics predicts that cold particles always stick to a surface, quantum mechanics predicts that they almost never do. The author shows that in some cases the probability of sticking is even lower than previously thought. In addition to being theoretically interesting, the phenomenon could be important in atomic mirrors or other "atom optics," atom trap experiments, and atomic clocks.

Journal article: Available to journalists on request

5) Negative friction from intermolecular force
A. Cohen and S. Mukamel
Physical Review Letters (to appear)

A force that acts between molecules can create negative friction, making molecules speed up when passing each other instead of slowing down. Though this van der Waals force is usually attractive, a new study shows that it may become repulsive if the interacting molecules are in excited states, or if they are out of thermodynamic equilibrium. This results in a greatly reduced, or even negative, friction. The study also found that the van der Waals force, typically considered the weakling of intermolecular forces, can become up to 50 times stronger under these non-equilibrium conditions. The theory may be applicable to cells, which run on interactions between chemically excited molecules, and in photosynthesis, in which optically excited molecules interact. The researchers also suggest the phenomenon may be useful in design of micromechanical systems and may lead to new ways to control matter on the nanoscale.

Journal article: Available to journalists on request

American Physical Society

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